The Qiqitori Blogs

There is very little code that I wrote myself in this project, but it’s the first time I’m looking at USB-HID on the protocol level and the end result is something mildly useful. So possibly worth a post? (Note: code is at the bottom of this post.)

All we’ll be doing here is modify an example from https://github.com/sekigon-gonnoc/Pico-PIO-USB, namely, the capture_hid_report example. You’ll need a good way to connect a USB-A port to your Raspberry Pi Pico. I’m using an old piece of hardware that was meant for use in desktop PCs to add USB ports on the back, internally connected to motherboard headers. (In the demo we’re using, USB D+ and D- are GPIO pins 0 and 1, respectively.)

Once you have your Pico flashed, open a serial terminal, and then connect a USB-HID gamepad. I’m using a fake PS3 controller. It looks very similar to a Sony PS3 controller but bears a “P3” mark instead of the PlayStation logo on the middle button. (I don’t know if it works with a real PS3.)
(I’m leaving out the wiring details here, but it’s all exactly as explained in the README.md in the repo.) I get the following messages when I plug in my controller. The “EP 0x81” messages are sent continuously.

Device 0 Connected
control in[complete]
Enumerating 054c:0268, class:0, address:1
control out[complete]
control in[complete]
control in[complete]
Manufacture:GASIA CORP.
control in[complete]
control in[complete]
Product:PLAYSTATION(R)3 Controller
control in[complete]
control in[complete]
control out[complete]
inum:0, altsetting:0, numep:2, iclass:3, isubclass:0, iprotcol:0, iface:0
        bcdHID:1.11, country:0, desc num:1, desc_type:34, desc_size:148
control out[error]
control in[complete]
                Report descriptor:05 01 09 04 a1 01 a1 02 85 01 75 08 95 01 15 00 26 ff 00 81 03 75 01 95 13 15 00 25 01 35 00 45 01 05 09 19 01 29 13 81 02 75 01 95 0d 06 00 ff 81 03 15 00 26 ff 00 05 01 09 01 a1 00 75 08  
                        epaddr:0x02, attr:3, size:64, interval:1
                        epaddr:0x81, attr:3, size:64, interval:1
...
054c:0268 EP 0x81:      01 00 00 00 00 00 80 80 80 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ef 10 00 00 00 00 23 6d 77 01 80 02 00 02 00 01 80 02 00 
054c:0268 EP 0x81:      01 00 00 00 00 00 80 80 80 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ef 10 00 00 00 00 23 6d 77 01 80 01 ff 01 ff 01 7f 01 ff 
054c:0268 EP 0x81:      01 00 00 00 00 00 80 80 80 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ef 10 00 00 00 00 23 6d 77 01 80 01 fe 01 fe 01 7e 01 fe 
054c:0268 EP 0x81:      01 00 00 00 00 00 80 80 80 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ef 10 00 00 00 00 23 6d 77 01 80 01 ff 01 ff 01 7f 01 ff 
054c:0268 EP 0x81:      01 00 00 00 00 00 80 80 80 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ef 10 00 00 00 00 23 6d 77 01 80 02 00 02 00 01 80 02 00
...

You can also use Linux tools such as usbhid-dump to get reports, but for some reason they look quite different from what I get here. Don’t know if the driver is doing something (odd, or not so odd) or if the Pico is doing something odd. (It’s quite likely that there’s a driver in Linux that configures the controller automatically. On the Pico you have to press the P3 button every time after plugging in, on Linux it lights up the first player LED immediately.)

Decoding the USB HID report descriptor

First of all, we don’t actually need to do all this. Pressing buttons and looking at the output makes it quite obvious what we have to do. But for let’s edify outselves anyway. Here’s a great tutorial on USB HID report descriptors: https://eleccelerator.com/tutorial-about-usb-hid-report-descriptors/. The same guy has a tool on their website that allows us to quickly decode the descriptor: https://eleccelerator.com/usbdescreqparser/.

When we paste in our descriptor, we get the following output:

0x05, 0x01,        // Usage Page (Generic Desktop Ctrls)
0x09, 0x04,        // Usage (Joystick)
0xA1, 0x01,        // Collection (Application)
0xA1, 0x02,        //   Collection (Logical)
0x85, 0x01,        //     Report ID (1)
0x75, 0x08,        //     Report Size (8)
0x95, 0x01,        //     Report Count (1)
0x15, 0x00,        //     Logical Minimum (0)
0x26, 0xFF, 0x00,  //     Logical Maximum (255)
0x81, 0x03,        //     Input (Const,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)
0x75, 0x01,        //     Report Size (1)
0x95, 0x13,        //     Report Count (19)
0x15, 0x00,        //     Logical Minimum (0)
0x25, 0x01,        //     Logical Maximum (1)
0x35, 0x00,        //     Physical Minimum (0)
0x45, 0x01,        //     Physical Maximum (1)
0x05, 0x09,        //     Usage Page (Button)
0x19, 0x01,        //     Usage Minimum (0x01)
0x29, 0x13,        //     Usage Maximum (0x13)
0x81, 0x02,        //     Input (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)
0x75, 0x01,        //     Report Size (1)
0x95, 0x0D,        //     Report Count (13)
0x06, 0x00, 0xFF,  //     Usage Page (Vendor Defined 0xFF00)
0x81, 0x03,        //     Input (Const,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)
0x15, 0x00,        //     Logical Minimum (0)
0x26, 0xFF, 0x00,  //     Logical Maximum (255)
0x05, 0x01,        //     Usage Page (Generic Desktop Ctrls)
0x09, 0x01,        //     Usage (Pointer)
0xA1, 0x00,        //     Collection (Physical)
0x75, 0x08,        //       Report Size (8)

// 63 bytes

This doesn’t look quite exactly the same as the examples in the tutorial. Below is my understanding, which may not be 100% correct, but makes sense to me at least. Let’s look at some output lines from the capture_hid_report example. The first line was:

054c:0268 EP 0x81:      01 00 00 00 00 00 80 80 80 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 ef 10 00 00 00 00 23 6d 77 01 80 02 00 02 00 01 80 02 00

First of all, all lines start with “054c:0268 EP 0x81”. That’s just added by the example code. Let’s go through the remaining numbers in the line (actually, just the bolded ones) and see how they relate to the descriptor we saw earlier.

01: report ID, same as the report ID defined in the descriptor.

00: this is the entire report that was described in the following extract from the descriptor, exactly 1 byte (8 bits):

0x75, 0x08,        //     Report Size (8)
0x95, 0x01,        //     Report Count (1)
0x15, 0x00,        //     Logical Minimum (0)
0x26, 0xFF, 0x00,  //     Logical Maximum (255)
0x81, 0x03,        //     Input (Const,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)

00 00 01 00: the (two) reports referenced in the following parts of the descriptor:

0x75, 0x01,        //     Report Size (1)
0x95, 0x13,        //     Report Count (19)
0x15, 0x00,        //     Logical Minimum (0)
0x25, 0x01,        //     Logical Maximum (1)
0x35, 0x00,        //     Physical Minimum (0)
0x45, 0x01,        //     Physical Maximum (1)
0x05, 0x09,        //     Usage Page (Button)
0x19, 0x01,        //     Usage Minimum (0x01)
0x29, 0x13,        //     Usage Maximum (0x13)
0x81, 0x02,        //     Input (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)
0x75, 0x01,        //     Report Size (1)
0x95, 0x0D,        //     Report Count (13)
0x06, 0x00, 0xFF,  //     Usage Page (Vendor Defined 0xFF00)
0x81, 0x03,        //     Input (Const,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)

This is actually two reports. One is 19 bits, the other is 13 bits. Together that’s exactly 32 bits. (This is similar to the three mouse buttons example in the tutorial, where we only have three buttons and therefore want to pad this to 8 bits.) Here, we have 19 buttons (though the controller only sports 16 physical buttons, unless I’m much mistaken), each of which is 1 bit, i.e., either pressed, or not. The remaining 13 bits are just to pad the report to make it easier to handle on the software side, or perhaps the padding is required somehow. (Looks like it is on Windows at least: https://stackoverflow.com/questions/65846159/usb-hid-report-descriptor-multiple-reports.)

And indeed, these four bytes (well, the first 19, er, 16 bits) do change when buttons are pressed. This is basically all we need to figure out how to use this controller with an MSX. We just need to figure out which button is responsible for which bit in the third and fourth bytes. (Unless we also wanted to translate analog stick input.)

What about the rest of the bytes? I don’t know how they relate to our descriptor above, but pressing buttons and moving the analog sticks makes it pretty obvious what they mean. We can see that the analog sticks are two bytes each, and there’s another byte for each button, and the value that comes up depends on how hard the button was pressed. I never knew that the △□○☓ buttons were pressure-sensitive on the PS3! Anyway, perhaps it’s possible that our descriptor is incomplete, as the frame they are transmitted in is limited to 64 (or maybe 63?) bytes?

So without going into any hardware details yet, here’s how we could modify the example code to translate our button presses to electric pulses on a GPIO pin. Here’s the part of the code that we’ll replace:

if (len > 0) {
  printf("%04x:%04x EP 0x%02x:\t", device->vid, device->pid,
        ep->ep_num);
  for (int i = 0; i < len; i++) {
    printf("%02x ", temp[i]);
  }
  printf("\n");
}

And here’s some of the code we could maybe replace the above block with. Note that this doesn’t actually consider the hardware details of the actual MSX interface yet, it’s just an example, so don’t copy this code. The actual code is in the last section of this blog post.

if (len > 0) {
  if (temp[0] == GAMEPAD_REPORT_ID) { // GAMEPAD_REPORT_ID is 1 in our case
    uint16_t button_state = (temp[2] << 8) | temp[3];
    if (button_state & BUTTON_UP) {
      gpio_put(BUTTON_UP_PIN, 1);
    } else {
      gpio_put(BUTTON_UP_PIN, 0);
    }
    if (button_state & BUTTON_DOWN) {
...
  }
}

My controller’s button mappings are as follows:

#define BUTTON_UP 0x1000
#define BUTTON_DOWN 0x4000
#define BUTTON_LEFT 0x8000
#define BUTTON_RIGHT 0x2000
#define BUTTON_A 0x40 // X or O, can't remember
#define BUTTON_B 0x20 // X or O, can't remember

MSX interface

Good news: the MSX joystick ports come with 5V and GND pins. According to https://www.msx.org/wiki/General_Purpose_port, we can draw up to 50 mA from these pins. Is that enough for the Pico and a controller? It is for mine! According to my measurements, the Pico + fake PS3 controller together draw about 40 mA. (Instrument’s display precision is 10 mA, so it could be up to 45 mA, or more if the instrument is inaccurate.) Note that many modern controllers (including real PS3 controllers) contain a battery, and will attempt to charge it. That will most likely take the current way above 50 mA.
(Note: I don’t think that drawing slightly more than 50 mA would be a huge problem at least on my MSX, which I’ve disassembled and reassembled a couple times.)

Slightly bad news (1): pressing buttons on MSX joysticks connects pins to pin 8, which is not GND. Pin 8 is “strobe” and can be GND or 5V. (Button pins are normally pulled high.)

Slightly bad news (2): on the MSX side, the joystick ports are actually GPIO ports and can be configured for both input (normal) and output! We don’t want the Pico to output when the MSX’s PSG is outputting too!

1: addressing this in software might be possible, albeit with a (very) short time lag. I don’t really know anything about this feature and have no easy way to test a software-based solution with MSX software that actually uses this feature.
2: I don’t think regular MSX joysticks worry about this. Some MSX systems may have safeguards in place.

Some measurements and wiring details

Using my multimeter’s rarely used ammeter mode, with the probes between STROBE and any button pin, I see a current of ~0.7 mA. (Two button pins, and the ammeter shows 1.5 mA. It probably goes up linearly the more buttons you press.) That’s quite a lot by modern standards!

On my MSX, there are two 74LS157 chips that implement joystick selection. The 74LS157’s outputs are directly connected to the PSG’s inputs. (Only one joystick’s state is visible to software at a time; a PSG register change is required to switch to the other one.) If we change the PSG’s I/O port’s direction, I think we’ll be pitting the 74LS157’s outputs against the PSG’s outputs. Doesn’t sound so great, eh.
Except, the 74LS157 has an \ENABLE pin! When the \ENABLE pin on a 74LS157 is disabled, the outputs will be high-Z, potentially allowing a signal coming from the PSG to make its way somewhere. Is there such functionality on the MB-H2? Answer after some probing: no. \ENABLE on these two chips is tied to GND.

While most button pins have 10K pull-up resistors (some seem to have 3.3K pull-up resistors), the STROBE pins are connected directly to PSG pins 8 and 9.

Deciding how to hook up the Pico to the joystick port

In real joysticks, when you press a button, you short the STROBE pin to the button pin, and STROBE can be HIGH or LOW. (When STROBE is HIGH, we short HIGH to HIGH, and nothing happens. Since on my MSX, as discussed above, the button pins are connected to 74LS157 inputs, they should always be pulled high, and never go low during device operation.) In real joysticks, when nothing is pressed, there is no connection, period. So that’s more like a tri-state affair, so instead of producing a HIGH level when nothing is pressed, we should set our Pico’s GPIO to high-Z (which we can do by setting it to INPUT).

Armed with the measurements above, I think we can hook up the Pico directly (let’s ignore 5V vs 3.3V for now), producing a LOW level when a button is pressed. We’ll have the Pico’s GPIO pin sink a little bit of current, but not too much. Even if all 6 buttons are pressed, the Pico shouldn’t sink more than around 4.5 mA across multiple pins. That’s okay, really. Even if we decided to make the Pico interface with two controllers, that’s still comfortably under our limit.

Now we could start thinking about reducing the 5V on the button pins to 3.3V on the Pico’s GPIO pins. But according to many accounts, 5V is okay if it’s sufficiently current-limited, which it is, in our case. So let’s ignore this for now.

Other MSX machines

(I will probably expand this section at some point.)

YIS-503

The YIS-503 (schematics, look at the circuits near the JOY1 and JOY2 connectors on the left) has 22K pull-up resistors and an MSX Engine. I doubt that MSX Engine chips (which have the PSG integrated) can be configured to destruct themselves.

Kuninet’s homebrew MSX

10k pull-up resistors. The rest of the wiring looks exactly like on the Hitachi MB-H2 as far as I can tell.

So does it actually work?

Yes. Here’s a pic of my trusty Hitachi MB-H2 MSX running its built-in sketch program, controlled through the fake PS3 controller. (Sorry, the computer’s still open from the probing I did earlier.)

Totally off-topic, but there’s a spot in this pic that has been cleaned up using my Pixel phone’s magic eraser. Can you see where it is? (I didn’t touch it up in an image editor afterwards.)

Problems

Plugging our contraption into the joystick port while the MSX is running crashes the machine! The screen goes very slightly dark for a split second too. Probably in-rush current. I’m pretty sure I blew my multimeter’s (200 mA) fuse trying to measure the current. Laff. (Having the controller already plugged in before powering on the computer works fine.)

Source code

Replace your Pico-PIO-USB/examples/capture_hid_report.c with the following code, (re-)make, flash, and you’re set. (I’m basing my modifications on git commit d00a10a8c425d0d40f81b87169102944b01f3bb3.)

#include <stdio.h>
#include <string.h>

#include "pico/stdlib.h"
#include "pico/multicore.h"
#include "pico/bootrom.h"

#include "pio_usb.h"

#define GAMEPAD_REPORT_ID 1

// 0x1: L2
// 0x2: R2
// 0x4: L1
// 0x8: R1
// 0x10: Triangle
// 0x20: Circle
// 0x40: X
// 0x80: Square?
// 0x100: ?
// 0x200: ?
// 0x400: R3
// 0x800: Start
// 0x1000: Up
// 0x2000: Right
// 0x4000: Down?
// 0x8000: Left?

#define BUTTON_UP 0x1000
#define BUTTON_DOWN 0x4000
#define BUTTON_LEFT 0x8000
#define BUTTON_RIGHT 0x2000
#define BUTTON_A 0x20
#define BUTTON_B 0x40
#define BUTTON_A_ALT 0x10
#define BUTTON_B_ALT 0x80

#define BUTTON_UP_PIN 16
#define BUTTON_DOWN_PIN 17
#define BUTTON_LEFT_PIN 18
#define BUTTON_RIGHT_PIN 19
#define BUTTON_A_PIN 20
#define BUTTON_B_PIN 21

static usb_device_t *usb_device = NULL;

void core1_main() {
  sleep_ms(10);

  // To run USB SOF interrupt in core1, create alarm pool in core1.
  static pio_usb_configuration_t config = PIO_USB_DEFAULT_CONFIG;
  config.alarm_pool = (void*)alarm_pool_create(2, 1);
  usb_device = pio_usb_host_init(&config);

  //// Call pio_usb_host_add_port to use multi port
  // const uint8_t pin_dp2 = 8;
  // pio_usb_host_add_port(pin_dp2);

  while (true) {
    pio_usb_host_task();
  }
}

static void gpio_setup()
{
  gpio_init(BUTTON_UP_PIN);
  gpio_init(BUTTON_DOWN_PIN);
  gpio_init(BUTTON_LEFT_PIN);
  gpio_init(BUTTON_RIGHT_PIN);
  gpio_init(BUTTON_A_PIN);
  gpio_init(BUTTON_B_PIN);

  gpio_set_dir(BUTTON_UP_PIN, GPIO_IN);
  gpio_set_dir(BUTTON_DOWN_PIN, GPIO_IN);
  gpio_set_dir(BUTTON_LEFT_PIN, GPIO_IN);
  gpio_set_dir(BUTTON_RIGHT_PIN, GPIO_IN);
  gpio_set_dir(BUTTON_A_PIN, GPIO_IN);
  gpio_set_dir(BUTTON_B_PIN, GPIO_IN);

  gpio_put(BUTTON_UP_PIN, 0);
  gpio_put(BUTTON_DOWN_PIN, 0);
  gpio_put(BUTTON_LEFT_PIN, 0);
  gpio_put(BUTTON_RIGHT_PIN, 0);
  gpio_put(BUTTON_A_PIN, 0);
  gpio_put(BUTTON_B_PIN, 0);
}

int main() {
  // default 125MHz is not appropreate. Sysclock should be multiple of 12MHz.
  set_sys_clock_khz(120000, true);

  stdio_init_all();
  printf("hello!");

  sleep_ms(10);

  multicore_reset_core1();
  // all USB task run in core1
  multicore_launch_core1(core1_main);

  gpio_setup();

  while (true) {
    if (usb_device != NULL) {
      for (int dev_idx = 0; dev_idx < PIO_USB_DEVICE_CNT; dev_idx++) {
        usb_device_t *device = &usb_device[dev_idx];
        if (!device->connected) {
          continue;
        }

        // Print received packet to EPs
        for (int ep_idx = 0; ep_idx < PIO_USB_DEV_EP_CNT; ep_idx++) {
          endpoint_t *ep = pio_usb_get_endpoint(device, ep_idx);

          if (ep == NULL) {
            break;
          }

          uint8_t temp[64];
          int len = pio_usb_get_in_data(ep, temp, sizeof(temp));

          if (len > 0) {
            if (temp[0] == GAMEPAD_REPORT_ID) {
              uint16_t button_state = temp[2] << 8 | temp[3];
              if (button_state & BUTTON_UP) {
                gpio_put(BUTTON_UP_PIN, 0);
                gpio_set_dir(BUTTON_UP_PIN, GPIO_OUT);
                printf("BUTTON_UP_PIN\n");
              } else {
                gpio_set_dir(BUTTON_UP_PIN, GPIO_IN);
              }

              if (button_state & BUTTON_DOWN) {
                gpio_put(BUTTON_DOWN_PIN, 0);
                gpio_set_dir(BUTTON_DOWN_PIN, GPIO_OUT);
                printf("BUTTON_DOWN_PIN\n");
              } else {
                gpio_set_dir(BUTTON_DOWN_PIN, GPIO_IN);
              }

              if (button_state & BUTTON_LEFT) {
                gpio_put(BUTTON_LEFT_PIN, 0);
                gpio_set_dir(BUTTON_LEFT_PIN, GPIO_OUT);
                printf("BUTTON_LEFT_PIN\n");
              } else {
                gpio_set_dir(BUTTON_LEFT_PIN, GPIO_IN);
              }

              if (button_state & BUTTON_RIGHT) {
                gpio_put(BUTTON_RIGHT_PIN, 0);
                gpio_set_dir(BUTTON_RIGHT_PIN, GPIO_OUT);
                printf("BUTTON_RIGHT_PIN\n");
              } else {
                gpio_set_dir(BUTTON_RIGHT_PIN, GPIO_IN);
              }

              if (button_state & BUTTON_A || button_state & BUTTON_A_ALT) {
                gpio_put(BUTTON_A_PIN, 0);
                gpio_set_dir(BUTTON_A_PIN, GPIO_OUT);
                printf("BUTTON_A_PIN\n");
              } else {
                gpio_set_dir(BUTTON_A_PIN, GPIO_IN);
              }

              if (button_state & BUTTON_B || button_state & BUTTON_B_ALT) {
                gpio_put(BUTTON_B_PIN, 0);
                gpio_set_dir(BUTTON_B_PIN, GPIO_OUT);
                printf("BUTTON_B_PIN\n");
              } else {
                gpio_set_dir(BUTTON_B_PIN, GPIO_IN);
              }
            }
          }
        }
      }
    }
    stdio_flush();
    sleep_us(10);
  }
}

Introduction and conclusion

I bought another Hitachi H2 MSX last year, mostly because I wanted the manual, which I’ve scanned. Unfortunately for my free time but fortunately for my, um, education in retro computing, this computer had issues with its video RAM. Often, the computer would boot up with a garbled screen. Resetting after a couple minutes would usually fix the issue. The video RAM is made by Toshiba, and is called TMM416P-2 (also marked 4116-2). If you have this memory, I’d recommend you look out for issues, because all eight ICs had the same issue, namely: crazy-ass noise on the -5V line. (How much noise is “crazy-ass” noise? In this case, it’s +-3V.) The noise sort of comes and goes, or at least gets stronger and weaker, randomly, which made it too hard for me to find a combination of capacitors to tame it. (Though it’s more likely to be present after turning the computer on after a long while.) I ended up socketing them all, replacing one that unfortunately died during the very professional desoldering process, and added 103 ceramic capacitors to (almost) every one, between the -5 and GND pins, which seems to have a slight positive effect. (The bottom part of the case has a hook that requires some clearance and prevents two of the chips from getting their capacitor.) I also replaced the zener diode with a 7905, which fit perfectly after bending the legs a little bit.

Details

The -5V rail for the 4116 VRAM chips is generated using a zener diode. Replacing this, or the capacitor on the rail, unfortunately didn’t have any effect. Hmm, odd!

Next, I decided to desolder the -5V pin on the first 4116 IC, and drive it using my own known good -5V supply (using a standard 7905 regulator). Result: noise both on the first chip and all the others. Hmm, odd!

Next, I did this for the rest of the 4116 ICs, and was able to see that each and every one generates noise.

Next, I decided to desolder all of them and individually test them on my 4116 tester. (They still produced the noise while in the tester.) I decided to desolder all of them because the H2 seemed to support 4416 ICs for the video RAM, and I happened to have some of those that were waiting to be put to use. I.e., there are holes of the right size, right next to the VDP, and the silkscreen on those holes says “TMS4416”. ;)

Well, today’s lesson is, do not necessarily trust the silkscreen. The TMS9928A doesn’t even support 4416 VRAM! The holes where the data pins go didn’t even have any traces on them.
The TMS9928A can be made to support 4416 RAM using a custom circuit, though. Maybe I should have implemented this circuit. I even bought the two required parts! But then decided against it for complexity management reasons.

Unfortunately, expecting to be able to use the 4416 slots, I had desoldered the original VRAM ICs in a rather brutish manner, losing vias and traces in the process, which meant that I needed to add a bunch of bodge wires to get them to work again. At least the bodge wires aren’t too complex to figure out, if at some point one of them decides to become loose again. I ended up keeping the original, noisy, RAM chips. But since they’re now all socketed, it shouldn’t be too hard to replace them at some point, if necessary.

Pictures

After

Before looking at the picture of the bodge wires below, please keep in mind that it is rude to stare.

Hi! My sabbatical ended and I’ve been working again since two months ago. Boo. However there’s this thing I just wanted to get off my chest, so I spent a few hours that I did not really have and wrote some code and this blog post about it!

Last year, I made a 4164/4116 DRAM tester for the Raspberry Pi Pico, which works just as you would expect, you program the Pico, place it on a breadboard, add some wires and something to drop the 5V to 3.3V for the Q output, place the 4164 or 4116 chip you’d like to test on the breadboard, connect a USB cable from the Pico to a computer, and look at the terminal output (or just the on-board LED). This is useful if you have already extracted a 4164 chip that you have determined to be bad. (I have written previously how you could determine whether a 4164 chip is bad, here and here.)

I previously made a “live” chip tester for the Arduino, capable of checking whether simple logic chips are doing what they’re supposed to be doing in a powered on system. I used the Arduino rather than the Pico because the Arduino is 5V tolerant. In this post, we’re not doing simple logic chips, but DRAM chips. And since we need to be super-fast, we’re using a Pico.

Executive summary

We “allocate” 64 KB of RAM on the Pico. We need to read in two 8-bit addresses, and combine them to a 16-bit address. If a write is being attempted, we write the same bit value into the appropriate address of Pico’s RAM. If a read is being attempted, we check if the DRAM’s output is the same as what we have in the Pico’s RAM. (We could also use 64 Kb of RAM on the Pico at a minimum, but as we’ll see in the next section we do not really have a lot of time for such shenanigans.)

To use this software, having an IC test clip would probably be very beneficial. I use these: https://www.kandh.com.tw/ic-test-clips-ic-test-clips.html. These are available in Akihabara from Akizuki: https://akizukidenshi.com/catalog/g/gC-04753/. If they aren’t available in your market, perhaps these somewhat expensive test clips from 3M might be more available: https://www.digikey.jp/ja/products/detail/3m/923700/3852.

Note: to use this software, you need to at least mostly know what you’re doing.

One note on hooking up the Pico directly to 5V components, as seen in the above pictures

Not guaranteed to not fry your Pico (note the double negation), do this at your own risk. Your Pico will possibly also draw more current than a normal TTL chip when driven above 3.6V or so, which could easily damage your precious hardware! (The current on the address pins will likely be supplied by a pair of 74LS157 chips, on the Q pins by the RAM chip, and the current on the RAM’s data in pin by the CPU or any other. \RAS and \CAS probably by custom logic chips.) Use a 74HCT245 between the Pico and the device you’d like to test.

Caveat 1

The Pico is very fast when compared to an 8-bit computer from the 1980s, but 4164 transition times are extremely fast too. If you look at a timing diagram for the 4164 (which you will find in any 4164 datasheet), you will notice that all transition times listed are on the order of <ten, tens, or low hundreds of nanoseconds. Most 4164 chips have a -20, -15, -12, or -10 suffix in their part number. This indicates the minimum allowable number of nanoseconds × 10 for the sum of all transitions. (If the DRAM is driven faster, it probably won’t work correctly. However, if it’s driven slower, most things will generally work out, though if your system e.g. reads the DRAM’s output too slowly it might be too late and not work out.)

The stock Pico runs at 125 MHz, which means that one CPU cycle is 8 ns. From hearsay, you can probably overclock any Pico to 200 MHz (clock cycles are 5 ns), and many people report that their Pico runs fine even at 400 MHz (clock cycles are 2.5 ns). For -15 DRAMs, you have 150/8 = 18.75 CPU cycles per transition if the DRAM is driven at its max speed. (Note: it isn’t on MSX machines, at least.) 18 CPU cycles isn’t a lot. Remember, we need to convert two 8-bit addresses to a 16-bit address, and then check if the newly read value matches our previously recorded value. Is that doable in 18 CPU cycles? I don’t think so, but I’m not an ARM assembly expert.

So, did I get it to work? Well… sort of but not quite.

Caveat 2

Note that there are many failure modes for DRAM chips. For example, if the chip gets super-hot within a few seconds, it’s probably shorted. I’d expect there to be a very low resistance between ground and another pin. Before hooking up the “live” tester I’m going to explain on this page, check for that kind of stuff. I would not recommend using the live tester on a chip that gets super-hot within seconds. You could risk melting your connectors, and if the short is not between VCC and GND, potentially also risk your Pico due to excessive current on a pin driven by the Pico.

Caveat 3

Untested with 4116 chips, only tested with my MSX’s main RAM.

Current status

The live tester successfully verifies that a TMS4164-15 DRAM chip under test in my stand-alone 4164 RAM tester is outputting the correct values. (There is no reason why it shouldn’t work with a 4116 chip. The tester certainly does! You just need to re-wire slightly. Also, 5V on the Pico, yeah, it doesn’t seem to “explode immediately.” But -5V or 12V? You’d better leave those pins unconnected!)

On a real system (my trusty Hitachi MB-H2 MSX) with TMS4164-15NL DRAM chips, the live tester manages just fine from power up, up until the first ~11000 comparisons (which is a split second), but at some point reads a 0 when it should have been a 1, and prints an error. Printing errors takes a long time at 115200 bps, so we go completely off the rails once we’ve encountered the first error. (That’s slightly configurable though, see “Lnobs” section for details.)

However, in the live tester’s “DEBUG” mode, it just collects a lot of samples into memory, and prints them out when the sample memory is full. Using a simple script (the Perl script included in the repo), I can then verify that all the samples check out. Note that the DEBUG code also prints out who many times it had to wait until it got data from the PIO. The answer is 0 times every time, which means that we’re too slow or almost too slow. (Sometimes there is a handful of mismatches, I’ll look into those at some point. Could be that we were just too slow, or the 5V is messing with the system ;D)

There’s a lot that could be improved, hence the “WIP” attribute in title. The first obvious improvement would be to try a little harder in the non-debug mode. The Pico has two CPU cores, and we’re only using one. We could attain more throughput by running according to the following scheme:

Core 1:
Wait for sample 1
Tell core 2 to wait for sample 2
Process sample 1
Wait for sample 3
Tell core 2 to wait for sample 4
Process sample 3
…

Core 2:
Wait for instructions from CPU 1
Wait for sample 2
Process sample 2
Wait for instructions from CPU 1
Wait for sample 4
Process sample 2
…

Another potential optimization would be to write the processing code in ARM assembly. (My experience with ARM assembly is mostly read-only, so not sure how much better I can get without spending way too much effort.)

Also I haven’t tried overclocking yet. Probably should!

Some more technical details

We use two PIO state machines. One waits for RAS high→low (“RAS SM”, and the other one waits for CAS high→low (“CAS SM”, which comes after the RAS transition.

Not all RAS transitions are followed by CAS transitions. For example, refresh is mostly RAS-only. In addition, though perhaps not used on the MSX(?), RAS transitions may be followed by multiple CAS transitions.

In the C code, we wait for events on the CAS SM, and then read from both the RAS SM’s FIFO and the CAS SM’s FIFO. In the PIO code, the CAS SM tells the RAS SM whether to push its address or not. (We could alternatively (maybe) always push and have the CPU make sure the FIFO never gets full, but my experiments in that regard didn’t go that well.)

There are a lot of defines that change the way the system works.

Knobs

• Setting PRINT_ERROR_THRESHOLD to something above 0 only starts printing errors after encountering that many errors.
• CORRECT_ERRORS causes the Pico’s memory to be updated when we encounter a mismatch
• VERBOSE_STATUS_LEDS causes the Pico to perform GPIO writes at GPIO16+ (or so, I recommend you check the source to find the exact GPIO pin number) to indicate whether we’re reading or writing. This isn’t very beneficial performance-wise.
• SWAP_RAS_AND_CAS_ADDRESSES: my Hitachi MB-H2 MSX applies the CPU’s A0-A7 to the RAM pins at RAS time, and A8-A15 at CAS time. When thinking “rows” and “columns”, most people would probably assume that “rows” use the more significant bits, but that is not necessarily the case, and it doesn’t matter. When operating in DEBUG mode, you’ll see accesses that are mostly linear if this define is set correctly. Otherwise each access will be 256 apart.

The code is at https://github.com/qiqitori/pico_live_chip_tester.

So… is this likely to find a fault?

I haven’t seen any DRAM chip failures (except on YouTube) where only some addresses were broken and the chip appeared to work otherwise. (SRAM chips are different story. I’m sort of planning on doing an SRAM tester too, but probably not too soon.) Most DRAM chips I’ve seen are an all-or-nothing affair. For all-or-nothing affairs, this chip tester is very likely to find the problem immediately, especially if you compare all 8 chips and only one is weird. For hypothetical chips with just a single address problem (or perhaps, a single broken row), either it’ll be difficult with the code not 100% working right now, or it might take several attempts and statistics.