PIC18F2480/2580/4480/4580
Setting the TXREQ bit does not initiate a message
transmission; it merely flags a message buffer as ready
for transmission. Transmission will start when the
device detects that the bus is available. The device will
then begin transmission of the highest priority message
that is ready.
24.5.4
PROGRAMMABLE AUTO-RTR
BUFFERS
In Mode 1 and 2, any of six programmable transmit/
receive buffers may be programmed to automatically
respond to predefined RTR messages without user
firmware intervention. Automatic RTR handling is
enabled by setting the TXnEN bit in the BSEL0 register
and the RTREN bit in the BnCON register. After this
setup, when an RTR request is received, the TXREQ
bit is automatically set and the current buffer content is
automatically queued for transmission as a RTR
response. As with all transmit buffers, once the TXREQ
bit is set, buffer registers become read-only and any
writes to them will be ignored.
When the transmission has completed successfully, the
TXREQ bit will be cleared, the TXBnIF bit will be set and
an interrupt will be generated if the TXBnIE bit is set.
If the message transmission fails, the TXREQ will remain
set, indicating that the message is still pending for trans-
mission and one of the following condition flags will be
set. If the message started to transmit but encountered
an error condition, the TXERR and the IRXIF bits will be
set and an interrupt will be generated. If the message lost
arbitration, the TXLARB bit will be set.
The following outlines the steps required to
automatically handle RTR messages:
1. Set buffer to Transmit mode by setting the
24.6.2
ABORTING TRANSMISSION
TXnEN bit to ‘1’ in BSEL0 register.
The MCU can request to abort a message by clearing
the TXREQ bit associated with the corresponding mes-
sage buffer (TXBnCON<3> or BnCON<3>). Setting the
ABAT bit (CANCON<4>) will request an abort of all
pending messages. If the message has not yet started
transmission, or if the message started but is inter-
rupted by loss of arbitration or an error, the abort will be
processed. The abort is indicated when the module
sets the TXABT bit for the corresponding buffer
(TXBnCON<6> or BnCON<6>). If the message has
started to transmit, it will attempt to transmit the current
message fully. If the current message is transmitted
fully and is not lost to arbitration or an error, the TXABT
bit will not be set because the message was transmit-
ted successfully. Likewise, if a message is being trans-
mitted during an abort request and the message is lost
to arbitration or an error, the message will not be
retransmitted and the TXABT bit will be set, indicating
that the message was successfully aborted.
2. At least one acceptance filter must be associ-
ated with this buffer and preloaded with the
expected RTR identifier.
3. Bit, RTREN in the BnCON register, must be set
to ‘1’.
4. Buffer must be preloaded with the data to be
sent as a RTR response.
Normally, user firmware will keep buffer data registers
up to date. If firmware attempts to update the buffer
while an automatic RTR response is in the process of
transmission, all writes to buffers are ignored.
24.6 CAN Message Transmission
24.6.1
INITIATING TRANSMISSION
For the MCU to have write access to the message buf-
fer, the TXREQ bit must be clear, indicating that the
message buffer is clear of any pending message to be
transmitted. At a minimum, the SIDH, SIDL and DLC
registers must be loaded. If data bytes are present in
the message, the Data registers must also be loaded.
If the message is to use extended identifiers, the
EIDH:EIDL registers must also be loaded and the
EXIDE bit set.
Once an abort is requested by setting the ABAT or
TXABT bits, it cannot be cleared to cancel the abort
request. Only CAN module hardware or a POR
condition can clear it.
To initiate message transmission, the TXREQ bit must
be set for each buffer to be transmitted. When TXREQ
is set, the TXABT, TXLARB and TXERR bits will be
cleared. To successfully complete the transmission,
there must be at least one node with matching baud
rate on the network.
© 2009 Microchip Technology Inc.
DS39637D-page 333
MICROCHIP [ MICROCHIP ]
