Flash AVR Amtel ATtiny Chips with the CH341A ISP Programmer

Weekend Project: I bricked a lot of ATtiny85 Digistump chips while I was tinkering with a custom bootloader. Here is how I unbricked them with less than $5 of hardware using the CH341A BIOS programmer.

The problem is that the CH341A is not designed for ATtiny chips, but for flashing motherboard BIOS chips. I bought one just for the included SOIC test clip, but I wanted to see if I could repurpose it for flashing an ATtiny85 chip. This post is a reference for myself about the commands and hardware hacks I had to do for when I have to do this again in half a year or later.

Hardware requirements:

  • An inexpensive CH341A EEPROM/BIOS programmer from AliExpress
  • A SOIC 8-pin test clip
  • A Linux machine running Debian/Ubuntu

I don’t have a dedicated AVR programmer, but I want to make a permanent AVR programmer rig instead of using the Arduino UNO ISP sketch trick (I have over 30 ATtiny85 chips and growing). Here’s how I accomplished this.

1) Map the CH341A 16 pin-outs to the ATtiny 8 pin-outs

With an ohmmeter, I mapped the pins with side labels to the socket pins. Now I know which socket pins correspond to the ATTiny function pins (e.g. MOSI, MISO, CS, etc.). Here is my sketch.

CH341A ISP programmer pin map to ATtiny chips
CH341A ISP programmer pin map to ATtiny chips

This jibes with the one schematic I found online for this common-yet-obscure ISP BIOS/EEPROM programmer.

CH341A MiniProgrammer ISP programmer schematic
CH341A MiniProgrammer ISP programmer schematic Credit: onetransistor.eu

2) Install libusb-1.0 (version 1.0.22+)

I’m running Linux Mint Bionic (18.04 LTS) which has only libusb 1.0.21. This is insufficient. The programming tool (next section) requires version 1.0.22+ because the API changed slightly. Disco has this version, so here is how I installed it from the Disco repo but stayed on Bionic (credits to George Shuklin).

3) Build the CH341A programmer tool

Trel725 wrote a small programming tool to make the CH341A flash ATtiny chips of all varieties. The only requirement is that libusb 1.0.22+ is installed. Simply clone the repo and run make in Linux. There will be a new file called chavrprog. Make sure it has execute permissions with chmod +x chavrprog. What it does is send bytes to the ATtiny and reads the responses in a way that enables flashing ATtiny chips. I’m so glad he made this tool.

Those steps again are:

4) Connect the CH341A connector pins to the SOIC test clip

I just used a breadboard and come colored wires to map the CH341A socket to the SOIC8 test clip given my sketch from above.

Map the CH341A socket to the SOIC test clip for ATtiny chips
Map the CH341A socket to the SOIC test clip for ATtiny chips
Closeup of breadboard wires in the SOIC8 test clip connector
Closeup of breadboard wires in the SOIC8 test clip connector

The colors are mapped to the CH341A as follows:

Notice the notch on the ribbon cable when inserting the colored wires into the ribbon cable connector.

Make sure the voltage jumper is set correctly. Pins 1 and 2 should be shorted with the supplied jumper for 3.3V operation.

5) Flash the ATtiny85 Digistump board

The red wire of test clip should line up with the little circle of the ATtiny chip – both denote pin 1. I tried a few times to get the clip to attach perfectly because the pins are so small. Notice the orientation of the Digistump in relation to the red wire of the SOIC8 test clip.

SOIC test clip on the ATtiny85 Digistump
SOIC test clip on the ATtiny85 Digistump

I could now flash the t85_default.hex from the micronucleus firmware repo located in https://github.com/micronucleus/micronucleus/tree/master/firmware/releases. I’ve actually forked this repo and made a special bootloader for the APA106 RGB LED. Either way, to flash the micronucleus bootloader all that is required is:

It worked great from the very first flash attempt. The on-board run light on the Digistump is lit awaiting a program for the bootloader. The Digistump is now unbricked.

6) Make a permanent AVR PCI programmer rig

The ISP programmer came with a little PCB to make custom expansion projects I suppose. I use that to permanently map the CH341A pins to the SOIC8 ribbon cable for use with ATtiny chips.

CH341A accessories
CH341A accessories

The plan is to use the empty PCB and create a hardware pin map between the CH341A socket and the ribbon cable connecting the SOIC8 test clip to the ATtiny chips. Only one cut needs to be made at pin 6 to isolate those two holes. The rest is straightforward soldering. The purple circles denote pins going down. The blue circles denote pins going up. I’ll only add a total of 12 pins, not 16 pins, to this PCB board.

Hardware pin mapping between the CH341A ISP programmer and the SOIC8 test clip for ATtiny chips
Hardware pin mapping between the CH341A ISP programmer and the SOIC8 test clip for ATtiny chips

Here I cut the connection between the holes at location 6 with an X-acto knife and added the wire jumpers like in the diagram above. I used the legs from a spare resistor I had lying around. They are thin so they leave enough room for the through-hole pins to fit.

Add wire jumpers from a spare resistor
Add wire jumpers from a spare resistor

The finished jumper board looks like this. Please forgive that there is still flux on the terminal base.

Completed CH341A PCB pin board
Completed CH341A PCB pin board

When the PCB is trimmed and clean, it looks like this.

CH341A PCB board trimmed and clean
CH341A PCB board trimmed and clean

7) Test the PCB rig

It’s good to test the wiring one more time to check for shorts. All the top pins should map to the top pins of the ATtiny chip and match the function pins on the side of the CH341A.

Confirm the wiring and function pins of the PCB board
Confirm the wiring and function pins of the PCB board

8) Flash an ATtiny85 Digistump using the new PCB

In the moment of truth, everything works fine. The new PCB is doing its job well and the first ATtiny85 was flashed successfully.

Successful flashing of an ATtiny85 with the CH341A and custom PCB
Successful flashing of an ATtiny85 with the CH341A and custom PCB

9) Make a unique connector

I know me and one day I’ll put the ribbon connector in the wrong way and burnout a chip. I took a pair of pins and trimmed down the leads. Then I put epoxy on the top, let it harden, and inserted it into the empty contacts of the ribbon connector. They are removable, but now the connector has only one way to connect to the modified PCB board for safety.

Make a unique ribbon connector with pins and epoxy
Make a unique ribbon connector with pins and epoxy

Success: This weekend I was able to repurpose a BIOS programmer to flash an ATtiny85 on a Digistump board using the SOIC8 test clip and a modified PCB. I was also able to flash about three dozen ATtiny85 chips in the span of about five minutes thanks to this ISP programmer.

I hope this is useful to anyone else trying something similar. There is very little information on the web about customizing this programmer to work with Amtel chips.

Successful batch flashing of three dozen ATtiny85 chips
Successful batch flashing of three dozen ATtiny85 chips