After building the new IO board, I started to realize that my idea to have the remote control in modular sections needed a redesign. Each module needed at least 2 17-pin connectors to provide communication from the previous module to the next and that was taking up valuable space on the small perfboard and that required components to be squashed together ( ultimately exposing my lack of fine soldering skills ).
I was looking at controlling multiple OLED screens and found a technique that used a multiplexer to allow the controller to talk to OLEDs with the same address. The multiplexer switches the I2C data line between each screen.
Using I2C got me thinking about how I could redesign the remote control modules with just 4 pins for the bus ( 2 for power and 2 for data ).
To communicate with each module, I needed a smaller microcontroller that would respond to I2C requests from the main MCU ( the ATmega328 ) and send data back. This meant that I could have the module MCU talk to a shift register to determine button presses or to light LEDs.
I chose the ATTiny85 because it was much smaller and didn’t require a separate oscillator for timing and also didn’t need associated smoothing capacitors for power. However, with fewer pins comes less flexibility. The shift register technique needs 3 pins for control ( data, shift and latch ) but I also needed an input pin for the button module. The ATTiny85 doesn’t have enough pins available to do that. By happenstance, I found that I had a solitary Johnson counter that has 10 pins for output and a one pin for clock. Each time the clock pin is taken high, the output pin is incremented so, for example, if the output pin is pin 1, a clock pulse moves it to pin 2 and so on from 0 to 9. When it reaches 9, it rolls back to 0 again. Additionally, there is a pin to reset the counter back to 0. This is important if you want to poll buttons sequentially. The reset allows you to start from a known state.
So, the module uses 2 pins ( clock and reset ) plus the data input line instead of 3 + input. That makes the Johnson counter perfect for the button input module.
I needed to make sure that the ATTiny85 could talk over the I2C bus to the main MCU.
With that testing done, I could then solder up a new Nerduino and button module. I was also able to test that the ATTiny85 could light an LED module. Given that I don’t care about any input on the LED module, I can go back to using a shift register for that module and still only use 3 pins on the module MCU.