ST92F124 datasheet(329/429 Pages) STMICROELECTRONICS

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CONTROLLER AREA NETWORK (bxCAN)

CONTROLLER AREA NETWORK (Cont’d)

10.10.4.7 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 CDGR register. This mode can be used for a

“Hot Selftest”, meaning the bxCAN can be tested

like in Loop Back mode but without affecting a run-

ning CAN system connected to the CANTX and

CANRX pins. In this mode, the CANRX pin is dis-

connected from the bxCAN and the CANTX pin is

held recessive.

Figure 146. bxCAN in Combined Mode

10.10.5 Functional Description

10.10.5.1 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 MCSR register.

Once the mailbox has left empty state, the soft-

ware 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 transmis-

sion. The transmission of the message of the

scheduled mailbox will start (enter transmit state)

when the CAN bus becomes idle. Once the mail-

box has been successfully transmitted, it will be-

come empty again. The hardware indicates a suc-

cessful transmission by setting the RQCP and

TXOK bits in the MCSR and CTSR registers.

If the transmission fails, the cause is indicated by

the ALST bit in the MCSR register in case of an Ar-

bitration 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 pri-

ority according to the arbitration of the CAN proto-

col. If the identifier values are equal, the lower

mailbox number will be scheduled first.

By Transmit Request Order:

The transmit mailboxes can be configured as a

transmit FIFO by setting the TXFP bit in the CMCR

the transmit request order.

This mode is very useful for segmented transmis-

sion.

Abort

A transmission request can be aborted by the user

setting the ABRQ bit in the MCSR register. In

pending or scheduled state, the mailbox is abort-

ed immediately. An abort request while the mail-

box is in transmit state can have two results. If the

mailbox is transmitted successfully the mailbox

becomes empty with the TXOK bit set in the

MCSR and CTSR registers. If the transmission

fails, the mailbox becomes scheduled, the trans-

mission is aborted and becomes empty with

TXOK cleared. In all cases the mailbox will be-

come empty again at least at the end of the cur-

rent transmission.

Non-Automatic Retransmission Mode

This mode has been implemented in order to fulfil

the requirement of the Time Triggered Communi-

cation option of the CAN standard. To configure

the hardware in this mode the NART bit in the

CMCR register must be set.

In this mode, each transmission is started only

once. If the first attempt fails, due to an arbitration

loss or an error, the hardware will not automatical-

ly restart the message transmission.

At the end of the first transmission attempt, the

hardware considers the request as completed and

sets the RQCP bit in the MCSR register. The result

of the transmission is indicated in the MCSR regis-

ter by the TXOK, ALST and TERR bits.

bxCAN

CANTX CANRX

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