In order to flash a microcontroller over USB, it needs something called a bootloader. This bootloader lives in a specific section of the flash memory, and allows you to load the actual application firmware (in this case, QMK) into the rest of the flash.
However, it can sometimes happen that the bootloader becomes corrupted and needs reflashing, or you may want to change the bootloader to another one. It's not possible to do this with the existing bootloader, because, of course, it is already running, and cannot overwrite itself. Instead, you will need to ISP flash the microcontroller.
There are several different kinds of bootloaders available for AVR microcontrollers. Most STM32 ARM-based microcontrollers already have a USB-capable bootloader in ROM, so generally do not need to be ISP flashed.
One of the following devices is required to perform the ISP flashing. The product links are to the official versions, however you can certainly source them elsewhere.
You'll also need some jumper wires to connect the ISP flasher and the target board. Some boards have an ISP header with the necessary pins broken out. If not, then you will need to temporarily solder the wires to the PCB -- usually to switch pins or directly to the MCU.
The wiring is fairly straightforward; for the most part, you'll be connecting like to like. Refer to the target MCU's datasheet for the exact RESET, SCLK, MOSI and MISO pins.
To use a 5V/16MHz Pro Micro as an ISP flashing tool, you will first need to load a special firmware onto it that emulates a hardware ISP flasher.
AVRDUDE Programmer: avrisp
AVRDUDE Port: Serial
| Pro Micro | Keyboard |
|---|---|
VCC |
VCC |
GND |
GND |
10 (B6) |
RESET |
15 (B1) |
SCLK |
16 (B2) |
MOSI |
14 (B3) |
MISO |
!> Note that the 10 pin on the Pro Micro should be wired to the RESET pin on the keyboard's controller. DO NOT connect the RESET pin on the Pro Micro to the RESET on the keyboard.
To use a Teensy 2.0 as an ISP flashing tool, you will first need to load a special firmware onto it that emulates a hardware ISP flasher.
AVRDUDE Programmer: avrisp
AVRDUDE Port: Serial
| Teensy | Keyboard |
|---|---|
VCC |
VCC |
GND |
GND |
B0 |
RESET |
B1 |
SCLK |
B2 |
MOSI |
B3 |
MISO |
!> Note that the B0 pin on the Teensy should be wired to the RESET pin on the keyboard's controller. DO NOT connect the RESET pin on the Teensy to the RESET on the keyboard.
SparkFun PocketAVR
Adafruit USBtinyISP
Thomas Fischl's USBasp
AVRDUDE Programmer: usbtiny / usbasp
AVRDUDE Port: usb
| ISP | Keyboard |
|---|---|
VCC |
VCC |
GND |
GND |
RST |
RESET |
SCLK |
SCLK |
MOSI |
MOSI |
MISO |
MISO |
!> The 5-pin "ICSP" header is for ISP flashing the PIC microcontroller of the Bus Pirate. Connect your target board to the 10-pin header opposite the USB connector instead.
AVRDUDE Programmer: buspirate
AVRDUDE Port: Serial
| Bus Pirate | Keyboard |
|---|---|
+5V |
VCC |
GND |
GND |
RST |
RESET |
CLK |
SCLK |
MOSI |
MOSI |
MISO |
MISO |
QMK Toolbox supports flashing both the ISP firmware and bootloader, but note that it cannot (currently) set the AVR fuse bytes for the actual ISP flashing step, so you may want to work with avrdude directly instead.
Setting up the QMK environment is highly recommended, as it automatically installs avrdude along with a host of other tools.
One of these files is what you will be ISP flashing onto the board. The default fuses are also listed.
If you're not sure what your board uses, look in the rules.mk file for the keyboard in QMK. The MCU and BOOTLOADER lines will have the values you need. It may differ between different versions of the board.
These are the factory default bootloaders shipped by Atmel (now Microchip). Note that the AT90USB64 and AT90USB128 bootloaders are slightly modified, due to a bug causing them to not enumerate properly in Windows 8 and later.
| MCU | Low | High | Extended | USB ID |
|---|---|---|---|---|
| ATmega16U4 | 0x5E |
0x99 / 0xD9 (JTAG disabled) |
0xF3 |
03EB:2FF3 |
| ATmega32U4 | 0x5E |
0x99 / 0xD9 (JTAG disabled) |
0xF3 |
03EB:2FF4 |
| AT90USB64 | 0x5E |
0x9B / 0xDB (JTAG disabled) |
0xF3 |
03EB:2FF9 |
| AT90USB128 | 0x5E |
0x99 / 0xD9 (JTAG disabled) |
0xF3 |
03EB:2FFB |
This is the default Arduino-style bootloader derived from the LUFA CDC bootloader, and is only for the ATmega32U4.
There are several variants depending on the vendor, but they all mostly work the same way. The SparkFun variants, for example, require the RESET pin to be grounded twice quickly in order to stay in bootloader mode for more than 750 ms.
| MCU | Low | High | Extended | USB ID |
|---|---|---|---|---|
| SparkFun Pro Micro (3V3/8MHz) | 0xFF |
0xD8 |
0xFE |
1B4F:9203 |
| SparkFun Pro Micro (5V/16MHz) | 0xFF |
0xD8 |
0xFB |
1B4F:9205 |
| SparkFun LilyPadUSB (and some Pro Micro clones) | 0xFF |
0xD8 |
0xFE |
1B4F:9207 |
| Pololu A-Star 32U4* | 0xFF |
0xD0 |
0xF8 |
1FFB:0101 |
| Adafruit Feather 32U4 | 0xFF |
0xD8 |
0xFB |
239A:000C |
| Adafruit ItsyBitsy 32U4 (3V3/8MHz)* | 0xFF |
0xD8 |
0xFB |
239A:000D |
| Adafruit ItsyBitsy 32U4 (5V/16MHz) | 0xFF |
0xD8 |
0xFB |
239A:000E |
| Arduino Leonardo* | 0xFF |
0xD8 |
0xFB |
2341:0036 |
| Arduino Micro* | 0xFF |
0xD8 |
0xFB |
2341:0037 |
?> Files marked with a * have combined Arduino sketches, which runs by default and also appears as a serial port. However, this is not the bootloader device.
This bootloader is primarily for keyboards originally designed for the PS2AVRGB firmware and Bootmapper Client. It is not recommended for use in new designs.
| MCU | Low | High | USB ID |
|---|---|---|---|
| ATmega32A | 0x0F |
0xD0 |
16C0:05DF |
USBaspLoader is a bootloader based on V-USB that emulates a hardware USBasp device. It runs on ATmega32A and ATmega328P MCUs.
Precompiled .hex files are generally not available, but you can compile it yourself by setting up the QMK environment and following Coseyfannitutti's guide for the appropriate MCU:
| MCU | Low | High | Extended | USB ID |
|---|---|---|---|---|
| ATmega32A | 0x1F |
0xC0 |
n/a | 16C0:05DC |
| ATmega328P | 0xD7 |
0xD0 |
0x04 |
16C0:05DC |
Note that some boards may have their own specialized build of this bootloader in a separate repository. This will usually be linked to in the board's readme.
Open a new Terminal window - if you are on Windows, use MSYS2 or QMK MSYS, not the Command Prompt. Navigate to the directory your bootloader .hex is in. Now it's time to run the avrdude command.
The syntax of avrdude is:
avrdude -c <programmer> -P <port> -p <mcu> -U flash:w:<filename>:i
<programmer>corresponds to the programmer type listed for each ISP flasher in the Hardware section, for exampleavrisp.<port>is the serial port that appears when you plug the ISP flasher in, if any. For some programmers this is simplyusb(or you can omit the-Pargument completely) since they do not operate as a serial device.- Windows:
COMx- check Device Manager, under the "Ports (COM & LPT)" section - Linux:
/dev/ttyACMx - macOS:
/dev/tty.usbmodemXXXXXX
- Windows:
<mcu>should be the lowercase name of the target AVR microcontroller, for exampleatmega32u4.<filename>is the absolute or relative path to the bootloader to be flashed, for exampleCaterina-Micro.hex.
You can also run man avrdude for more information.
If all goes well, you should get output similar to the following:
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.00s
avrdude: Device signature = 0x1e9587 (probably m32u4)
avrdude: NOTE: "flash" memory has been specified, an erase cycle will be performed
To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: reading input file "Caterina-Micro.hex"
avrdude: writing flash (32730 bytes):
Writing | ################################################## | 100% 11.58s
avrdude: 32730 bytes of flash written
avrdude: verifying flash memory against Caterina-Micro.hex:
avrdude: load data flash data from input file Caterina-Micro.hex:
avrdude: input file Caterina-Micro.hex contains 32730 bytes
avrdude: reading on-chip flash data:
Reading | ################################################## | 100% 10.33s
avrdude: verifying ...
avrdude: 32730 bytes of flash verified
avrdude: safemode: Fuses OK (E:CB, H:D8, L:FF)
avrdude done. Thank you.
This is a slightly more advanced topic, but may be necessary if you are switching from one bootloader to another (for example, Caterina to Atmel/QMK DFU on a Pro Micro). Fuses control some of the low-level functionality of the AVR microcontroller, such as clock speed, whether JTAG is enabled, and the size of the section of flash memory reserved for the bootloader, among other things. You can find a fuse calculator for many AVR parts here.
!> WARNING: Setting incorrect fuse values, in particular the clock-related bits, may render the MCU practically unrecoverable without high voltage programming (not covered here)! Make sure to double check the commands you enter before you execute them.
To set the fuses, add the following to the avrdude command:
-U lfuse:w:0xXX:m -U hfuse:w:0xXX:m -U efuse:w:0xXX:m
where the lfuse, hfuse and efuse arguments represent the low, high and extended fuse bytes as listed in the Hardware section.
?> You may get a warning from avrdude that the extended fuse byte does not match what you provided when reading it back. If the second hex digit matches, this can usually be safely ignored, because the top four bits of this fuse do not actually exist on many AVR parts, and may read back as anything.
For mass production purposes, it is possible to join the bootloader and QMK firmware together into a single file, due to the way the Intel Hex format works:
- Open the QMK firmware and bootloader
.hexfiles in a text editor. - Remove the last line of the QMK firmware (which should be
:00000001FF- this is just an "end of file" marker). - Paste the contents of the bootloader
.hexfile onto a new line at the end of the QMK firmware file, with no empty lines between. - Save it as a new file, for example
<keyboard>_<keymap>_production.hex.
You can then ISP flash this combined firmware instead, which allows you to skip the extra step of flashing the QMK firmware over USB.