CAN modules UM0404
420/564 DocID13284 Rev 2
The Disabled Automatic Retransmission mode is enabled by programming bit DAR in the
CAN Control Register to one. In this operation mode the programmer has to consider the
different behavior of bits TxRqst and NewDat in the Control Registers of the Message
Buffers:
• When a transmission starts bit TxRqst of the respective Message Buffer is reset, while
bit NewDat remains set.
• When the transmission completed successfully bit NewDat is reset.
When a transmission failed (lost arbitration or error) bit NewDat remains set. To restart the
transmission the CPU has to set TxRqst back to one.
21.7.4 Test mode
The Test Mode is entered by setting bit Test in the CAN Control Register to one. In Test
Mode the bits Tx1, Tx0, LBack, Silent and Basic in the Test Register are writable. Bit Rx
monitors the state of pin CAN_RxD and therefore is only readable. All Test Register
functions are disabled when bit Test is reset to zero.
21.7.5 Silent mode
The CAN Core can be set in Silent Mode by programming the Test Register bit Silent to
one.
In Silent Mode, the C-CAN is able to receive valid data frames and valid remote frames, but
it sends only recessive bits on the CAN bus and it cannot start a transmission. If the CAN
Core is required to send a dominant bit (ACK bit, overload flag, active error flag), the bit is
rerouted internally so that the CAN Core monitors this dominant bit, although the CAN bus
may remain in recessive state. The Silent Mode can be used to analyze the traffic on a CAN
bus without affecting it by the transmission of dominant bits (Acknowledge Bits, Error
Frames). Figure 177 shows the connection of signals CAN_TxD and CAN_RxD to the CAN
Core in Silent Mode.
Figure 177. CAN core in silent mode
In ISO 11898-1, the Silent Mode is called the Bus Monitoring Mode.
21.7.6 Loop back mode
The CAN Core can be set in Loop Back Mode by programming the Test Register bit LBack
to one. In Loop Back Mode, the CAN Core treats its own transmitted messages as received
CAN_TxD CAN_RxD
Tx Rx
CAN Core
C-CAN
•
•
=1
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