This test implements a basic four I2C command set to control an ATtiny85 MCU from an ESP8266 through the serial console. In those commands where there are data replies, there is a plain CRC-8 check implemented.
User (serial console) --> ESP8266 --> Attiny85
a - (STDPB1_1) Set ATtiny85 PB1 = 1
s - (STDPB1_0) Set ATtiny85 PB1 = 0
d - (STANAPB3) Set ATtiny85 PB3 = PWMx (the command asks for a PWM value input)
f - (READADC2) Read ATtiny85 ADC2 (the reply has 2 data bytes + 1 CRC byte)