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ISP Flashing Guide

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.

Hardware

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.

Pro Micro as ISP

SparkFun Pro Micro

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

Wiring

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.

Teensy 2.0 as ISP

PJRC Teensy 2.0

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

Wiring

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 / USBtinyISP

SparkFun PocketAVR
Adafruit USBtinyISP

!> SparkFun PocketAVR and USBtinyISP DO NOT support AVR chips with more than 64 KiB of flash (e.g., the AT90USB128 series). This limitation is mentioned on the shop page for SparkFun PocketAVR and in the FAQ for USBtinyISP. If you try to use one of these programmers with AT90USB128 chips, you will get verification errors from avrdude, and the bootloader won't be flashed properly (e.g., see the issue #3286).

AVRDUDE Programmer: usbtiny
AVRDUDE Port: usb

Wiring

ISP Keyboard
VCC VCC
GND GND
RST RESET
SCLK SCLK
MOSI MOSI
MISO MISO

USBasp

Thomas Fischl's USBasp

AVRDUDE Programmer: usbasp
AVRDUDE Port: usb

Wiring

ISP Keyboard
VCC VCC
GND GND
RST RESET
SCLK SCLK
MOSI MOSI
MISO MISO

Bus Pirate

Adafruit Bus Pirate

!> 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

Wiring

Bus Pirate Keyboard
+5V VCC
GND GND
RST RESET
CLK SCLK
MOSI MOSI
MISO MISO

Software

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.

Bootloader Firmware

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.

Atmel DFU

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

Caterina

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.

BootloadHID (PS2AVRGB)

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

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.

Flashing the Bootloader

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 example avrisp.
  • <port> is the serial port that appears when you plug the ISP flasher in, if any. For some programmers this is simply usb (or you can omit the -P argument 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
  • <mcu> should be the lowercase name of the target AVR microcontroller, for example atmega32u4.
  • <filename> is the absolute or relative path to the bootloader to be flashed, for example Caterina-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.

Setting the Fuses

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.

Creating a "Production" Firmware

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:

  1. Open the QMK firmware and bootloader .hex files in a text editor.
  2. Remove the last line of the QMK firmware (which should be :00000001FF - this is just an "end of file" marker).
  3. Paste the contents of the bootloader .hex file onto a new line at the end of the QMK firmware file, with no empty lines between.
  4. 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.