Freescale Semiconductor MPC5200B ユーザーズマニュアル

Freescale Semiconductor

19-29

flag, and generates a receive interrupt 

 to the CPU

1

. The user’s receive handler has to read the received 

message from the RxFG and then reset the RXF flag to acknowledge the interrupt and to release the foreground buffer. A new message, which 
can follow immediately after the IFS field of the CAN frame, is received into the next available RxBG. If the MSCAN receives an invalid 
message in its RxBG (wrong identifier, transmission errors etc.) the actual contents of the buffer will be over-written by the next message. 
The buffer will then not be shifted into the FIFO.

When the MSCAN module is transmitting, the MSCAN receives its own transmitted messages into the background receive buffer, RxBG, but 
does not shift it into the receiver FIFO, generate a receive interrupt, or acknowledge its own messages on the CAN bus. The exception to this 
rule is in loop back mode 

 where the MSCAN treats its 

own messages exactly like all other incoming messages. The MSCAN receives its own transmitted messages in the event that it loses 
arbitration

2

. If arbitration is lost, the MSCAN must be prepared to become a receiver.

An overrun condition occurs when all receive message buffers in the FIFO are filled with correctly received messages with accepted identifiers 
and another message is correctly received from the bus with an accepted identifier. The latter message is discarded and an error interrupt with 
overrun indication is generated if enabled 

. The MSCAN is still able to transmit messages while the receiver 

FIFO being filled, but all incoming messages are discarded. As soon as a receive buffer in the FIFO is available again, new valid messages 
will be accepted.

The MSCAN Identifier Acceptance Registers (

) define the acceptable patterns of the standard or extended identifier (ID10 - ID0 or ID28 - ID0). Any of these bits can be marked 

‘don’t care’ in the MSCAN Identifier Mask Registers 

.

A filter hit is indicated to the application software by a set Receive Buffer Full flag (RXF=1) and three bits in the CANIDAC register 

. These Identifier Hit flags (IDHIT2-0) clearly identify 

the filter section that caused the acceptance. They simplify the application software’s task to identify the cause of the receiver interrupt. In 
case more than one hit occurs (two or more filters match), the lower hit has priority.

A very flexible programmable generic identifier acceptance filter has been introduced to reduce the CPU interrupt loading. The filter is 
programmable to operate in four different modes

3

: 

• Two identifier acceptance filters, each to be applied to a) the full 29 bits of the extended identifier and to the following bits of 

the CAN 2.0B frame: Remote Transmission Request (RTR), Identifier Extension (IDE), and Substitute Remote Request 
(SRR) or b)

4

 the 11 bits of the standard identifier plus the RTR and IDE bits of the CAN 2.0A/B messages. This mode 

implements two filters for a full length CAN 2.0B compliant extended identifier. 

 shows how the first 32-bit filter 

bank (CANIDAR0-3, CANIDMR0-3) produces a filter 0 hit. Similarly, the second filter bank (CANIDAR4-7, CANIDMR4-7) 
produces a filter 1 hit.

• Four identifier acceptance filters, each to be applied to a) the 14 most significant bits of the extended identifier plus the SRR 

and IDE bits of CAN 2.0B messages or b) the 11 bits of the standard identifier, the RTR and IDE bits of CAN 2.0A/B 
messages. 

 shows how the first 32-bit filter bank (CANIDAR0-3, CANIDMR0-3) produces filter 0 and 1 hits. 

Similarly, the second filter bank (CANIDAR4-7, CANIDMR4-7) produces filter 2 and 3 hits.

• Eight identifier acceptance filters, each to be applied to the first 8 bits of the identifier. This mode implements eight 

independent filters for the first 8 bits of a CAN 2.0A/B compliant standard identifier or a CAN 2.0B compliant extended 
identifier. 

 shows how the first 32-bit filter bank (CANIDAR0-3, CANIDMR0-3) produces filter 0 to 3 hits. Similarly, 

the second filter bank (CANIDAR4-7, CANIDMR4-7) produces filter 4 to 7 hits.

• Closed filter. No CAN message is copied into the foreground buffer RxFG, and the RXF flag is never set.

1. The receive interrupt occurs only if not masked. A polling scheme can be applied on RXF also.
2. Reference the Bosch CAN 2.0A/B protocol specification dated September 1991 for details.
3. For a better understanding of references made within the filter mode description, reference the Bosch specification dated September 1991 

which details the CAN 2.0A/B protocol.

4. Although this mode can be used for standard identifiers, it is recommended to use the four or eight identifier acceptance filters for standard 

identifiers