MCP2515
3.3
Initiating Transmission
3.0
3.1
MESSAGE TRANSMISSION
Transmit Buffers
In order to initiate message transmission, the
TXBnCTRL.TXREQ bit must be set for each buffer to
be transmitted. This can be accomplished by:
The MCP2515 implements three transmit buffers. Each
of these buffers occupies 14 bytes of SRAM and are
mapped into the device memory map.
• Writing to the register via the SPI write command
• Sending the SPI RTS command
The first byte, TXBnCTRL, is a control register
associated with the message buffer. The information in
this register determines the conditions under which the
message will be transmitted and indicates the status of
the message transmission (see Register 3-1).
• Setting the TXnRTS pin low for the particular
transmit buffer(s) that are to be transmitted
If transmission is initiated via the SPI interface, the
TXREQ bit can be set at the same time as the TXP
priority bits.
Five bytes are used to hold the standard and extended
identifiers, as well as other message arbitration
information (see Register 3-3 through Register 3-7).
The last eight bytes are for the eight possible data
bytes of the message to be transmitted (see
Register 3-8).
When TXBnCTRL.TXREQ is set, the TXBnCTRL.ABTF,
TXBnCTRL.MLOA and TXBnCTRL.TXERR bits will be
cleared automatically.
Note:
Setting the TXBnCTRL.TXREQ bit does
not initiate a message transmission. It
merely flags a message buffer as being
ready for transmission. Transmission will
start when the device detects that the bus
is available.
At a minimum, the TXBnSIDH, TXBnSIDL and
TXBnDLC registers must be loaded. If data bytes are
present in the message, the TXBnDm registers must
also be loaded. If the message is to use extended
identifiers, the TXBnEIDm registers must also be
loaded and the TXBnSIDL.EXIDE bit set.
Once the transmission has completed successfully, the
TXBnCTRL.TXREQ bit will be cleared, the
CANINTF.TXnIF bit will be set and an interrupt will be
generated if the CANINTE.TXnIE bit is set.
Prior to sending the message, the MCU must initialize
the CANINTE.TXInE bit to enable or disable the
generation of an interrupt when the message is sent.
If
the
message
transmission
fails,
the
TXBnCTRL.TXREQ will remain set. This indicates that
the message is still pending for transmission and one
of the following condition flags will be set:
Note:
The TXBnCTRL.TXREQ bit must be clear
(indicating the transmit buffer is not
pending transmission) before writing to
the transmit buffer.
• If the message started to transmit but encoun-
tered an error condition, the TXBnCTRL.TXERR
and the CANINTF.MERRF bits will be set and an
interrupt will be generated on the INT pin if the
CANINTE.MERRE bit is set
3.2
Transmit Priority
Transmit priority is a prioritization within the MCP2515
of the pending transmittable messages. This is
independent from, and not necessarily related to, any
prioritization implicit in the message arbitration scheme
built into the CAN protocol.
• If the message is lost, arbitration at the
TXBnCTRL.MLOA bit will be set
Note:
If
One-shot
mode
is
enabled
(CANCTRL.OSM), the above conditions
will still exist. However, the TXREQ bit will
be cleared and the message will not
attempt transmission a second time.
Prior to sending the SOF, the priority of all buffers that
are queued for transmission is compared. The transmit
buffer with the highest priority will be sent first. For
example, if transmit buffer 0 has a higher priority setting
than transmit buffer 1, buffer 0 will be sent first.
3.4
One-Shot Mode
If two buffers have the same priority setting, the buffer
with the highest buffer number will be sent first. For
example, if transmit buffer 1 has the same priority
setting as transmit buffer 0, buffer 1 will be sent first.
One-shot mode ensures that a message will only
attempt to transmit one time. Normally, if a CAN
message loses arbitration, or is destroyed by an error
frame, the message is retransmitted. With One-shot
mode enabled, a message will only attempt to transmit
one time, regardless of arbitration loss or error frame.
There are four levels of transmit priority. If
TXBnCTRL.TXP<1:0> for a particular message buffer
is set to 11, that buffer has the highest possible priority.
If TXBnCTRL.TXP<1:0> for a particular message
buffer is 00, that buffer has the lowest possible priority.
One-shot mode is required to maintain time slots in
deterministic systems, such as TTCAN.
© 2005 Microchip Technology Inc.
Preliminary
DS21801D-page 15
MICROCHIP [ MICROCHIP ]
