You could be at the limit of what is possible, and both I2C and SPI could be too much for the ATmega chip. If you scroll down here: then you see that a SPI Slave is just an interrupt that handles one byte. It would be nice if you can switch to other boards, however, some boards are bad I2C Slaves. I plan to assemble a small prototype with the usb capable chip and one satellite - I actually had one together years ago but dropped the project due to cost - but I’m not sure what will come up when I expand two 8. So, I’m looking for help assessing pro/con on these two protocols in this role. If I set them up as I2C slaves now they obviously can’t communicate with another peripheral as the bus master. For this small amount of data the extra speed is perhaps a luxury but I’d also like to keep the I2C bus on each satellite micro open for future expansion.
I know SPI is faster, and maybe less memory intensive (?), but my search results indicate that 328s aren’t great as SPI slaves… something about read speed and not having an interrupt that is fast enough to catch the first few bytes. I’ll be passing, say, 25 bytes back and forth pretty much constantly. I have enough pins for either protocol and everything will be tightly packed in one box. I’m using the 328s essentially as pin expanders but I’ll be making use of both interrupt pins, analog reading, some PID control outputting high frequency PWM… more than than just a few extra gpio… 8 identical setups talking to a 9th device. I have a project where one micro controller with usbMIDI functionality will be passing info back and forth with 8 other micros (in this case standalone 328s with arduino bootloader).
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