STM8S datasheet(384/467 Pages) STMICROELECTRONICS

384 / 467 page

Controller area network (beCAN)

RM0016

384/467

DocID14587 Rev 14

Note:

As the Tx line is still active in this mode, be aware that it can disturb the communication on

the CAN bus.

23.5.3

Loop back combined with silent mode

It is also possible to combine Loop Back mode and Silent mode by setting the LBKM and

SILM bits in the CAN_DGR register. This mode can be used for a “Hot Selftest”, meaning

the beCAN can be tested like in Loop Back mode but without affecting a running CAN

system connected to the CANTX and CANRX pins. In this mode, the CANRX pin is

disconnected from the beCAN and the CANTX pin is held recessive.

Figure 145. beCAN in combined mode

23.6

Functional description

23.6.1

Transmission handling

In order to transmit a message, the application must select one empty transmit mailbox, set

up the identifier, the data length code (DLC) and the data before requesting the transmission

by setting the corresponding TXRQ bit in the CAN_MCSR register. Once the mailbox has

exited empty state, the software no longer has write access to the mailbox registers.

Immediately after the TXRQ bit has been set, the mailbox enters pending state and waits to

become the highest priority mailbox, see Transmit Priority. As soon as the mailbox has the

highest priority it will be scheduled for transmission. The transmission of the message of

the scheduled mailbox will start (enter transmit state) when the CAN bus becomes idle.

Once the mailbox has been successfully transmitted, it will become empty again. The

hardware indicates a successful transmission by setting the RQCP and TXOK bits in the

CAN_MCSR and CAN_TSR registers.

If the transmission fails, the cause is indicated by the ALST bit in the CAN_MCSR register in

case of an Arbitration Lost, and/or the TERR bit, in case of transmission error detection.

Transmit priority

By identifier:

When more than one transmit mailbox is pending, the transmission order is given by the

identifier of the message stored in the mailbox. The message with the lowest identifier value

has the highest priority according to the arbitration of the CAN protocol. If the identifier

values are equal, the lower mailbox number will be scheduled first.

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