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Debugging FAQ

This page details various common questions people have about troubleshooting their keyboards.

Debugging :id=debugging

Your keyboard will output debug information if you have CONSOLE_ENABLE = yes in your rules.mk. By default the output is very limited, but you can turn on debug mode to increase the amount of debug output. Use the DEBUG keycode in your keymap, use the Command feature to enable debug mode, or add the following code to your keymap.

void keyboard_post_init_user(void) {
  // Customise these values to desired behaviour
  debug_enable=true;
  debug_matrix=true;
  //debug_keyboard=true;
  //debug_mouse=true;
}

Debugging Tools

There are two different tools you can use to debug your keyboard.

Debugging With QMK Toolbox

For compatible platforms, QMK Toolbox can be used to display debug messages from your keyboard.

Debugging With hid_listen

Prefer a terminal based solution? hid_listen, provided by PJRC, can also be used to display debug messages. Prebuilt binaries for Windows,Linux,and MacOS are available.

Sending Your Own Debug Messages :id=debug-api

Sometimes it's useful to print debug messages from within your custom code. Doing so is pretty simple. Start by including print.h at the top of your file:

#include "print.h"

After that you can use a few different print functions:

  • print("string"): Print a simple string.
  • uprintf("%s string", var): Print a formatted string
  • dprint("string") Print a simple string, but only when debug mode is enabled
  • dprintf("%s string", var): Print a formatted string, but only when debug mode is enabled

Debug Examples

Below is a collection of real world debugging examples. For additional information, refer to Debugging/Troubleshooting QMK.

Which matrix position is this keypress?

When porting, or when attempting to diagnose pcb issues, it can be useful to know if a keypress is scanned correctly. To enable logging for this scenario, add the following code to your keymaps keymap.c

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
  // If console is enabled, it will print the matrix position and status of each key pressed
#ifdef CONSOLE_ENABLE
    uprintf("KL: kc: 0x%04X, col: %u, row: %u, pressed: %b, time: %u, interrupt: %b, count: %u\n", keycode, record->event.key.col, record->event.key.row, record->event.pressed, record->event.time, record->tap.interrupted, record->tap.count);
#endif 
  return true;
}

Example output

Waiting for device:.......
Listening:
KL: kc: 169, col: 0, row: 0, pressed: 1
KL: kc: 169, col: 0, row: 0, pressed: 0
KL: kc: 174, col: 1, row: 0, pressed: 1
KL: kc: 174, col: 1, row: 0, pressed: 0
KL: kc: 172, col: 2, row: 0, pressed: 1
KL: kc: 172, col: 2, row: 0, pressed: 0

How long did it take to scan for a keypress?

When testing performance issues, it can be useful to know the frequency at which the switch matrix is being scanned. To enable logging for this scenario, add the following code to your keymaps config.h

#define DEBUG_MATRIX_SCAN_RATE

Example output

  > matrix scan frequency: 315
  > matrix scan frequency: 313
  > matrix scan frequency: 316
  > matrix scan frequency: 316
  > matrix scan frequency: 316
  > matrix scan frequency: 316

hid_listen Can't Recognize Device

When debug console of your device is not ready you will see like this:

Waiting for device:.........

Once the device is plugged in then hid_listen finds it you will get this message:

Waiting for new device:.........................
Listening:

If you can't get this 'Listening:' message try building with CONSOLE_ENABLE=yes in [Makefile]

You may need privileges to access the device an OS like Linux. Try sudo hid_listen.

On many Linux distros you can avoid having to run hid_listen as root by creating a file called /etc/udev/rules.d/70-hid-listen.rules with the following content:

SUBSYSTEM=="hidraw", ATTRS{idVendor}=="abcd", ATTRS{idProduct}=="def1", TAG+="uaccess", RUN{builtin}+="uaccess"

Replace abcd and def1 with your keyboard's vendor and product id, letters must be lowercase. The RUN{builtin}+="uaccess" part is only needed for older distros.

Can't Get Message on Console

Check:

  • hid_listen finds your device. See above.
  • Enable debug by pressing Magic+d. See Magic Commands.
  • Set debug_enable=true. See Debugging
  • Try using print function instead of debug print. See common/print.h.
  • Disconnect other devices with console function. See Issue #97.