RM0319 datasheet(198/368 Pages) STMICROELECTRONICS

198 / 368 page

Controller area network ports (CAN)

RM0319

198/368

Doc ID 022640 Rev 3

several transmit messages are assigned to the same Message object (when the number of

message object is not sufficient), the whole message object has to be configured before the

transmission of this message is requested.

The transmission of any number of message objects may be requested at the same time,

they are transmitted subsequently according to their internal priority. Messages may be

updated or set to not valid any time, even when their requested transmission is still pending.

The old data will be discarded when a message is updated before its pending transmission

has started.

Depending on the configuration of the message object, the transmission of a message may

be requested autonomously by the reception of a remote frame with a matching identifier.

15.4.3

Using the disabled automatic retransmission mode

According to the CAN Specification (see ISO11898, Recovery Management section), the

CAN controller provides means for automatic retransmission of frames that have lost

artbitration or that have been disturbed by errors during transmission. The frame

transmission service will not be confirmed to the user before the transmission is

successfully completed. By default, the means for automatic retransmission is enabled. It

can be disabled in order to allow CAN to work within a time triggered CAN environment

(TTCAN, see ISO11898-1).

The disabled automatic retransmission mode is enabled by programming the DAR bit in the

CAN Control register to one. In this operation mode, the programmer has to consider the

different behaviour of TxRqst and NewDat bits in the control register of the message buffers:

●

When a transmission starts, the TxRqst bit of the respective message buffer is reset,

while the NewDat bit remains set.

●

When the transmission completed successfully, the NewDat bit is reset.

When a transmission has failed (lost arbitration or error), the NewDat bit remains set. To

restart the transmission, the CPU has to set TxRqst back to one.

15.4.4

Using the test mode

To enter the test mode, the Test bit of the CAN Control register should be set 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 CAN_RX pin and therefore is only readable. All test register functions

are disabled when the Test bit is reset to zero.

15.4.5

Using the silent mode

The CAN core can be set in silent mode by programming the Silent bit of Test register to

one. In silent mode, the 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 the dominant bit, although the CAN bus

may remain in recessive state. The silent mode can be used to analyse the traffic on a CAN

bus without affecting it by the transmission of dominant bits (Acknowledge Bits, Error

Frames). The following figure shows the connection of signals CAN_TX and CAN_RX to the

CAN core in silent mode.

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