NXP Semiconductors UM10237

User manual Rev. 02 — 19 December 2008 492 o f 792

Chapter 18: LPC24XX CAN controllers CAN1/2

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

Controllers start a Remote Frame transmission with the same identifier simultaneously.

For reasons of compatibility no DLC > 8 should be used. If a value greater than 8 is

selected, 8 bytes are transmitted in the data frame with the Data Length Code specified in

DLC. The range of the Data Byte Count is 0 to 8 bytes and is coded as follows:

(11)

Table 433. Transmit Frame Information Register (CAN1TFI[1/2/3] - address

0xE004 40[30/40/50], CAN2TFI[1/2/3] - 0xE004 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.

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

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.

31 FF If this bit is 0, the next tr ansmit message will be sent with an 11bit

Identifier (standard frame format), while if it’s 1, the message will be

sent with a 29 bit Identifier (extended frame format).

DataByteCount DLC=