Atmel Xplained Evaluation Board AT32UC3A3-XPLD AT32UC3A3-XPLD 데이터 시트
제품 코드
AT32UC3A3-XPLD
488
32072H–AVR32–10/2012
AT32UC3A3
23.8.9.1
Packet Error Checking
Each SMBus transfer can optionally end with a CRC byte, called the PEC byte. Writing a one to
CMDR.PECEN enables automatic PEC handling in the current transfer. Transfers with and with-
out PEC can freely be intermixed in the same system, since some slaves may not support PEC.
The PEC LFSR is always updated on every bit transmitted or received, so that PEC handling on
combined transfers will be correct.
CMDR.PECEN enables automatic PEC handling in the current transfer. Transfers with and with-
out PEC can freely be intermixed in the same system, since some slaves may not support PEC.
The PEC LFSR is always updated on every bit transmitted or received, so that PEC handling on
combined transfers will be correct.
In master transmitter mode, the master calculates a PEC value and transmits it to the slave after
all data bytes have been transmitted. Upon reception of this PEC byte, the slave will compare it
to the PEC value it has computed itself. If the values match, the data was received correctly, and
the slave will return an ACK to the master. If the PEC values differ, data was corrupted, and the
slave will return a NACK value. The DNAK bit in SR reflects the state of the last received
ACK/NACK value. Some slaves may not be able to check the received PEC in time to return a
NACK if an error occurred. In this case, the slave should always return an ACK after the PEC
byte, and some other mechanism must be implemented to verify that the transmission was
received correctly.
all data bytes have been transmitted. Upon reception of this PEC byte, the slave will compare it
to the PEC value it has computed itself. If the values match, the data was received correctly, and
the slave will return an ACK to the master. If the PEC values differ, data was corrupted, and the
slave will return a NACK value. The DNAK bit in SR reflects the state of the last received
ACK/NACK value. Some slaves may not be able to check the received PEC in time to return a
NACK if an error occurred. In this case, the slave should always return an ACK after the PEC
byte, and some other mechanism must be implemented to verify that the transmission was
received correctly.
In master receiver mode, the slave calculates a PEC value and transmits it to the master after all
data bytes have been transmitted. Upon reception of this PEC byte, the master will compare it to
the PEC value it has computed itself. If the values match, the data was received correctly. If the
PEC values differ, data was corrupted, and SR.PECERR is set. In master receiver mode, the
PEC byte is always followed by a NACK transmitted by the master, since it is the last byte in the
transfer.
data bytes have been transmitted. Upon reception of this PEC byte, the master will compare it to
the PEC value it has computed itself. If the values match, the data was received correctly. If the
PEC values differ, data was corrupted, and SR.PECERR is set. In master receiver mode, the
PEC byte is always followed by a NACK transmitted by the master, since it is the last byte in the
transfer.
The PEC byte is automatically inserted in a master transmitter transmission if PEC is enabled
when NBYTES reaches zero. The PEC byte is identified in a master receiver transmission if
PEC is enabled when NBYTES reaches zero. NBYTES must therefore be written with the total
number of data bytes in the transmission, including the PEC byte.
when NBYTES reaches zero. The PEC byte is identified in a master receiver transmission if
PEC is enabled when NBYTES reaches zero. NBYTES must therefore be written with the total
number of data bytes in the transmission, including the PEC byte.
In combined transfers, the PECEN bit should only be written to one in the last of the combined
transfers. Consider the following transfer:
transfers. Consider the following transfer:
S, ADR+W, COMMAND_BYTE, ACK, SR, ADR+R, DATA_BYTE, ACK, PEC_BYTE, NACK, P
This transfer is generated by writing two commands to the command registers. The first com-
mand is a write with NBYTES=1 and PECEN=0, and the second is a read with NBYTES=2 and
PECEN=1.
mand is a write with NBYTES=1 and PECEN=0, and the second is a read with NBYTES=2 and
PECEN=1.
Writing a one to the STOP bit in CR will place a STOP condition on the bus after the current
byte. No PEC byte will be sent in this case.
byte. No PEC byte will be sent in this case.
23.8.9.2
Timeouts
The TLOWS and TLOWM fields in SMBTR configure the SMBus timeout values. If a timeout
occurs, the master will transmit a STOP condition and leave the bus. The SR.TOUT bit is set.
occurs, the master will transmit a STOP condition and leave the bus. The SR.TOUT bit is set.
23.8.9.3
SMBus ALERT Signal
A slave can get the master’s attention by pulling the TWALM line low. The TWIM will then set the
SR.SMBALERT bit. This can be set up to trigger an interrupt, and software can then take the
appropriate action, as defined in the SMBus standard.
SR.SMBALERT bit. This can be set up to trigger an interrupt, and software can then take the
appropriate action, as defined in the SMBus standard.