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CAN Controller (CAN)
Chapter 19
Preliminary User’s Manual U17566EE1V2UM00
19.11.3
Automatic block transmission (ABT)
The automatic block transmission (ABT) function is used to transmit two or
more data frames successively with no CPU interaction. The maximum
number of transmit message buffers assigned to the ABT function is eight
(message buffer numbers 0 to 7).
By setting the CnCTRL.OPMODE2 to CnCTRL.OPMODE0 bits to 010B,
“normal operation mode with automatic block transmission function” (hereafter
referred to as ABT mode) can be selected.
To issue an ABT transmission request, define the message buffers by software
first. Set the CnMCONFm.MA0 bit (1) in all the message buffers used for ABT,
and define all the buffers as transmit message buffers by setting the
CnMCONFm.MA2 to CnMCONFm.MA0 bits to 000B. Be sure to set the same
ID for the message buffers for ATB even when that ID is being used for all the
message buffers. To use two or more IDs, set the ID of each message buffer by
using the CnMIDLm and CnMIDHm registers. Set the CnMDLCm and
CnMDATA0m to CnMDATA7m registers before issuing a transmission request
for the ABT function.
After initialization of message buffers for ABT is finished, the CnMCTRLm.RDY
bit needs to be set (1). In the ABT mode, the CnMCTRLm.TRQ bit does not
have to be manipulated by software.
After the data for the ABT message buffers has been prepared, set the
CnGMABT.ABTTRG bit to 1. Automatic block transmission is then started.
When ABT is started, the TRQ bit in the first message buffer (message buffer
0) is automatically set to 1. After transmission of the data of message buffer 0
is finished, the TRQ bit of the next message buffer, message buffer 1, is set
automatically. In this way, transmission is executed successively.
A delay time can be inserted by program in the interval in which the
transmission request (TRQ bit) is automatically set while successive
transmission is being executed. The delay time to be inserted is defined by the
CnGMABTD register. The unit of the delay time is DBT (data bit time). DBT
depends on the setting of the CnBRP and CnBTR registers.
During ABT, the priority of the transmission ID is not searched. The data of
message buffers 0 to 7 is sequentially transmitted. When transmission of the
data frame from message buffer 7 has been completed, the ABTTRG bit is
automatically cleared to 0 and the ABT operation is finished.
If the RDY bit of an ABT message buffer is cleared during ABT, no data frame
is transmitted from that buffer, ABT is stopped, and the ABTTRG bit is cleared.
After that, transmission can be resumed from the message buffer where ABT
stopped, by setting the RDY and ABTTRG bits to 1 by software. To not resume
transmission from the message buffer where ABT stopped, the internal ABT
engine can be reset by setting the CnGMABT.ABTCLR bit to 1 while ABT
mode is stopped and the ABTTRG bit is cleared to 0. In this case, transmission
is started from message buffer 0 if the ABTCLR bit is cleared to 0 and then the
ABTTRG bit is set to 1.
An interrupt can be used to check if data frames have been transmitted from all
the message buffers for ABT. To do so, the CnMCTRLm.IE bit of each
message buffer except the last message buffer needs to be cleared (0).
If a transmit message buffer other than those used by the ABT function
(message buffers 8 to 31) is assigned to a transmit message buffer, the priority
of the message to be transmitted is determined by the priority of the
transmission ID of the ABT message buffer whose transmission is currently
held pending and the transmission ID of the message buffers other than those
used by the ABT function.
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