CONTROLLER AREA NETWORK (bxCAN)
CONTROLLER AREA NETWORK (Cont’d)
10.10.4.7 Loop Back combined with Silent
Mode
bitration Lost, and/or the TERR bit, in case of
transmission error detection.
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.
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.
Figure 146. bxCAN in Combined Mode
By Transmit Request Order:
The transmit mailboxes can be configured as a
transmit FIFO by setting the TXFP bit in the CMCR
register. In this mode the priority order is given by
the transmit request order.
bxCAN
Tx
Rx
This mode is very useful for segmented transmis-
sion.
Abort
=1
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.
CANTX CANRX
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.
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.
If the transmission fails, the cause is indicated by
the ALST bit in the MCSR register in case of an Ar-
328/426
9
STMICROELECTRONICS [ ST ]
