PIC18F6585 Datasheet, PDF (333/496 Pages) Microchip Technology – 64/68/80-Pin High-Performance, 64-Kbyte Enhanced Flash Microcontrollers with ECAN Module

PIC18F6585/8585/6680/8680
23.5.3 PROGRAMMABLE TRANSMIT/
RECEIVE BUFFERS
The ECAN module implements six new buffers: B0-B5.
These buffers are individually programmable as either
transmit or receive buffers. These buffers are available
only in Mode 1 and 2. As with dedicated transmit and
receive buffers, each of these programmable buffers
occupies 14 bytes of SRAM and are mapped into SFR
memory map.
Each buffer contains one Control register (BnCON),
four Identifier registers (BnSIDL, BnSIDH, BnEIDL,
BnEIDH), one Data Length Count register (BnDLC)
and eight Data Byte registers (BnDm). Each of these
registers contains two sets of control bits. Depending
on whether the buffer is configured as transmit or
receive, one would use the corresponding control bit
set. By default, all buffers are configured as receive
buffers. Each buffer can be individually configured as
transmit or receive buffers by setting the corresponding
TXENn bit in the BSEL0 register.
When configured as transmit buffers, user firmware
may access transmit buffers in any order similar to
accessing dedicated transmit buffers. In receive config-
uration, with Mode 1 enabled, user firmware may also
access receive buffers in any order required. But in
Mode 2, all receive buffers are combined to form a sin-
gle FIFO. Actual FIFO length is programmable by user
firmware. Access to FIFO must be done through the
FIFO pointer bits (FP<4:0>) in the CANCON register. It
must be noted that there is no hardware protection
against out of order FIFO reads.
23.5.4 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 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.
The following outlines the steps required to
automatically handle RTR messages:
1. Set buffer to Transmit mode by setting TXnEN
bit to ‘1’ in BSEL0 register.
2. At least one acceptance filter must be associ-
ated with this buffer and preloaded with
expected RTR identifier.
3. Bit RTREN in 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 buffer while
an automatic RTR response is in process of
transmission, all writes to buffers are ignored.
23.6 CAN Message Transmission
23.6.1 INITIATING TRANSMISSION
For the MCU to have write access to the message
buffer, 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.
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.
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.
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 transmission 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.
 2004 Microchip Technology Inc.
DS30491C-page 331