Requirement

• low power : 3V3 or lower, not 5V powered
• at least 7 GPIOs, 9 preferred.  We don't want to read the status of the switches with an ADC.
• as small as possible
• cheap
• easy to solder
• Availability : 2nd sourced
• PlatformIO / Arduino support

The Arduino support seriously limits the choices.  8MHz on a 1V8 AVR-architecture is out of spec.  Higher supply voltages are needed, which imply the addition of a boost converter when powering with two AA-batteries.

Using AVRISPmkII on Linux requires a rules-file for udev:

SUBSYSTEM!="usb_device", ACTION!="add", GOTO="avrisp_end"

# Atmel Corp.JTAG ICE mkII
ATTRS{idVendor}==”03eb”, ATTRS{idProduct}==”2103″, MODE:=”660″, GROUP:=”dialout”

# Atmel Corp. AVRISP mkII
ATTRS{idVendor}==”03eb”, ATTRS{idProduct}==”2104″, MODE:=”660″, GROUP:=”dialout”

# Atmel Corp. Dragon
ATTRS{idVendor}==”03eb”, ATTRS{idProduct}==”2107″, MODE:=”660″, GROUP:=”dialout”

LABEL=”avrisp_end”

Save this file as /etc/udev/rules.d/60-avrisp.rules

Choices

Bare bone MCU

The smallest/cheapest MCU that PlatformIO supports is the ATtiny24A/44A/84A family.  They can be had for €0.80 to €0.90 a piece.  SOIC-14 will be the preferable package.

This can be used with PlatformIO.  See here for a demo-setup.

The ATtiny441 and ATtiny841 are pin-compatible parts with some extra features such as two USARTS (HardwareSerial ports).

Arduino Pro Mini 328 - 3.3V/8MHz

• Newer versions have four extra GPIOs : pin A4, A5, A6 & A7.  These are located two by two on the right hand side of the MCU.  Older versions have only two extra GPIOs.
• Requires an FTDI serial USB converter for programming.
  • Warning! : When using a TTL-3V3-WE cable, the signal levels are 3V3, but the VCC level is 5V! The problem is that this FTDI VCC pin is directly connected to the 3V3 VCC pin on the Arduino.
  • Either disconnect FTDI VCC or connect it to the RAW pin of the Arduino Pro Mini.
  • 5V on VCC is no problem for the Arduino Pro Mini, but it might be disastrous to your connected 3V3 peripherals.
• 33 x 18mm
• fits in a DIP (2x12 pins, 0.6" row spacing) socket.  Height can be reduced by inserting the headers from top side and solder them from bottom side.
• Sources for DIP socket :
  • ED24DT
  • A 24-LC-TT
  • A 24-LC-TR
• Where to buy : AliExpress 32341782884
• Getting started : Sparkfun : Using the Arduino Pro Mini 3.3V
• Burning the bootloader.  There's a bootloader present upon delivery.  In case you removed it and want to program it again:
  
  • How to burn the bootloader in an Arduino Nano using an Arduino UNO
  • Burn a New Bootloader - Arduino Pro Mini