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Chapter 16
CAN Controller
User’s Manual U16504EE1V1UD00
Figure 16-45:
Transmit Control Register (2/2)
The TXAn bits (n = 0, 1) have a dual function:
1. The CPU can request an abort by writing a “1” into the bit.
2. The DCAN signals whether such an request is still pending. The bit is cleared at the same time
when the TXRQn bit (n = 0, 1) is cleared.
The abort process does not affect any rules of the CAN protocol. A frame already started will continue
to its end.
An abort operation can cause different results dependent on the time it is issued.
d) When an abort request is recognized by the DCAN before the start of the arbitration for transmit,
the TXCn bit (n = 0, 1) is reset showing that the buffer was not send to other nodes.
e) When the abort request is recognized during the arbitration and the arbitration is lost afterwards,
the TXCn bit (n = 0, 1) is reset showing that the buffer was not send to other nodes.
f) When the abort request is recognized during frame transmission and the transmission ends with
an error afterwards, the TXCn bit (n = 0, 1) is reset showing that the buffer was not send to other
nodes.
g) When the abort request is recognized during the frame transmission and transmission ends with-
out error. The TXCn bit (n = 0, 1) is set showing a successful transfer of the data. I.e the abort
request was not issued.
In all cases the TXRQn bit and the TXAn bit (n = 0, 1) bit will be cleared at the end of the abort opera-
tion, when the transmit buffer is available again.
Cautions: 1. The bits are cleared when the INIT bit in CANC register is set.
2. Writing a 0 to TXAn (n = 0, 1) bit has no influence
3. Do not perform read-modify-write operations on TCR.
The TXCn bit (n = 0, 1) are updated at the end of every frame transmission or abort.
The transmit request bits are checked by the DCAN immediately before the frame is started. The order
in which the TXRQn bit (n = 0, 1) will be set does not matter as long as the first requested frame is not
started on the bus.
The TXRQn bit (n = 0, 1) have dual function:
1. Request the transmission of a transmit buffer.
2. Inform the CPU whether a buffer is available or if it is still occupied by a former transmit
request.
Setting the transmission request bit requests the DCAN to sent the buffer contents onto the bus. The
DCAN clears the bit after completion of the transmission. Completion is either a normal transfer without
error or an abort request.
TXRQn
Transmission Request Flag
0
Write: no influence
Read: transmit buffer is free
1
Write: request transmission for buffer n
Read: transmit buffer is occupied by former transmit request