AVR SPI Slave

Copyright (c) 2019 Warren Taylor.

Disclaimer

The following documentation in no way guarantees a secure system.

Overview

The original plan was to connect peripherals directly to the ESP8266 data pins but I ran out of pins very quickly. Since I had an unused ATmega168 lying around I though "what the heck" and connected the two chips via SPI with the ESP8266 as the master and the ATmega as the slave. In additions to providing a larger number of data pins this scheme also provided 5v tolerance.

This AVR SPI Slave simply receives a byte value from the controlling ESP8266 and then sets the output of Port D to this by value. In other words the ESP8266 is setting the eight Port D output pins via SPI. That's all.

However, a huge advantage for the future is that with a bit more programming in the ESP8266 the AVR can be programmed Over-the-Air, which means it can be made to do anything.

This in not an Arduino Sketch. This is very simple native AVR firmware. Why? Because I didn't know how easy native AVR programming was until I tried it.

The source for this project can be found at https://github.com/tsi-software/Secure_ESP8266_MQTT_poc

Requirements

• ATmega168 (or similar)
• avr-gcc
• AVR ISP (In-System Programming) device

Wiring up the AVR

I’m going to defer to two other web sites that do an excellent job of describing how to wire-up and program the AVR.

• http://ediy.com.my/index.php/projects/item/86-minimal-arduino-with-8mhz-internal-clock
• http://www.ladyada.net/learn/avr/index.html

Programming the AVR

Connect you development computer to the ATmega168 as described above. Then, from this "avr_spi_slave" directory, run:

make program_fuses
make program

Note: "make program_fuses" only needs to be run once, not each time the source file changes.

AVR References

• http://ediy.com.my/index.php/projects/item/86-minimal-arduino-with-8mhz-internal-clock
• http://www.ladyada.net/learn/avr/index.html
• https://en.wikipedia.org/wiki/AVR_microcontrollers
• https://www.arduino.cc/en/Tutorial/ArduinoISP
• https://www.arduino.cc/en/Main.ArduinoISP
• http://maxembedded.com/2015/06/setting-up-avr-gcc-toolchain-on-linux-and-mac-os-x/
• https://www.microchip.com/mplab/avr-support/avr-and-arm-toolchains-c-compilers
• https://gcc.gnu.org/wiki/avr-gcc
• https://www.nongnu.org/avr-libc/
• http://www.engbedded.com/fusecalc/