Janky-ass USB-C to USB-A adapter

Argh, I hate laptops that don’t have USB-A ports. Especially today, as I forgot my adapters at the office. Again!

I happened to have a micro USB connector with exposed conductors. I happened to have a micro USB to USB-C adapter. I have a (non-janky!) USB port with Dupont connectors. One of my best purchases from Hard Off ever. And of course I have plenty of test clips.

(I’m actually supposed to have a USB-C connector with exposed conductors, I just did a couple months ago. But I just can’t find it. Maybe haven’t seen it since moving house.)

Let’s tape it down for electrical safety

And because the power wires on this connector look rather anemic I used a powered hub. A USB 1.1 hub, BTW. Not sure if this is too janky for the blazing speeds of USB 2.0. (Also a great purchase from Hard Off, BTW.) Microcontrollers like the RP2040 and ESP32S3 only support USB 1.1 so these things are useful again these days. E.g. USB (UVC) cameras set a different descriptor when connected via USB 1.1 vs. USB 2.0. Note: my particular hub wouldn’t work until I connected the VCC wire. I don’t think this is required for all hubs.

Simple Apple 2 PSU repair

Well, I finally had a chance to see a few of those infamous RIFA metallized paper capacitors first hand!

Yesterday, I had a look at two Apple IIe machines. One was from 1987. This computer works and is in really good condition and bears the signature of Steve Wozniak. I thought it would be a good idea to go and check for RIFA capacitors in there, but there weren’t any. Here’s a picture of its really clean power supply:

I am guessing that those two white things are the noise suppression caps.

The other Apple IIe doesn’t work. The person running this machine immediately switched off the mains line when smoke started coming out of its power supply. That’s good! The capacitor didn’t go short, and the fuse was still intact. Maybe it even reduced the amount of smoke and brown juice sprayed about its perimeter.

From the symptoms I’d correctly assumed that the owner had witnessed a RIFA capacitor explosion, so I came prepared by buying some replacement caps! I’d expected to have to bodge them due their size difference, but to my surprise, the PSU board had three holes for the noise suppression caps. The RIFA cap was using the outer holes (“holes 1 and 3”), and my smaller cap fit perfectly into holes 2 and 3! Cool beans.

Original RIFA cap (pin spacing of around 2 cm I believe) and replacement (1.5 cm). (The replacement cap has its X2-certification on the other side.)

The PSU had two RIFA caps. The one pictured above is “intact”, though it has a huge crack in its plastic case. Here are the pictures that we’ve all been waiting for, the exploded cap:

The ejected brown juice wasn’t too bad and cleaned up quite nicely.

Here is the computer put back together again, already performing advanced calculations.

The end.

Edit 2024-07-11: one more, an Apple II Plus

This guy’s power supply was riveted. What the actual? I think the screws used were some tamper-proof kind, too. The rivets were drilled out and the screws still wouldn’t come out, beyond a millimeter or so. So the screws were drilled out too. It was a bit of a nightmare. The label on the power supply said Astec AA1040. There were no RIFA caps in it.

Apple 1 (replica) power supply build and repair

A while ago an acquaintance asked me to build a power supply for his replica Apple 1 board. He had all parts on hand (in fact, they were the exact parts mentioned in the Apple 1 documentation and schematics, Stancor P-380 and Stancor P-8667). My acquaintance probably knows a lot more than me, so I basically just did what he asked: I drilled holes into a sufficiently nice piece of wood from the 100-yen shop, soldered connections to the transformers (I believe they were probably salvaged, but luckily still had wires attached), added a fuse, soldered an AC cable into the mix, and soldered the Apple 1 power connector. Doing all this took between 4 and 8 hours, I don’t really remember. Here’s a picture of the assembled power supply.

With multimeters hooked up to each of the voltage rails (-12V, -5V, 5V, 12V), I hid behind a big rock and pushed down on an ACME plunger detonator. The explosion… didn’t take place, and the voltages were all good. Phew! Next I hooked up a period-correct monochrome CRT, and saw an image! A very jumpy image. It was possible to adjust HSYNC on the back of the CRT, which stabilized the image, but it still didn’t look correct. Pressing RESET or any other key on the keyboard didn’t do anything.

Stable (i.e., not rolling) but incorrect image

(Note on fuses: my fuse is on the primary side, mains voltage here is about 100V. I blew a 0.3A fuse on power up, then a 0.5A fuse on power up, and am currently using a 2.5A fuse. Maybe I’ll try 1.0A or 1.5A. Or maybe I should go and look at slow blow fuses.)

Before studying the schematics, I fired up an oscilloscope and had a look at the frequencies of the video signal. Here’s what I found:

Those small yellow blobs in regularly-spaced intervals close to the center line are the wannabe horizontal blank periods. They are supposed to have a frequency of 15.something KHz. In the above picture, I moved the oscilloscope’s vertical cursors further apart such that I get a frequency somewhat close to 15 (12.1 KHz), and as you can see we get 6 horizontal blanks in that period! This means the video signal is much too fast. I also looked for the vertical blank. It’s supposed to be 60 Hz and measured as 243 Hz. (Probably actually 240 Hz.) (At this point it’s worth worrying that we might be overclocking the CPU as well, but that wasn’t the case, it was running at a safe speed.)

At this point it was time to print out the schematics and study them. Here’s a picture of my annotated Apple 1 schematics that I used to debug the problem:

Teletype section (Note that I cannot guarantee that all my annotations are 100% correct.)
CPU section (nothing much annotated here)

Some repair guidelines

There isn’t all that much repair information on the Apple 1, but it’s not too complicated. The computer is basically two devices, a teletype that generates video, and a “computer” section that has the CPU, ROM, and memory. The two devices are linked through an MC6820 chip. There’s one more link; we also get the clock signal for the CPU from teletype section.

One more thing: when you turn on the Apple 1, the CPU isn’t live yet, because there is no reset circuit! (You have to reset the CPU manually using a special key on the keyboard.) So anything that looks too garbage-y right on power up isn’t likely to have much to do with the CPU.

Repair

After studying the schematics, I fired up an oscilloscope and traced the oscillator’s output, through the flip-flop, through various counter stages. At this point, a little luck, or a different strategy, would have helped me find the problem rather quickly, but it took at least an hour (probably more) of poking around until I found something obviously out of the ordinary: the “TC” output of the 74160 decade-counter IC was very low. Man, if it had been a Q output I’d have found it in minutes, heh.

74160’s TC output

I removed other chips the TC signal goes to, removed the 74160, checked for shorts to adjacent pins on the board, but couldn’t find anything. Which means the bad signal is produced by the 74160. Perhaps it would work with a slightly higher voltage, so maybe we’ll keep it. So we decided to see if we could get some replacement 74160 ICs (none of that Low-Power Schottky rubbish), and fortunately someone was selling them on Yahoo Auctions! We procured a few and the replacement 74160 made the problem go away!

(74LS160s or even modern HCTs/ACTs/whathaveyous would have likely worked too, but this replica board is proudly populated with original non-LS chips from mostly the 1970s. It’s all fun and games until they say good-bye!)

There were no more defects and we were able to boot into Wozmon and load BASIC from tape.

How to boot into BASIC

After powering up, clear the screen and reset the CPU using the dedicated keys. You should get a ‘\’ prompt. Type C100R and enter. This runs the cassette firmware. You should get “C100: A9*”. Then type in E000.EFFFR and at the same time you press enter, start playing the tape. (This loads the BASIC interpreter into memory at 0xE000 to 0xEFFF.) When the audio stops and you get ‘\’ you can start the BASIC interpreter by typing E000R and enter. The BASIC prompt is ‘>’.

The acrylic case

For some reason, the acrylic case that was purchased to put this system into didn’t have the screw holes in the right location, so I had the pleasure of adding more screw holes into the case. (Not just that, the lid needed some trimming in order to fit the cassette board, and we also decided to mod the air inlet/outlet a bit.) And though I didn’t have any experience drilling holes into acrylic (or even a lot of experience drilling anything, really), it actually went really well, using these drill bits specially made for use with acrylic: https://www.amazon.co.jp/dp/B00A630ZRE: Acrysunday アクリル板専用ビット.

It all worked out, mostly. When I did the four holes for the fan I unfortunately didn’t take into account that when screwing the nuts onto the bolts, the nut will obviously add a millimeter (or so) of height vs. just a naked bolt. (The case isn’t that high, so I just had a couple of millimeters I could move the holes up or down.) Putting the nuts on the two top bolts will prevent the lid (the one pictured above) from fitting properly, it would rest on the nuts instead of on the bottom case.

We added some heatsinks on all of the counter ICs, all of which were running rather hot (about 65 degrees after 10 minutes of operation), a powerful fan to blow air on the main voltage regulator (which sucks air through a filter so we don’t end up with a bunch of dust inside), and made a make-shift connector for the video port (J2).

Makeshift video connector using screw terminals: general idea
Makeshift video connector: placed on J2 and connected to TV input

How to sniff IR codes with an ESP32 microcontroller without writing a single line of code

I’ve been developing software for the ESP32 and ESP32S3 “professionally” for about one year now. I still like the Raspberry Pi Pico better, but the ESP32 line of microcontrollers is cool too. And if you need wireless and don’t really care too much about power consumption, the ESP32 (or -S3 or what have you) is a nice thing to have around.

The examples in IDF (the name of the ESP32 SDK) are really great, and you can do a bunch of stuff just by taking an example and changing a couple lines.

I’m a huge fan of the Raspberry Pi Pico’s PIO, and the ESP32 has something that is slightly similar, as-in “programmable IO”. It’s called “RMT”, and in examples/peripherals/rmt, we have a few examples that make use of this bit of silicon. The examples/peripherals/rmt/ir_nec_transceiver example is the one we look at in this article.

Basically, all you have to do is flash this to your dev board and connect an IR receiver module’s output pin to GPIO19 (and its VCC and GND to VCC and GND. Be sure to hook it up to 3.3V, not 5V.) If you want to do what this example is supposed to do (output the same signal back through an infrared LED on GPIO18), feel free. But if you just want to sniff codes, leave the infrared LED disconnected and look at the UART output.

Note that the output of the IR receiver is expected to be inverted compared to the actually sent signal. I think most IR receivers invert the signal, so you’ll most likely be fine.

Build steps

If your dev board isn’t a standard ESP32, but e.g. an ESP32S3, you first have to do:

idf.py set-target esp32s3

Then you do:

idf.py flash monitor

This will build the example, flash it to the dev board, and start a monitor.

Then you’ll see a bunch of output like this:

NEC frame start---
{0:218},{1:0}
---NEC frame end: Unknown NEC frame

But if you press a key on a remote control, you may see something like this:

NEC frame start---
{0:9120},{1:4426}
{0:632},{1:498}
{0:622},{1:512}
{0:628},{1:499}
{0:631},{1:525}
{0:606},{1:498}
{0:632},{1:525}
{0:605},{1:501}
{0:628},{1:529}
{0:602},{1:1633}
{0:615},{1:1657}
{0:610},{1:1646}
{0:610},{1:1646}
{0:610},{1:1646}
{0:611},{1:1649}
{0:605},{1:1650}
{0:606},{1:1649}
{0:607},{1:1652}
{0:603},{1:529}
{0:599},{1:530}
{0:596},{1:503}
{0:635},{1:1646}
{0:609},{1:1647}
{0:609},{1:522}
{0:608},{1:497}
{0:633},{1:522}
{0:609},{1:1647}
{0:608},{1:1648}
{0:607},{1:1650}
{0:606},{1:501}
{0:627},{1:529}
{0:600},{1:1616}
{0:642},{1:1647}
{0:610},{1:0}
---NEC frame end: Address=FF00, Command=CE31

And there you have the address and the command! Note that by default, extended NEC codes are allowed. (However, the remote control I used here generates a non-extended NEC code, where the second byte in the address (and command) is the first byte but inverted. I.e., 0xFF == ^0x00, 0xCE == ^0x31.)

And that’s all! If you output this using an IR LED (make sure you use a resistor), you will be able to control the device in question using your microcontroller. Note that you will probably have to move the LED rather close to the device you’re trying to control (depending on your resistor value). Note that you shouldn’t really exceed 30 mA per GPIO. I understand that most IR LEDs are good up to 100 mA, but the ESP32’s GPIO pins aren’t. You’ll need to amplify the signal if you want to go higher than 30 mA.

Sniffing IR (NEC) codes on the Raspberry Pi Pico

The Pico has a very similar demo, pico-examples/pio/ir_nec/ir_loopback/ir_loopback.c. If all you want to do is sniffing, it may work a little nicer if you comment out the sending stuff (everything from “// create a 32-bit frame and add it to the transmit FIFO” to “sleep_ms(100);”), as well as the sleep_ms(900) at the bottom. (Otherwise you’ll have to wait a little bit until your IR code shows up.) Also, it won’t do extended NEC codes. You’d have to modify nec_receive.c a little bit.

Building a new ZX80 / Making PCBs in Inkscape / ZX80 replica Gerber files

Jump to ZX80 replica PCB Gerber files

A while ago, I watched a series of videos by DrMattRegan on the ZX80 and was very impressed with the uber-hacks the designers put in there. I’m not going to go into much detail here, but here is a set of facts that may pique your interest:

  • The CPU’s A6 pin is permanently shorted to its \INT pin
  • The ZX80 has only 1 KB of RAM
  • The RAM is SRAM but the CPU’s internal DRAM refresh counter is not wasted

Additionally, the ZX81 was one of the first retro computers I ever repaired (article 1, article 2), which made the prospects of getting a ZX80 of my own even more attractive to me.

You have a lot of options if you want to build a ZX80 yourself. First of all, all the ICs (except the 2114 SRAM) can be bought new. There are multiple PCB designs that you can download for free, some electrically equivalent, some using chips that are a little more common than the ones on the original PCB. (For example, apart from the 2114 RAM, the 74LS93 is pretty rare because it is essentially useless nowadays. It was probably cheaper than the other 74-series counter ICs about 50 years ago, because a lot of “maybe nice to have” features were removed. It can easily be replaced using a different 74-series counter IC, and these days there really is no price difference.)

One thing that I really liked about the ZX81 that I worked on was the curved traces and the absence of solder mask on the traces. On this page by Grant Searle, you can find instructions to re-create the original PCB. There is a PDF that contains high-res images that look like this:

These can be printed out and turned into PCBs. I’m not much of a chemist, and even if the etching and tinning processes went well, I’d still need to do a bunch of drilling and then plate the newly drilled vias. Nowadays, there are companies such as PCBWay and JLCPCB that can do all this for you, and in an automated fashion, and for very attractive prices! (I do not intend to steer you away from etching PCBs yourself, in fact it always impresses me when people make PCBs themselves!)

However, these companies (currently?) do not accept bitmaps (as far as I know), they want Gerber files. So I traced the (600 dpi) bitmaps in Grant Searle’s PDF file and sent them out. I chose JLCPCB after some research because I found a pic of a board manufactured by JLCPCB that didn’t have any solder mask applied, and it looked pretty much the way I wanted it! However, I’m sure that PCBWay could do the same.

In order to get a board without solder mask, you have to get rid of the solder mask layer in the SVG or Gerber file, and you probably should also add a comment stating that this is intentional. I added this in my order, no guarantees the Chinese is correct: “The Gerber files don’t have a solder mask. This is intentional. Please do not apply a solder mask. Gerber文件没有焊膜。这是有意的。请不要应用焊膜。 我们有会说中文的人,所以如果你有什么问题,可以说中文。” I ordered 5 boards (which is the minimum order), lead-free HASL, white silkscreen (“ink-jet/screen printing silkscreen”), no gold fingers (not present on the original PCB either), regular PCB thickness (1.6 mm), regular outer copper weight (1 oz), regular via covering (tented), no castellated holes. JLCPCB’s customer service recommended I switch to ENIG, but it worked out fine with regular lead-free HASL. The price was $18.02 plus $11.70 shipping, minus the first time discount ($5).

When exporting the Gerber files in KiCad, you should follow the PCB manufacturer’s recommended settings. JLCPCB and probably most other places have a support page for this, this is JLCPCB’s.

A glimpse of the outcome

Tracing bitmapped PCB foils

I used Inkscape to trace the PCBs in the PDF. Tracing is easy, you just load a bitmap and then go to “Path” -> “Trace Bitmap…”. But how do you convert that into Gerber files? Well, there is an extension that converts SVG files to KiCad files, svg2shenzhen.

It still took me many, many hours though. Why?

Converting traced holes to actual drill holes

svg2shenzhen expects “circle” shapes in a layer called “Drill”. If that layer doesn’t exist and/or it doesn’t contain circles, there won’t be any drill holes in your Gerber file. I wrote a very simple Inkscape extension to do this: Converting paths to circles in Inkscape

But first, we need to separate out the holes into a single layer. To do this, we first break apart the path containing all the traces (“Path” -> “Break Apart”). Then everything that isn’t connected together, and everything overlapping other things will exist as separate paths. Holes overlap other things and thus will be separate paths. And they will be obscured by the outer path. Now you just click on the surrounding path and remove it, perhaps like this:

One more consideration is hole size. Though the details are a little hazy now, selecting multiple circles and changing their size all at once didn’t work very well for me in Inkscape, so I just edited the SVG file directly.

The traced circles are paths that more or less look like circles, sure. But they’re often slightly oval or otherwise unshapely and not all the same size. So when you (after using the extension) crack open the SVG in a text editor, you may see that the radii are slightly different everywhere:

holes_circles.svg:    <circle
holes_circles.svg-       cx="50.392669494501675"
holes_circles.svg-       cy="193.60723742169256"
holes_circles.svg-       r="0.3154674216925315"
holes_circles.svg:       id="circle12107" />
holes_circles.svg:    <circle
holes_circles.svg-       cx="64.40136232421558"
holes_circles.svg-       cy="193.60780999999997"
holes_circles.svg-       r="0.31595000000000084"
holes_circles.svg:       id="circle12109" />
holes_circles.svg:    <circle
holes_circles.svg-       cx="78.4180010571009"
holes_circles.svg-       cy="193.60854169447143"
holes_circles.svg-       r="0.3165816944714095"
holes_circles.svg:       id="circle12111" />

I just did a find and replace operation here and used holes sizes that seemed good to me (after JLCPCB support telling me that my holes were rather small). And damn, by sheer dumb luck, I chose the best hole sizes ever (0.4 mm) and my jumper wires fit right into them without using clips; they make almost perfect contact. Maybe 0.375 mm or so would have been even better? (The other hole sizes I chose were 0.8 mm and 1 mm.) (Note: PCB manufacturers don’t have every drill bit in the universe, so 0.4 mm and 0.375 mm may just end up being the same drill bit.)

Excellent hole size.

Update 2024-04-19: the holes for the headphone/microphone/power connectors are too tight though.
:(

Adhering to minimum spacing constraints

PCB manufacturers require that certain spacing constraints are observed. For example, traces may need to be 0.125 mm apart. The original bitmap you have may or may not adhere to the spacing constraints. I don’t know whether the bitmap in the PDF adheres to them! But I certainly do know that my order was rejected because the traces in my Gerber file weren’t quite up to snuff.

How would I even find out what my minimum spacing is? Well, I found this page by yet another company that makes PCBs: https://instantdfm.bayareacircuits.com/. This page analyzes your Gerber file and sends you a snazzy PDF with close-ups of your horrible transgressions. This is a screenshot from an early version of my Gerber files:

2.97 mil… that’s 0.075438 mm. Less than 0.125 mm!

So I manually fixed this location and re-submitted, and sure enough, there were plenty of other similar narrow gaps. So I decided there must be a way to get Inkscape to find these critical regions, and this is what I came up with:

I just gave every object a 0.125/2 mm = 0.0625 mm border, and then zoomed in a bunch to look for objects that were touching each other. (Actually I probably added some to that value, but a couple months have passed and I don’t remember.)

Border color is set to red here. Lots of… intimate traces!

Mistakes

As mentioned earlier, in Inkscape, when you break apart a path that contains “holes”, you’ll end up with a large mass obscuring the holes. Here’s a video of exactly that:

Look at the large black regions with holes, especially near the top left. The holes will “disappear” (they will be obscured) after breaking apart the path.

Well, I failed to notice that a number of holes within these black regions had been obscured. And thus they came out like this on the PCB:

IC3 and IC14 have their left pins all sewn together. There were a couple similar spots elsewhere, but you get the idea.

While annoying, I was able to fix this using a utility knife. Boy, that utility knife’s blades went dull quick. (Luckily, my utility knife is one where you can just snap off consumed blade chunks. Having done this for the first time in my life, it was quite scary, to be honest. I did it behind a glass window.)

Very beautiful, I know. However, once you have soldered the IC sockets, all this is mostly hidden underneath the sockets.

Procurement and assembly

After taking care of these mishaps, it was time to do a bunch of other stuff, such as celebrating Christmas and the New Year, and at some point bite the bullet and solder sockets and resistors and capacitors. I soldered everything using lead-free solder, and most or maybe even all of the components I put on the PCB are lead-free too, including the Z80! So maybe this is the first RoHS-compatible ZX81?! (Except the 2114 SRAM chips most likely aren’t RoHS-compatible. We’ll talk about those in a later section, BTW.)

I had half of the 74-series logic chips in stock. (From when I got Ben Eater’s DIY 8-bit computer, which I never assembled. I used the breadboards and some of the other components for a host of other things though!) The other half came from a small electronics shop right next to my office in Machida, サトー電気. I wanted a RoHS Z80 (print on chip ends in -PEG rather than -PEC), and bought it off Amazon.

Update 2024-04-19: Zilog/Littelfuse are reportedly discontinuing the original Z80 CPUs after almost 50 years of production! Maybe get them while they’re still available. Only source so far: https://www.mouser.com/PCN/Littelfuse_PCN_Z84C00.pdf (zilog.com still seems to list everything as “active” at the time of this writing)

For the ROM I’m just using my EEPROM that I’ve been using in other projects. Since it’s huge and has a slightly different pinout, I’m currently using a breadboard as an adapter.

Putting jumper wires in IC sockets permanently damages the sockets. Here I am plugging the jumper wires into a sacrificial socket that I plug into the soldered socket. This way the soldered socket doesn’t get damaged.

The only thing I couldn’t get anywhere, including Akihabara, is the 6.5 MHz oscillator. Many people use a 6.5536 MHz crystal in their ZX80 builds (hmm, I’ve seen a number like that before!) because the 6.5 MHz ones are pretty rare. But even those didn’t seem to exact in my neck of the woods. I ordered a 6.5 MHz oscillator off Digikey (through Marutsu) and will probably have it in a few days. (By the way, the original ZX80 used a 6.5 MHz ceramic resonator, but these seem to be just as unavailable. But who knows, maybe I could have gotten a 6 MHz one and filed it down a bit to get it to 6.5 MHz? But crystals work even better, and are probably a little better for the environment because ceramic resonators are made of lead zirconate titanate. Note that they are exempted from RoHS regulations and can be labeled RoHS3-compliant as long as their leads do not contain lead, I guess.) So I’m using a Raspberry Pi Pico to generate the clock signal for now.

I had some problems with the clock though:

  • I forgot to solder on R20. This resistor is needed in the clock circuit. Fixed using jumper wires.
  • I changed the Pico’s output pin from GPIO0 to GPIO2 and failed to re-wire. I then suspected the Z80 was bad and tested it on a breadboard with the data pins tied to ground to emulate NOPs. Eventually figured out the problem (dur). It was difficult to figure out at first because the circuitry on the PCB picked up the output from GPIO2 as noise, and amplified it to produce something like a clock signal. Except that this was a very dirty clock signal (see image below)! It is interesting that the Z80 seems to detect that the clock is a little cuckoo, and while it works for a split second, it quickly calls it quits and the address bus freezes in a random state. That seems like a useful thing to be aware of! (Could of course be that this is not an intentional feature, or that it’s just seeing a HALT instruction or something.)
  • While testing the Z80 on the breadboard, I set the Pico’s clock output to 3.25 MHz. I then forgot to change it back to 6.5 MHz.
Temporary R20 fix
Dirty clock that quickly made the Z80 seize up.

RAM problem

The 2114 RAM chips originally came out of the Commodore PET that I fixed two years ago. I removed them from the PET because they were a little faulty, but kept them because they still mostly worked. Lucky! I had three, and hoped two of them would work well enough to at least get the ZX80 booted. Well, I picked a lucky one and an unlucky one. I didn’t see anything on the composite output. (Which at the time of writing is just a hole in the PCB. The ZX80 and early versions of the ZX81 don’t produce a back porch, but that problem is somehow fixed or alleviated or otherwise rendered irrelevant inside the modulator. The problem just manifests itself when you decide that the wire leading into the modulator is now going to be composite output. Back when I was looking at the ZX81 at the computer museum in Oume, I used a 555-based circuit that I got from here to fix the problem.)

Not knowing whether it’s a RAM problem or a serious problem with the PCB, I broke out my Pico-based logic analyzer. This time I didn’t bother adding resistor dividers because all Picos I have (I think I have four) turned out to be a least a little 5V-tolerant, and I wouldn’t be using the logic analyzer for a long time.

I needed to make some minor modifications to the logic analyzer code: the reset circuit in the ZX80 uses a 220K pull-up resistor (it’s very high value because it is also a 1 second (or so) RC delay circuit), and the Pico by default has pull downs active on its input GPIOs. These pull downs are much lower in value than 220K, effectively keeping reset asserted forever. (They are surprisingly low!) So the init code now looks like this:

    stdio_init_all();
    gpio_init_mask(ALL_REGULAR_GPIO_PINS);
    gpio_set_dir_masked(ALL_REGULAR_GPIO_PINS, GPIO_IN);
    gpio_disable_pulls(TRIGGER_PIN); // default is pull down; this pull down is much higher in value the the zx80's reset circuit's pull-up and therefore holds the cpu in reset

Of course, it would probably be even better if we disabled all pulls on all GPIOs. Anyway, running the logic analyzer, I quickly found a RAM problem with bit 4.

Here is the logic analyzer output with the problematic signals:

    115       6 0283 40
    116      16 0283 2a
    117       1 0f10 2a
    118      21 0f50 00
    119       6 0284 00
    120      27 0284 0a
    121       6 0285 0a
    122      27 0285 40
    123       8 000a 40
    124      25 000a 21
    125       8 000b 21
    126      25 000b 40

The first number is just the line number, the second number is the number of times the third and fourth numbers are repeated in the logic analyzer output (which is not in sync with the clock, but much faster). You can ignore everything that is less than around 10. In the above output, we are fetching and then executing the instruction at 0x0283~0x0285. Here is the relevant assembly source:

        ld iy,04000h            ;0273   fd 21 00 40     . ! . @ 
        ld hl,04028h            ;0277   21 28 40        ! ( @ 
        ld (04008h),hl          ;027a   22 08 40        " . @ 
        ld (hl),080h            ;027d   36 80   6 . 
        inc hl                  ;027f   23      # 
        ld (0400ah),hl          ;0280   22 0a 40        " . @ 
        ld hl,(0400ah)          ;0283   2a 0a 40        * . @

So we can see that we are putting 0x4028 into hl, and then incrementing hl, which means that hl should now be 0x4029. The instruction at 0x280~0x282 puts hl into 0x400a, and the instruction at 0x0283~0x0285 reads it back from the same address. So we should be putting 0x4029 in there (though not shown above, we are) and should be reading 0x4029 back. But the relevant parts of the logic analyzer output are like this:

    124      25 000a 21
    126      25 000b 40

Don’t worry about this showing 000a and 000b rather than 400a and 400b. I just don’t have the higher address lines connected to the logic analyzer. We’re reading back 0x4021! That’s missing the fourth bit. So I put in the other RAM chip and bam! I see a lovely 15.something KHz signal on the through hole that would normally be occupied by one of the modulator’s leads! (Well, actually I saw a 7.65 KHz signal because I forgot to change the Pico’s clock output back to 6.5 MHz, but while that was very puzzling for a bit, it was very easy to fix.)

The computer resting on top of that old laptop is the newer one!
It’s working! Never mind the Hitachi MB-H2 in the reflection.
I’m currently using two of these smartphone stylus…es to operate the keyboard on the PCB (one for shift and the other for every other key).

There’s a problem though: I’m not able to press keys like ” and many other shifted keys. I haven’t properly looked into that problem yet. However, I did do a quick internet search and found someone with the same problem:

For example, all of 1 to 0 keys work but shifted, only 1 and 0 work. Some other shifted keys don’t work but there doesn’t seem to be a consistent pattern on each row. The display still flickers when say shift 2 or shift 9 (or other combinations) is pressed, just nothing else happens.

https://sinclairzxworld.com/viewtopic.php?p=32893&sid=b108eefd0c71a1bedd148b3049db2673#p32893

But their problem was apparently caused by using super long wires on their keyboard. I’m not doing that as far as I know?!

As my EEPROM is currently mounted on a breadboard and my EEPROM is 512 KB while the ZX80’s ROM is just 4 KB, I have plenty of space left. So I put the ZX81 ROM image at address 0x10000, which means I just need to change the wire on the EEPROM’s A16 pin from 0V to 5V to change to that ROM. And it works too! Plus, the keyboard layout is different, which means I’m able to access the ” key and print out a proper message rather than just numbers:

To do

  • Solder crystal oscillator (when I get it) and R20, and maybe headphone/microphone jacks, etc.
  • Backporch generator
  • Make a proper ROM adapter?
  • Get all shifted keys to work
  • RAM expansion
  • Try to load some software
  • Make the keyboard more ergonomic
  • Maybe get some kind of case

KiCad / Gerber files

Here are the SVG and KiCad files, with the above mentioned shorting issues most likely fixed. Some important notes:

  • I do not have permission from Grant Searle nor from the copyright holders of the original ZX80 PCB to post anything like this, and if either of these parties asks me to take my files down, I intend to comply swiftly. (I’d just need to be sure that you are who you claim to be. I apologize in advance for any grievance caused and will apologize again if grievance is actually caused.)
  • I used Grant Searle’s replica foils as a base and traced them in Inkscape. I performed some manual and some automated tweaks. This version of the board is likely a less faithful replica of the original board than Grant Searle’s foils. (I don’t think anything’s shifted more than even 1 mm though. Though maybe the holes are quite different. I didn’t encounter any problems with the drill holes while soldering though.)
  • I haven’t tested (i.e. manufactured) this version of the KiCad files, I have only ordered one set of the ZX80 boards, with the shorted pins. This issue should be fixed now, and I hope I didn’t add any new issues.

PS3 controller on the Switch (or Xbox, etc.?)

Last year, I posted a couple of articles on using a USB-HID controller on an MSX and on a Nintendo Switch. I also posted an article on a PS3 controller repair.

A happy Pico enjoying a round of Mario Kart. (This particular Pico is doing something else, but it’s content watching its brother play.)

Well, it’s time to put 2 and 2 together and get that silly PS3 controller to work on the Switch! (Or on other devices supported by GP2040-CE, which as I understand includes the Xbox and PS4/PS5?) To do this, we just have to realize that the PS3 controller works on Linux. So we just need to figure out how Linux does it. And a quick Google query (though I can’t remember the words I chose) surfaced up an old patch from 2007 introducing PS3 support to the Linux kernel:

Add the USB HID quirk HID_QUIRK_SONY_PS3_CONTROLLER. This sends an
HID_REQ_GET_REPORT to the the PS3 controller to put the device into
‘operational mode’.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4a1a4d8b87389e35c3af04c0d0a95f6a0391b964

Looking into the patch, we see that there isn’t much to it, it’s just, as the description says, sending a single request to the controller:

+	result = usb_control_msg(dev, usb_rcvctrlpipe(dev, 0),
+				 HID_REQ_GET_REPORT,
+				 USB_DIR_IN | USB_TYPE_CLASS |
+				 USB_RECIP_INTERFACE,
+				 (3 << 8) | 0xf2, ifnum, buf, 17,
+				 USB_CTRL_GET_TIMEOUT);

So, how could we do this in 1) our code that we used for the MSX, and 2) in GP2040-CE?

Modifying Pico-PIO-USB to work with the PS3 controller

Pico-PIO-USB isn’t very over-engineered yet(?!), and we can just modify the enumerate_device function in pio_usb_host.c to check if we’re seeing a PS3 controller, and send the request. Near the bottom of this function, we send the “get_hid_report_descrpitor_request” to the USB device. Immediately after we add our own code, as below:

diff --git a/src/pio_usb_host.c b/src/pio_usb_host.c
index 864d048..0efc567 100644
--- a/src/pio_usb_host.c
+++ b/src/pio_usb_host.c
@@ -1018,6 +1018,21 @@ static int enumerate_device(usb_device_t *device, uint8_t address) {
         printf("\n");
         stdio_flush();
 
+        // We need to send a special HID request otherwise the contoller won't do anything
+        if ((device->vid == 0x054c) && (device->pid == 0x0268)) {
+          usb_setup_packet_t get_hid_report_ps3_controller_request =
+              GET_HID_REPORT_PS3_CONTROLLER;
+          control_in_protocol(
+              device, (uint8_t *)&get_hid_report_ps3_controller_request,
+              sizeof(get_hid_report_ps3_controller_request), rx_buffer, LINUX__SIXAXIS_REPORT_0xF2_SIZE);
+          printf("\t\tPS3 controller response:");
+          for (int i = 0; i < LINUX__SIXAXIS_REPORT_0xF2_SIZE; i++) {
+            printf("%02x ", device->control_pipe.rx_buffer[i]);
+          }
+          printf("\n");
+          stdio_flush();
+        }
+
       } break;
       default:
         break;

This requires the following definitions to be added in usb_definitions.h. I just used the same names used in Linux (not the ancient 2007 version, but something recent):

diff --git a/src/usb_definitions.h b/src/usb_definitions.h
index c345bdd..845fbbc 100644
--- a/src/usb_definitions.h
+++ b/src/usb_definitions.h
@@ -302,10 +302,16 @@ enum {
   USB_REQ_REC_OTHER = 0x03,
 };
 
+// some constants taken from Linux source code (.../ch9.h, .../hid.h)
+#define LINUX__USB_REQ_GET_DESCRIPTOR 0x06
+#define LINUX__HID_REQ_GET_REPORT 0x01
+#define LINUX__HID_FEATURE_REPORT 0x02
+#define LINUX__SIXAXIS_REPORT_0xF2_SIZE 17
+
 #define GET_DEVICE_DESCRIPTOR_REQ_DEFAULT                                      \
-  { USB_REQ_DIR_IN, 0x06, 0, 0x01, 0, 0, 0x12, 0 }
+  { USB_REQ_DIR_IN, LINUX__USB_REQ_GET_DESCRIPTOR, 0, 0x01, 0, 0, 0x12, 0 }
 #define GET_CONFIGURATION_DESCRIPTOR_REQ_DEFAULT                               \
-  { USB_REQ_DIR_IN, 0x06, 0, 0x02, 0, 0, 0x09, 0 }
+  { USB_REQ_DIR_IN, LINUX__USB_REQ_GET_DESCRIPTOR, 0, 0x02, 0, 0, 0x09, 0 }
 #define SET_CONFIGURATION_REQ_DEFAULT                                          \
   { USB_REQ_DIR_OUT, 0x09, 0, 0, 0, 0, 0, 0 }
 #define SET_ADDRESS_REQ_DEFAULT                                                \
@@ -313,10 +319,13 @@ enum {
 #define SET_HID_IDLE_REQ_DEFAULT                                               \
   { USB_REQ_TYP_CLASS | USB_REQ_REC_IFACE, 0x0A, 0, 0, 0, 0, 0, 0 }
 #define GET_HID_REPORT_DESCRIPTOR_DEFAULT                                      \
-  { USB_REQ_DIR_IN | USB_REQ_REC_IFACE, 0x06, 0, 0x22, 0, 0, 0xff, 0 }
+  { USB_REQ_DIR_IN | USB_REQ_REC_IFACE, LINUX__USB_REQ_GET_DESCRIPTOR, 0, 0x22, 0, 0, 0xff, 0 }
+#define GET_HID_REPORT_PS3_CONTROLLER                               \
+  { USB_REQ_DIR_IN | USB_REQ_TYP_CLASS | USB_REQ_REC_IFACE, LINUX__HID_REQ_GET_REPORT, 0xf2, LINUX__HID_FEATURE_REPORT+1, 0, 0, LINUX__SIXAXIS_REPORT_0xF2_SIZE, 0 }
+
 #define GET_HUB_DESCRPTOR_REQUEST                                              \
   {                                                                            \
-    USB_REQ_DIR_IN | USB_REQ_TYP_CLASS | USB_REQ_REC_DEVICE, 0x06, 0, 0x29, 0, \
+    USB_REQ_DIR_IN | USB_REQ_TYP_CLASS | USB_REQ_REC_DEVICE, LINUX__USB_REQ_GET_DESCRIPTOR, 0, 0x29, 0, \
         0, 8, 0                                                                \
   }
 #define GET_HUB_PORT_STATUS_REQUEST

And that’s all!

So, will the same modifications work in GP2040-CE too? No, while GP2040-CE uses Pico-PIO-USB to get USB host functionality to work at all while the Pico is busy being a USB device, protocol-y stuff is handled by TinyUSB, which by the way is included in the Pico SDK. Looking at pspassthrough.cpp for inspiration, I found that if we just add the following code to our previously modified version of src/addons/keyboard_host.cpp, our PS3 controller wakes up from its daze and starts sending our inputs!

diff --git a/src/addons/keyboard_host.cpp b/src/addons/keyboard_host.cpp
index 59bc85e..88d82da 100644
--- a/src/addons/keyboard_host.cpp
+++ b/src/addons/keyboard_host.cpp
@@ -2,6 +2,8 @@
 #include "storagemanager.h"
 #include "usbhostmanager.h"
 
+uint8_t report_buffer[64];
+
 bool KeyboardHostAddon::available() {
     const KeyboardHostOptions& keyboardHostOptions = Storage::getInstance().getAddonOptions().keyboardHostOptions;
     return keyboardHostOptions.enabled &&
@@ -77,6 +79,10 @@ void KeyboardHostAddon::preprocess() {
 }
 
 void KeyboardHostAddon::mount(uint8_t dev_addr, uint8_t instance, uint8_t const* desc_report, uint16_t desc_len) {
+  uint8_t* buf = report_buffer;
+  tuh_hid_get_report(dev_addr, instance, 0xf2, HID_REPORT_TYPE_FEATURE, buf, 17);
+
   _keyboard_host_enabled = true;
 }
 
Works for me!

Commodore SR-37 calculator “repair” and review

Before Commodore made computers, they made typewriters, and later calculators. I scored one such calculator, one that was listed as “non-functional”. The repair itself was very quick, as I had expected.

Repair

Honestly, it was just the power adapter. And some gunk in the keys. I’ll spare you the details on the gunk for today. Let’s see what’s wrong with the plug. Here’s the picture from the listing:

昭和の頃 赤色表示電卓 コモドール Commodore SR-37 難有品(^00WH10A_画像1
The pic from the listing.

Why would you bother to take a picture with the AC adapter cord connected to the calculator but the AC adapter not plugged in? Well, while I didn’t really think too much of it when I saw the listing, I quickly found the answer after it arrived here: it’s damn impossible to get out of there!

Until I got out some pliers and turned it left and right while pulling a little bit for a while. The cable was really sticky, and I believe (though this is half a year ago already) pretty much glued the connector to the device.

So, did they use a bit of an unusual plug shape? Yes, they did! It looks a bit like a mono headphone connector, maybe a bit thicker? (That reminds me, the ZX81’s power connector probably uses something similar.) But anyway, my trusty 28 in 1 “28 in 3” plug set (https://www.amazon.co.jp/gp/product/B01NCN3P3B/) contained something that fit beautifully, and applying power at about 9V, the calculator sprang to life.

Original connector (top) and replacement connector
Glorious LED display
Device’s innards. I didn’t take it apart any further than this, but cleaned up the gunk. Apparently didn’t take an “after” picture though, so you’ll just have to trust me that it looked as clean as a whistle afterwards. Maybe.

Review

So you are thinking of buying a calculator, and hey, you’ve always wanted to show off how cool retro tech can be. Someone nearby is selling a retro calculator with an LED display. It looks fantastic! But will it be useful?

The Commodore SR-37 does pack a lot of functions, maybe not quite as many as a modern scientific calculator, but it’s not far away. (For example, you can’t easily convert degrees to radians.)

So let’s see how useful this thing is. I’ve thought of some expectations the modern calculator user may have that aren’t quite fulfilled by this device:

ExpectationTrue:
False:
Doesn’t use power when turned off using power switch on device.
Doesn’t use a lot of power. So for example not 1.4 W even when you make it display 8888888888888888888888.8.
Doesn’t turn off the display to conserve power after a short while.
Doesn’t take a long time to compute, e.g., exponentials or roots. Definitely not like 2 seconds!
Comes with a boring LCD rather than a beautiful LED display.
Table 1.1: table of broken expectations

The main problem in my opinion is that it uses a lot of power. Even if you turn it off, it’ll use some power (100 mA or so). At least my model didn’t come with a place to put in batteries so it’s external power only. The fact that this device is basically slightly on all the time (unless you unplug the cord or have a switch nearby), I’m a bit concerned for its longevity. So I don’t think I’ll use it much. :(

Besides, what I really need is a calculator that is really good at converting between bases, like kcalc. I’m not sure such a device even exists!

Christmas 2023: DIY electronics kit for children

Wow, two weeks have passed since Christmas. Wow, we’re one week into 2024. Happy New Year!

I have an 8-year-old niece and I thought I’d give her an electronics kit for Christmas. Well, it seems that in modern electronics kits, electronic components are covered in a bunch of plastic, making everything look kind of childish. They are a little safer I suppose because they are too big to be swallowed. But at least I (as a child) didn’t really like stuff with a “Fisher-Price” look, I tended to want the real deal. So I decided to make my own electronics kit with my own manual! I just put in a few components I mostly had on hand (I also washed them before, just in case I’d used them somewhere dirty before):

  • 5 red LEDs with a low forward voltage (around 1.7V), this way you can show that they light up (barely) with very fresh alkaline batteries. (Note: most red LEDs have a higher forward voltage of 1.85V or so, and wouldn’t light up at all)
  • 5 blue LEDs
  • 5 white LEDs
  • 5 100 uF (or so) electrolytic capacitors
  • 5 470 uF (or so) electrolytic capacitors
  • 5 470 ohm (or so) resistors
  • 5 2n2222 transistors
  • 2 CR2032 batteries
  • 2 CR2032 battery holders (the ones I bought can just barely fit into a breadboard)
  • Optional: 4x AA/AAA battery holder and AA/AAA batteries (referenced in text, so make sure to remove reference if you don’t want to include them)
  • Some wires
  • 1 breadboard

I didn’t include a multimeter, but if the child’s/person’s household doesn’t have one, it might make sense to include one. (The manual doesn’t really fully explain how to use multimeters, however.)

In the manual, the first experiment makes use of the fact that blue and white LEDs have a forward voltage that is quite compatible with CR2032 coin cells. The experiment just sandwiches a coin cell between an LED’s legs.

Then the manual explains that this doesn’t work with red LEDs and a resistor is used to limit the current. (Actually it will probably be okay for a while because coin cells don’t give a lot of current.) Some effort is made to explain voltage, current, and resistance, but (hopefully) on a level that is (possibly, barely) understandable by an 8-year-old.

A little later, capacitors are explained a bit. And transistors. The final experiment is a circuit that has some twinkling LEDs. The following is a similar circuit, just to give you an idea:

Old video shot on a potato.

And here’s the completed set, just before wrapping it:

DIY electronics kit, complete with “Instuction Manual” (d’oh!)

The manual is in .odt format (can be opened in LibreOffice and similar) and you can download it below. It includes some copyrighted pictures from other sites (the battery pic, the diode symbol(s), the transistor pic, the capacitor pic, and the empty breadboard pics). The front page image is AI-generated but heavily edited. Note: LibreOffice may have a bug that prevents emoji from being included in exported PDF files. My printed booklet ended up not containing emoji and I noticed only much later. :(

I made a booklet using the following LibreOffice print settings and a stapler:

Make sure to select “A4” and “Landscape” before selecting “Brochure”, otherwise LibreOffice might get the layout wrong.

If you want to customize the manual for somebody else, you should read through the entire thing. Edit it to your heart’s content. There is at least one reference to “8-year-olds”, maybe do a search. There are references to “dad or uncle” and “parents”. The “license” is “public domain”. Feel free to give credit, but you don’t have to.

Displaying any image on an MSX, loading from a ROM

This article is basically just a note that I can come back to in case I forget some details.

The tool on this page (Japanese) takes an image file and adds dithering and stuff: https://nazo.main.jp/prog/retropc/gcmsx.html

The output is something that can be BLOAD’ed straight to VRAM memory using BLOAD’s S parameter. Didn’t even know that option existed!

This means we can easily convert this to a ROM by adding a loader that pokes everything into VRAM. We just need to get rid of the BSAVE header at the start (or ignore it in the loader), which looks like this according to http://www.faq.msxnet.org/suffix.html#BIN:

byte 0  : ID byte #FE
byte 1+2: start-address
byte 3+4: end-address
byte 5+6: execution-address

So we just cut off 8 bytes at the beginning, e.g. by doing:

tail -c +8 msx_20231111232339933.SC2 > foo.bin

And the loader in assembly (z80asm-flavor) could look like this:

SetVdpWrite: macro high low ; from http://map.grauw.nl/articles/vdp_tut.php
	ld a,low
	out (0x99),a
	ld a,high+0x40
	out (0x99),a
endm

vpoke: macro value
; 	ld a,value ; not needed in this implementation
	out (0x98),a
; 	nop ; nops not needed in this implementation
; 	nop
; 	nop
endm

	org 0x4000
	db "AB" ; magic number
	dw entry_point
	db 00,00,00,00,00,00,00,00,00,00,00,00 ; ignored

entry_point:
	ld a,2
	call 0x005f
	SetVdpWrite 00 00
	ld hl,data
loop:
	ld a,(hl)
	vpoke a
	inc hl
	ld a,h
	cp end>>8
	jr nz,loop
	ld a,l
	cp end&0xff
	jr nz,loop
inf:
	jr inf
data:
	incbin "foo.bin" ; read data from file foo.bin
end:
	ds 0x8000-$ ; fill remainder of 16 KB chunk with 0s

Of course, this approach is quite wasteful; we need almost 16 KB of memory to display any image, even if it’s mostly empty.

Converting paths to circles in Inkscape

Or alternatively: how to get svg2shenzhen to recognize your drill paths as drill holes

(My) rationale: there is an Inkscape extension called svg2shenzhen. This extension creates Gerber and KiCad files that can be used to create printed circuit boards, from standard SVG files. Also, this extension has a funny name. Older, DIY printed circuit boards are just a high-DPI bitmap. Using Inkscape and this extension, you can trace the bitmap and then convert it to Gerber. However, most PCBs (especially old PCBs) need to have holes drilled. The drill locates are just circles in the bitmap, and after tracing, they’re just paths. The svg2shenzhen extension (at the time of this writing) detects circles in a certain layer as locations that need to be drilled, but not paths.

I’m not an Inkscape expert, but AFAIK Inkscape (at the time of this writing) doesn’t have a built-in tool to convert (mostly) circular paths to circles. So I wrote a simple extension that does this! It works fine on Linux. Not so sure about Windows.

Extensions are made of only two files, a file that describes the extension, and the extension code (which is in Python in many cases). These two files just have to be placed into the location shown in Edit -> Preferences -> System -> User extensions, which in my case is ~/.config/inkscape/extensions/.

Here are the two files, you can copy them into a text editor like Kate or gedit or Notepad, what have you, and save them into the above directory. I recommend keeping my file names, path2circle.inx and path2circle.py. Note, some skeleton code in path2circle.py was generated by ChatGPT, though it was quite wrong. That’s where some of the verbose comments and extraneous code came from.

path2circle.inx:

<?xml version="1.0" encoding="UTF-8"?>
<inkscape-extension 
    xmlns="http://www.inkscape.org/namespace/inkscape/extension">
    <name>Path2Circle</name>
    <id>user.path2circle</id>
    <effect>
        <object-type>all</object-type>
        <effects-menu>
            <submenu name="Custom"/>
        </effects-menu>
    </effect>
    <script>
        <command location="inx" interpreter="python">path2circle.py</command>
    </script>
</inkscape-extension>

path2circle.py:

import inkex
from inkex import Circle

class Path2Circle(inkex.EffectExtension):
    def effect(self):
        # Iterate through all the selected objects in the SVG
        for node in self.svg.selection:
            # Check if the object is a path (or any other object type)
            if node.tag.endswith("path"):
                # Get the bounding box of the object
                x, y, width, height = self.get_object_dimensions(node)
                x = x.minimum
                y = y.minimum
                # with open('/path/to/debug/directory/debug_output.txt', 'a') as f:
                    # print(f"Object Dimensions: x={x}, y={y}, width={width}, height={height}", file=f)
                layer = self.svg.get_current_layer()
                diameter = min(width, height)
                layer.add(self.add_circle(x, y, diameter/2))

    def add_circle(self, x, y, radius):
        """Add a circle at the given location"""
        elem = Circle()
        elem.center = (x+radius, y+radius)
        elem.radius = radius
        return elem

    def get_object_dimensions(self, object_node):
        # Get the bounding box of the object
        bbox = object_node.bounding_box()

        # Extract the bounding box coordinates
        x = bbox.x
        y = bbox.y
        width = bbox.width
        height = bbox.height

        return x, y, width, height

if __name__ == '__main__':
    Path2Circle().run()

To use the extension, you probably first need to restart Inkscape. (You do not need to restart Inkscape after changing the extension, however.) Select all the paths you’d like to convert, and then hit Extensions -> Custom -> Path2Circle. Note: the extension doesn’t actually care if the paths even remotely look like circles, so make sure to select the correct paths. You can easily modify the extension to calculate the radius differently, or e.g. replace paths with other objects, such as squares, rectangle, or ellipses. Let me know if you need help doing that.