
RM0008
Inter-integrated circuit (I
2
C) interface
Doc ID 13902 Rev 12
745/1096
26.3.8
Packet error checking
A PEC calculator has been implemented to improve the reliability of communication. The
PEC is calculated by using the C(x) = x
8
+ x
2
+ x + 1 CRC-8 polynomial serially on each bit.
●
PEC calculation is enabled by setting the ENPEC bit in the I2C_CR1 register. PEC is a
CRC-8 calculated on all message bytes including addresses and R/W bits.
–
In transmission: set the PEC transfer bit in the I2C_CR1 register after the TxE
event corresponding to the last byte. The PEC will be transferred after the last
transmitted byte.
–
In reception: set the PEC bit in the I2C_CR1 register after the RxNE event
corresponding to the last byte so that the receiver sends a NACK if the next
received byte is not equal to the internally calculated PEC. In case of Master-
Receiver, a NACK must follow the PEC whatever the check result. The PEC must
be set before the ACK pulse of the current byte reception.
●
A PECERR error flag/interrupt is also available in the I2C_SR1 register.
●
If DMA and PEC calculation are both enabled:-
–
In transmission: when the I
2
C interface receives an EOT signal from the DMA
controller, it automatically sends a PEC after the last byte.
–
In reception: when the I
2
C interface receives an EOT_1 signal from the DMA
controller, it will automatically consider the next byte as a PEC and will check it. A
DMA request is generated after PEC reception.
●
To allow intermediate PEC transfers, a control bit is available in the I2C_CR2 register
(LAST bit) to determine if it is really the last DMA transfer or not. If it is the last DMA
request for a master receiver, a NACK is automatically sent after the last received byte.
●
PEC calculation is corrupted by an arbitration loss.
26.4 I
2
C interrupts
The table below gives the list of I
2
C interrupt requests.
Table 188.
I
2
C Interrupt requests
Interrupt event
Event flag
Enable control bit
Start bit sent (Master)
SB
ITEVFEN
Address sent (Master) or Address matched (Slave)
ADDR
10-bit header sent (Master)
ADD10
Stop received (Slave)
STOPF
Data byte transfer finished
BTF
Receive buffer not empty
RxNE
ITEVFEN and ITBUFEN
Transmit buffer empty
TxE