PROGRAMMER.md

Programming the AVR128Dx

Contents:

• Overview
• USB to Serial
• Wiring
• Adding a Reset Button

Protocol

The AVR128Dx series of microcontrollers use a proprietary programming and debugging interface, UDPI (Unified Programming and Debugging Interface). You can find detailed specifications in the datasheet, but the gist is:

• one-wire protocol (based on run o' the mill UART)
• fixed frame format with 8-bits per frame
• one (even) parity bit
• two stop bits
• baud detection via the SYNCH frame

There are some other goodies and nuances, but it's a relatively simple protocol with only a handful of commands for this target architecture.

Programmers...

Probably, the easiest / most reliable way to program this device is with a purpose-built programmer and standard tooling...

...in my case, I wanted to get started with what I had lying around and work in a familiar dev environment (vim + make + avr-gcc), and not wait for anything to arrive in the mail...

As it turns out, all you need is pymcuprog and a USB-to-Serial converter — which we will adapt to work as a UDPI interface.

For the USB-to-Serial, you have a few options:

• buy a dedicated converter
• use another MCU with multiple serial ports (:white_check_mark: this is what I'm doing)
• use another MCU and bit-bang or else bend another peripheral type into a second serial port

USB to Serial

In my case, I used an Arduino Pro Micro clone that I had lying around (but any device with multiple serial ports will work). I don't normally work in the Arduino IDE or with the Arduino et al libs, but, in this case, it's pretty handy. I created the following "sketch" and uploaded it to my Pro Mini:

// Simple serial passthrough:
// - 115200 baud
// - even parity
// - two stop bits
void setup() {
  Serial.begin(115200, SERIAL_8E2);
  Serial1.begin(115200, SERIAL_8E2);
}

// relay whatever we get on "Serial" to "Serial1" and vice versa:
void loop() {
  if (Serial.available()) {
    Serial1.write(Serial.read());
  }

  if (Serial1.available()) {
    Serial.write(Serial1.read());
  }
}

Wiring

All we need to do to make the UART work as a UDPI programmer is to put a Schottky diode from Serial 1 Rx to Tx, like so (if you don't have a Schottky handy, a 1k resistor can be pressed into service):

Adding a Reset Button

To add a reset button:

• add a push-button from pin 18 (!reset) to ground
• put a 100n cap between pin 18 and ground

This is the setup I have: