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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
304 of 571
NXP Semiconductors
UM10316
Chapter 21: LPC29xx CAN 0/1
5.
CAN transmit buffers
The CAN controller contains three transmit buffers. Each of these has a length of four
32-bit words and can store one complete CAN message.
The transmit buffer-status bits TBS3, TBS2, TBS1 in the CAN controller status register
CCSTAT signal which of the three transmit buffers is available and ready to be filled with
data for the next transmit messages.
5.1 Transmit buffer layout
The transmit buffers are located in the address range from CANC Base 030h to 05Ch.
The buffer layout is subdivided into message-information, identifier and data registers.
The message info register includes the Tx frame info describing frame format, data length
and whether it is a remote or a data frame. In addition, a Tx priority field allows definition
of a priority for each transmit buffer (see
for more details).
The identifier register contains the message ID. Depending on the chosen frame format,
an 11-bit identifier for standard frame format (SFF) or a 29-bit identifier for extended frame
format (EFF) then follows.
Remark:
Unused bits in the ID field have to be defined as 0.
Data registers A and B contain the message data bytes.
The number of data fields used in a message is coded with the data-length code DLC in
the message info register. At the start of a remote frame transmission the DLC is not
considered because the RTR bit is 1 (= remote).
This forces the number of transmitted/received data bytes to be 0. The DLC must be
specified correctly to avoid bus errors, which can occur if two CAN controllers
simultaneously start a remote frame transmission with the same identifier. For reasons of
compatibility
no
DLC greater than eight should be used. If a value greater than eight is
selected, eight bytes are transmitted in the data frame with the DLC specified in the
message info register.
Fig 71. General structure of a bit-period
clk(sys)
CAN:
t
TSEG2
t
TSEG1
t
SYNCSEG
Sync.
Seg.
TSEG1
TSEG2
Sync.
Seg .
TSEG1
Baud rate prescaler
t
clk(sys)
t
scl
e.g.
BRP =
00000001b
TSEG1 =
0101b
010b
nominal bit time
TSEG2 =