38.7.2.3 Filtering on Data Bytes
When the filter is configured to receive standard frames, the EID part of the filter and mask object can be selected to filter the data bytes. The DNCNT[4:0] bits in the C1CONL register are used to select how many bits in the data bytes are compared. Table 38-7 explains how many data bits are compared and which filter bits and data bits are compared.
If DNCNTx is:
- ‘
0’, then data byte filtering is disabled. - Nonzero, the filtering will commence on as many data bits as specified in DNCNTx. A filter hit will require matching of the SIDx bits and a match of n data bits with the filter’s EID[0:17] bits. Data Byte 0[7] is always compared to EID[0], Data Byte 0[6] to EID[1], Data Byte 2[6] to EID[17].
- Greater than 18, indicating that the user-selected number of bits is greater than the total number of EIDx bits. The filter comparison will terminate with the 18th bit of the data.
- Greater than 16 and the received message has DLC = 2, indicating a payload of two data bytes. The filter comparison will terminate with the 16th bit of the data.
- Greater than 8 and the received message has DLC = 1, indicating a payload of one data byte. The filter comparison will terminate with the 8th bit of the data.
- Greater than 0 and the received message has DLC = 0, indicating no data payload. The filter comparison will terminate with the identifier.
| DNCNT[4:0] | Received Message Data Bits to be Compared Byte [bits] | EIDx Bits Used for Acceptance Filter |
|---|---|---|
00000 |
No Comparison | No Comparison |
00001 |
Data Byte 0[7] | EID[0] |
00010 |
Data byte 0[7:6] | EID[0:1] |
00011 |
Data byte 0[7:5] | EID[0:2] |
00100 |
Data byte 0[7:4] | EID[0:3] |
00101 |
Data byte 0[7:3] | EID[0:4] |
00110 |
Data byte 0[7:2] | EID[0:5] |
00111 |
Data byte 0[7:1] | EID[0:6] |
01000 |
Data byte 0[7:0] | EID[0:7] |
01001 |
Data byte 0[7:0] and Data Byte 1[7] | EID[0:8] |
01010 |
Data byte 0[7:0] and Data Byte 1[7:6] | EID[0:9] |
01011 |
Data byte 0[7:0] and Data Byte 1[7:5] | EID[0:10] |
01100 |
Data byte 0[7:0] and Data Byte 1[7:4] | EID[0:11] |
01101 |
Data byte 0[7:0] and Data Byte 1[7:3] | EID[0:12] |
01110 |
Data byte 0[7:0] and Data Byte 1[7:2] | EID[0:13] |
01111 |
Data byte 0[7:0] and Data Byte 1[7:1] | EID[0:14] |
10000 |
Data byte 0[7:0] and Data Byte 1[7:0] | EID[0:15] |
10001 |
Byte 0[7:0] and Byte 1[7:0] and Byte 2[7] | EID[0:16] |
10010 to 11111 |
Byte 0[7:0] and Byte 1[7:0] and Byte 2[7:6] | EID[0:17] |
Figure 38-14
illustrates how the first 18 data bits of the received message data payload are compared
with the corresponding EIDx bits of the message acceptance filter (EID[17:0] bits in the
C1FLTOBJxH/L registers). The IDE bit of the received message must be
‘0’.
