I2C hängt, sendet manchmal nur 7 bit

External Slave Device Hanging the Bus by Holding SDA Low 

A problematic scenario can arise if the processor/I2C module gets reset 

while it is in the middle of mastering a transfer. In this scenario the 

external slave might be holding SDA low to transmit a 0 (or ACK). In this 

case it will not release SDA until it gets another falling edge on SCL. 

Even in this case it's not until it tries to transmit a '1' that it will 

actually release SDA after seeing SCL fall. The end result is that the bus 

will hang. If the I2C tries to initiate a new transfer it will hit 

an "arbitration lost" condition because SDA won't match the address it's 

sending. 

There are a couple ways to recover from this scenario. 

Option 1: For devices that mux the SCL/SDA pins with GPIO, the easiest 

thing is to configure the pins for GPIO operation and toggle SCL until the 

slave releases SDA. At this point you should be able to resume normal 

operation. 

Option 2: Many devices don't mux SCL/SDA with GPIO since the I2C I/O cells 

are often special open drain cells. A workaround has been reported to work 

even on these devices. By configuring the I2C for "free data format" and 

then reading a byte the I2C will immediately start sending clocks to input 

data (rather than trying to send an address). This can be used to free up 

the bus. 

For devices that actually boot from I2C you need to be careful about this 

scenario. Since the above workarounds rely on software they will not be 

possible if the device cannot boot! For this case you need to do something 

different. For example, if you can control the timing of the reset you 

could make sure you only reset the device when no I2C transaction is in 

progress. A different option would be to build your board such that the

processor reset power cycles the I2C EEPROM/flash (or simply resets it, 

but most EEPROMs/flashes don't have a reset pin).

  Aber Du hast doch die Probleme im laufenden Betrieb und Dein "Master"

  bekommt nicht wiederholt einen Reset?

  Option 2: Many devices don't mux SCL/SDA with GPIO since the I2C I/O 

  cells are often special open drain cells. A workaround has been reported

  to work even on these devices. By configuring the I2C for "free data 

  format" and then reading a byte the I2C will immediately start sending 

  clocks to input data (rather than trying to send an address). This can

  be used to free up the bus.