#528 ATtiny IxpandO

Using an ATtiny85 to drive the Boldport IxpandO MCP23017-based I²C IO expansion board.

Build

Notes

I like Arduino - mainly because it is always surprising to find how much can be done with such a constrained microprocessor. But sometimes even an ATmega328 is overkill, in which case it is fun to try and squeeze a project into an ATtiny instead.

One of the challenges with the ATtiny is that the Arduino Wire library is not supported, so there’s no direct support for I²C.

A leading alternative is the Adafruit TinyWireM I²C for the ATtiny, so I wanted to test it out. The library already comes with examples for how to use it with temperature sensors, LCDs and RTCs, so I though I’d try something different - the MCP23017 port expander, which could be a useful tool in cases where the ATtiny has all the processing power and memory required, but it just needs more I/O.

To test this, I’m using the Boldport IxpandO - an input/output expansion board based on Microchip’s MCP23017 (BoldportClub Project #16).

Code Modifications

I’m using the ATtinyController.ino sketch for the demo, which is a modified version of the demo sketch I used with an Arduino when I first built the Boldport IxpandO.

Key modifications:

a different library name, although I’ve obfusticated this with a #define Wire TinyWireM
the extended form of requestFrom is not supported, so readRegister needed to be re-written with more fundamental commands.
the hardware I²C SDA pin is shared with INT0, so I couldn’t directly use the MCP23017 interrupt. While I think I could switch to pin change interrupts, for now I’ve just replaced the interrupt-driven input with polling

Construction

It’s pretty simple! On the ATtiny85 PDIP/SOIC/TSSOP package, hardware I²C is enabled on pins 5 and 7:

Pin	Function	Other functions
5	SDA	USCK/SCL/ADC1/T0/INT0/PCINT2
7	SCK	MOSI/DI/AIN0/OC0A/OC1A/AREF/PCINT0

I programmed the ATtiny85 with my DiY programmer shield for an Arduino Uno running the Arduino ISP sketch. Because of the use of pins 5 and 7 during programming, it means the ATtiny can’t run the sketch successfully while still on the programming board.

Breadboard

Schematic

Testing things out on a breadboard:

ATtinyController_bb_build

Building an Adapter Board

To clean things up a bit, I made a small mounting board for the ATtiny in a DIP8 adapter, with a micro USB adapter for 5V supply. The 8-pin header allows the IxpandO to be connected with press fit.

adapter_board