SAM4L Datasheet, PDF (747/1185 Pages) ATMEL Corporation – ATSAM ARM-based Flash MCU

ATSAM4L4/L2
1. If SMBus mode and PEC is used, NBYTES must be set up with the number of bytes to
receive. This is necessary in order to know which of the received bytes is the PEC byte.
NBYTES can also be used to count the number of bytes received if using DMA.
2. Receive a byte. Set SR.BTF when done.
3. Update NBYTES. If CR.CUP is written to one, NBYTES is incremented, otherwise
NBYTES is decremented. NBYTES is usually configured to count downwards if PEC is
used.
4. After a data byte has been received, the slave transmits an ACK or NAK bit. For ordi-
nary data bytes, the CR.ACK field controls if an ACK or NAK should be returned. If PEC
is enabled and the last byte received was a PEC byte (indicated by NBYTES equal to
zero), The TWIS will automatically return an ACK if the PEC value was correct, other-
wise a NAK will be returned.
5. If STOP is received, SR.TCOMP will be set.
6. If REPEATED START is received, SR.REP will be set.
The TWI transfers require the receiver to acknowledge each received data byte. During the
acknowledge clock pulse (9th pulse), the master releases the data line (HIGH), enabling the
slave to pull it down in order to generate the acknowledge. The master polls the data line during
this clock pulse.
The SR.RXRDY bit indicates that a data byte is available in the RHR. The RXRDY bit is also
used as Receive Ready for the Peripheral DMA Controller receive channel.
Figure 28-10. Slave Receiver with One Data Byte
TWD
S
DADR
WA
DATA
AP
TCOMP
RXRDY
Read RHR
Figure 28-11. Slave Receiver with Multiple Data Bytes
TWD S
DADR
WA
DATA n
A DATA (n+1) A DAT A (n+m)-1 A DATA (n+m) A P
TCOMP
RXRDY
Read RHR
DATA n
Read RHR
DATA (n+1)
Read RHR
DAT A (n+m)-1
Read RHR
DATA (n+m)
28.8.6
Interactive ACKing Received Data Bytes
When implementing a register interface over TWI, it may sometimes be necessary or just useful
to report reads and writes to invalid register addresses by sending a NAK to the host. To be able
to do this, one must first receive the register address from the TWI bus, and then tell the TWIS
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