avr-pico-programmer

A programmer for the AVR architecture microcontroller, specifically the ATTiny84A by Microchip, for the Raspberry Pi Pico.

• Turns the Raspberry Pi Pico into an AVR programmer via SPI programming mode.
• Instructions are streamed using the serial port to the Raspberry Pi Pico via USB which handles the rest.
• Verifies flashed pages after programming to confirm a successful flash.

Problem

• Initially, the ATTiny did not respond to any of my SPI programming commands, eventually after a bit of back and fourth with Microchip support, we figured out that the ATTiny does not support programming via 3.3V which the Raspberry Pi Pico uses for GPIO logic; all SPI pins must use a logic voltage of 5V, simply supplying Vcc with 5V is not enough.
  • Luckily I managed to bypass the requirement for a 5V programming device by using a 3V3 5V logic converter which works by converting 3.3V signals to 5V signals, which effectively acts as a translator between the Pico and the ATTiny (PS. Any 3.3V to 5V logic converter PCB with support for at least 4 ports (MISO, MOSI, SCK, RESET) should work).
  • It should be noted that not all ATTiny/AVR controllers have this restriction, some could be programmed with just the Pico's 3.3V signals, in which case translation through a logic converter is not needed; however as far as I am concerned the ATTiny84A does need 5V logic.

How to use

Flashing software

• You can either use the released .uf2 binary file and drop it onto the Raspberry Pi Pico which has the compiled source code, or compile it yourself with the Raspberry Pi Pico SDK for C/C++.

Physical setup

• You simply need to connect all the SPI and other relavent pins from the Raspberry Pi Pico to the logic converter, and from the logic converter to the ATTiny.

• I created a schematic which demonstrates this with a Raspberry Pi Pico and the ATTiny84A, if you're using another AVR controller you should be able to infer which pins to connect from the relavent AVR datasheet:

  In case the schematic is unclear: VBUS-HV&VCC | GND-GND&GND | 3V3_OUT-LV | GP19-LV3-HV3-PA6 | GP18-LV2-HV2-PA4 | GP17-LV1-HV1-PB3 | GP16-LV4-HV4-PA5

Programming

• The tool is designed so that you can also program firmware using a serial port client, which you can do so by using a tool such as PuTTY and pasting in the hex of each individual byte of the firmware, each being followed by a space (" ") - after all hex bytes have been sent, press enter and the programmer will begin erasing and flashing the AVR controller.
• Alternatively, I have written a client for this in Node.js which takes in a binary file of the firmware and automatically interfaces with the Pico programmer:

Motivation

• A while back I got hold of an ATTiny84A but had no idea how to program it, later realising I needed an Arduino or a dedicated AVR programmer, hence decided to try to program it via a Raspberry Pi Pico.
• I started looking through the ATTiny datasheet and attempted to program it according to the serial programming section.
• It served as a fun project as I was interested in the AVR architecture and wanted to challenge myself by writing a programmer for it from scratch.

Writing code for AVR

• This is a topic on it's own and may include a guide on programming the ATTiny84A controller as well as others, but in general you will need to install the AVR GCC compiler, AVR assembly compiler, as well as the headers for AVR programming.
• Due to the nature of how the AVRs registers and control flow works, you will likely not have to write any assembly and can write the whole thing in C as the AVR GCC compiler already handles the interrupt vector table and layout of your program automatically - all you need to include is a main function written by yourself.
• A sample script in C which toggles the PA0 pin on and off every second using busy wait (I'm not entirely sure how it works since I have yet to completely learn the RISC/AVR architecture):

  #include <avr/io.h>
  #include <util/delay.h>
  
  int main() {
      DDRA = _BV(PA0);
  	
      while (1) {
          // Toggle port A.
          PORTA ^= _BV(PA0);
  
          // Busy wait 1 second.
          _delay_ms(1000);
      }
  
      return 0;
  }

• I then compiled and turned it into a .bin microcode binary file with the following bash sequence on Ubuntu (F_CPU is the clock speed of the controller in Hz required by the delay.h header):
  • avr-gcc -DF_CPU=1000000 -mmcu=attiny84a -O2 test.c -o fw.elf
  • avr-objcopy -O binary fw.elf fw.bin
  • To see and read the underlying microcode bytes:
    • xxd -i fw.bin