I2C bus recovery
I wonder how useful would it be to add a bus recovery mechanism to the software I2C implementation. It would be something similar to what is discussed here: https://github.com/esp8266/Arduino/issues/1025
The particular use case is when either the slave or the master get reset in the middle of a transmission, and leave the bus in inconsistent state, usually with the other party pulling one of the lines down and preventing communication. This can easily happen when we are doing aggressive power management, entering deep sleep or cutting power to the sensors.
The proposed solution is to strobe the clock line several times and send a NACK, to make sure all the devices on the bus get to finish the last byte they were sending.
extmod/machine_i2c: Add clock stretching support
When the clock is too fast for the i2c slave, it can temporarily hold
down the scl line to signal to the master that it needs to wait. The
master should check the scl line when it is releasing it after
transmitting data, and wait for it to be released.
This change has been tested with a logic analyzer and an i2c slace
implemented on an atmega328p using its twi peripheral, clocked at 8Mhz.
Without the change, the i2c communication works up to aboy 150kHz
frequency, and above that results in the slave stuck in an unresponsive
state. With this change, communication has been tested to work up to
400kHz.
This fixes #1948