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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
345 of 808
NXP Semiconductors
UM10360
Chapter 16: LPC17xx CAN1/2
8.
CAN controller operation
8.1 Error handling
The CAN Controllers count and handle transmit and receive errors as specified in CAN
Spec 2.0B. The Transmit and Receive Error Counters are incriminated for each detected
error and are decremented when operation is error-free. If the Transmit Error counter
contains 255 and another error occurs, the CAN Controller is forced into a state called
Bus-Off. In this state, the following register bits are set: BS in CANxSR, BEI and EI in
CANxIR if these are enabled, and RM in CANxMOD. RM resets and disables much of the
CAN Controller. Also at this time the Transmit Error Counter is set to 127 and the Receive
Error Counter is cleared. Software must next clear the RM bit. Thereafter the Transmit
Error Counter will count down 128 occurrences of the Bus Free condition (11 consecutive
recessive bits). Software can monitor this countdown by reading the Tx Error Counter.
When this countdown is complete, the CAN Controller clears BS and ES in CANxSR, and
sets EI in CANxSR if EIE in IER is 1.
The Tx and Rx error counters can be written if RM in CANxMOD is 1. Writing 255 to the
Tx Error Counter forces the CAN Controller to Bus-Off state. If Bus-Off (BS in CANxSR) is
1, writing any value 0 through 254 to the Tx Error Counter clears Bus-Off. When software
clears RM in CANxMOD thereafter, only one Bus Free condition (11 consecutive
recessive bits) is needed before operation resumes.
8.2 Sleep mode
The CAN Controller will enter sleep mode if the SM bit in the CAN Mode register is 1, no
CAN interrupt is pending, and there is no activity on the CAN bus. Software can only set
SM when RM in the CAN Mode register is 0; it can also set the WUIE bit in the CAN
Interrupt Enable register to enable an interrupt on any wake-up condition.
The CAN Controller wakes up (and sets WUI in the CAN Interrupt register if WUIE in the
CAN Interrupt Enable register is 1) in response to a) a dominant bit on the CAN bus, or b)
software clearing SM in the CAN Mode register. A sleeping CAN Controller that wakes up
Table 316. CAN Wake-up Flags register (CANWAKEFLAGS - address 0x400F C114) bit
description
Bit
Symbol
Function
Reset
Value
0
-
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
NA
1
CAN1WAKE
Wake-up status for CAN channel 1
.
Read: when 1, indicates that a falling edge has occurred on the
receive data line of CAN channel 1.
Write: writing a 1 clears this bit.
0
2
CAN2WAKE
Wake-up status for CAN channel 2
.
Read: when 1, indicates that a falling edge has occurred on the
receive data line of CAN channel 2.
Write: writing a 1 clears this bit.
0
31:3 -
Reserved, user software should not write ones to reserved bits.
The value read from a reserved bit is not defined.
NA