we've had some troubles with TWI/I2C after waking up from sleep with the Atmel Xmega256A3. Instead of digging into the details of TWI/I2C we've decided to use the supplied twi_master_driver from Atmel attached to AVR1308 application note.
The problem is one or a few failed TWI transactions just after waking up from sleep. On the I2C-bus connected to the XMega we have a few potentiometers, a thermometer and an RTC. The XMega acts as the only master on the bus.
We use the sleep functions found in AVRLIBC:
{code for turning of VCC to all I2C connected devices}
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_enable();
sleep_cpu();
{code for turning on VCC to all I2C connected devices}
The XMega as woken from sleep by the RTC which sets a pin high. After the XMega is woken from sleep, we want to set a value on one of the potentiometers, but this fails. For some reason, the TWI-transaction result is TWIM_RESULT_NACK_RECEIVED instead of TWIM_RESULT_OK in the first transaction. After that everything seems to work again.
Have we missed anything here? Is there any known issues with the XMega, sleep and TWI? Do we need to reset the TWI of clear any flags after waking from sleep?
Best regards Fredrik
There is a common problem on I2C/TWI where the internal state machine gets stuck in an intermediate state if a transaction is not completed fully. The slave then does not respond correctly when addressed on the next transaction. This commonly happens when the master is reset or stops outputting the SCK signal part way through the read or write. A solution is to toggle the SCK line manually 8 or 9 times before starting any data transactions so the that the internal state machines in the slaves are all reset to the start of transfer point and they are all then looking for their address byte.