Programming A Message Acceptance Filter - Intel 8XC196K Series User Manual

8XC196K x , J x , CA USER'S MANUAL

12.4.4 Programming a Message Acceptance Filter

The mask registers provide a method for developing an acceptance filtering strategy. Without a
filtering strategy, a message object could accept an incoming message only if their identifiers
were identical. The mask registers allow a message object to ignore one or more bits of incoming
message identifiers, so it can accept a range of message identifiers.
The standard global mask register (Figure 12-9) applies to messages with standard (11-bit) mes-
sage identifiers, while the extended global mask register (Figure 12-10) applies to messages with
extended (29-bit) identifiers. The message 15 mask register (Figure 12-11) provides an additional
filter for message object 15, to allow it to accept a greater range of message identifiers than mes-
sage objects 1–14 can. Clear a mask bit to accept either a zero or a one in that position.
The CAN controller applies the appropriate global mask to each incoming message identifier and
checks for an acceptance match on message objects 1–14. If no match exists, it then applies the
message 15 mask and checks for a match on message object 15.
CAN_SGMSK
(87C196CA)
Program the CAN standard global mask (CAN_SGMSK) register to mask ("don't care") specific
message identifier bits for standard message objects.
15
87C196CA MSK20
7
MSK28
Bit Bit
Number Mnemonic
15:13 MSK20:18
12:8 —
7:0 MSK28:21
Figure 12-9. CAN Standard Global Mask (CAN_SGMSK) Register
12-18
MSK19 MSK18 —
MSK27 MSK26 MSK25
ID Mask
These bits individually mask incoming message identifier (ID) bits.
0 = mask the ID bit (accept either "0" or "1")
1 = accept only an exact match
Reserved; for compatibility with future devices, write zeros to these bits.
ID Mask
These bits individually mask incoming message identifier (ID) bits.
0 = mask the ID bit (accept either "0" or "1")
1 = accept only an exact match
Address: 1E07H, 1E06H
Reset State: Unchanged
— —
MSK24 MSK23 MSK22
Function
8
— —
0
MSK21