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M0A21/M0A23 Series
May 06, 2022
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Rev 1.02
M0
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SE
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H
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ANUAL
Disabled Automatic Retransmission
In accordance with the CAN Specification (see ISO11898, 6.3.3 Recovery Management), the C_CAN
provides means for automatic retransmission of frames that have lost arbitration or have been disturbed
by errors during transmission. The frame transmission service will not be confirmed to the user before
the transmission is successfully completed. This means that, by default, automatic retransmission is
enabled. It can be disabled to enable the C_CAN to work within a Time Triggered CAN (TTCAN, see
ISO11898-1) environment.
The Disabled Automatic Retransmission mode is enabled by setting the Disable Automatic
Retransmission (DAR bit (CAN_CON[5])) to one. In this operation mode, the programmer has to
consider the different behavior of bits TxRqst (CAN_IFn_MCON[8]) and NewDat (CAN_IFn_MCON[15])
of the Message Buffers:
When a transmission starts, bit TxRqst of the respective Message Buffer is cleared, while
bit NewDat remains set.
When the transmission completed successfully, bit NewDat is cleared.
When a transmission fails (lost arbitration or error), bit NewDat remains set.
To restart the transmission, the software should set the bit TxRqst again.
6.16.6 Test Mode
Test Mode is entered by setting the Test bit (CAN_CON[7]). In Test Mode, bits Tx1 (CAN_TEST[6]),
Tx0 (CAN_TEST[5]), LBack (CAN_TEST[4]), Silent (CAN_TEST[3]) and Basic (CAN_TEST[2]) are
writeable. Bit Rx (CAN_TEST[7]) monitors the state of the CAN_RX pin and therefore is only readable.
All Test Register functions are disabled when the Test bit is cleared.
Silent Mode
The CAN Core can be set in Silent Mode by programming the Silent bit (CAN_TEST[3]) 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, Error Frames), 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. Figure 6.16-2
shows the connection of signals CAN_TX and CAN_RX to the CAN Core in Silent Mode.
C_CAN
= 1
CAN_TX
CAN_RX
CAN_Core
Tx
Rx
Figure 6.16-2 CAN Core in Silent Mode
Loop Back Mode
The CAN Core can be set in Loop Back Mode by programming the Test Register bit LBack
(CAN_TEST[4]) to one. In Loop Back Mode, the CAN Core treats its own transmitted messages as
received messages and stores them in a Receive Buffer (if they pass acceptance filtering). Figure 6.16-3
shows the connection of signals, CAN_TX and CAN_RX, to the CAN Core in Loop Back Mode.