MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 22-15
22.3.3.4 RX Mask Registers
These registers are used as acceptance masks for received frame ID. Three masks are defined: a global
mask, used for RX buffers 0–13 and 16–63, and two extra masks dedicated for buffers 14 and 15. The
meaning of each mask bit is the following:
• Mask bit = 0: the corresponding incoming ID bit is “don’t care.”
• Mask bit = 1: the corresponding ID bit is checked against the incoming ID bit, to see if a match
exists.
Note that these masks are used both for standard and extended ID formats. The value of mask registers
should not be changed while in normal operation. Locked frames which had matched a MB through a mask
may be transferred into the MB (upon release) but may no longer match. Table 22-9 shows some examples
of ID masking for standard and extended message buffers.
0123456789101112131415
R0000000000000000
W
Reset0000000000000000
Reg Addr Base + 0x0008
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
RTIMER
W
Reset0000000000000000
Reg Addr Base + 0x0008
Figure 22-5. Free Running Timer (CANx_TIMER)
Table 22-9. Mask Examples for Standard/Extended Message Buffers
Base ID
ID28.................ID18
IDE
Extended ID
ID17......................................ID0
Match
MB2 ID 1 1 1 1 1 1 1 1 0 0 0 0
MB3 ID 1 1 1 1 1 1 1 1 0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
MB4 ID 0 0 0 0 0 0 1 1 1 1 1 0
MB5 ID 0 0 0 0 0 0 1 1 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
MB14 ID 1 1 1 1 1 1 1 1 0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
RX Global Mask 1 1 1 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 1
RX Msg in
1
1 1 1 1 1 1 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 3
RX Msg in
2
1 1 1 1 1 1 1 1 0 0 1 0 2
RX Msg in
3
1 1 1 1 1 1 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0
RX Msg in
4
0 1 1 1 1 1 1 1 0 0 0 0
RX Msg in
5
0 1 1 1 1 1 1 1 0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 14
RX 14 Mask 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0
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