UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 368 of 841
NXP Semiconductors
UM10360
Chapter 16: LPC176x/5x CAN1/2
The values for the reserved bits of the CANxTFI register in the Transmit Buffer should be
set to the values expected in the Receive Buffer for an easy comparison, when using the
Self Reception facility (self test), otherwise they are not defined.
The CAN Controller consist of three Transmit Buffers. Each of them has a length of 4
words and is able to store one complete CAN message as shown in Figure 54
.
The buffer layout is subdivided into Descriptor and Data Field where the first word of the
Descriptor Field includes the TX Frame Info that describes the Frame Format, the Data
Length and whether it is a Remote or Data Frame. In addition, a TX Priority register allows
the definition of a certain priority for each transmit message. Depending on the chosen
Frame Format, an 11-bit identifier for Standard Frame Format (SFF) or an 29-bit identifier
for Extended Frame Format (EFF) follows. Note that unused bits in the TID field have to
be defined as 0. The Data Field in TDA and TDB contains up to eight data bytes.
Automatic transmit priority detection
To allow uninterrupted streams of transmit messages, the CAN Controller provides
Automatic Transmit Priority Detection for all Transmit Buffers. Depending on the selected
Transmit Priority Mode, internal prioritization is based on the CAN Identifier or a user
defined "local priority". If more than one message is enabled for transmission (TR=1) the
internal transmit message queue is organized such as that the transmit buffer with the
lowest CAN Identifier (TID) or the lowest "local priority" (TX Priority) wins the prioritization
and is sent first. The result of the internal scheduling process is taken into account short
before a new CAN message is sent on the bus. This is also true after the occurrence of a
transmission error and right before a re-transmission.
Tx DLC
The number of bytes in the Data Field of a message is coded with the Data Length Code
(DLC). At the start of a Remote Frame transmission the DLC is not considered due to the
RTR bit being '1 ' (remote). This forces the number of transmitted/received data bytes to
be 0. Nevertheless, the DLC must be specified correctly to avoid bus errors, if two CAN
Table 330. CAN Transmit Frame Information register (CAN1TFI[1/2/3] - address 0x4004 40[30/40/50], CAN2TFI[1/2/3] -
0x4004 80[30/40/50]) bit description
Bit Symbol Function Reset
Value
RM
Set
7:0 PRIO If the TPM (Transmit Priority Mode) bit in the CANxMOD register is set to 1, enabled Tx
Buffers contend for the right to send their messages based on this field. The buffer with the
lowest TX Priority value wins the prioritization and is sent first.
x
15:8 - Reserved. 0
19:16 DLC Data Length Code. This value is sent in the DLC field of the next transmit message. In
addition, if RTR = 0, this value controls the number of Data bytes sent in the next transmit
message, from the CANxTDA and CANxTDB registers:
0000-0111 = 0-7 bytes
1xxx = 8 bytes
0X
29:20 - Reserved. 0
30 RTR This value is sent in the RTR bit of the next transmit message. If this bit is 0, the number of
data bytes called out by the DLC field are sent from the CANxTDA and CANxTDB registers.
If this bit is 1, a Remote Frame is sent, containing a request for that number of bytes.
0X
31 FF If this bit is 0, the next transmit message will be sent with an 11-bit Identifier (standard frame
format), while if it’s 1, the message will be sent with a 29-bit Identifier (extended frame
format).
0X
To Next Page
Loading...
