Controller Area Network (CAN) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 656

7 6 5 4 3 2 1 0

Reset value
= 0x00

0 0 0 0 0 TX2 TX1 TX0

TX2 , TX0: transmit buffer k ( = 0,1, or 2) select bits

0 = the associated message buffer is deselected.
1 = the associated message buffer is selected, if it is the lowest-numbered bit.

k

Figure 13.27 ■ MSCAN transmitter buffer select register (CAN x TBSEL, x 5 0, 1, 2, 3, or 4)

When one wants to get the next available transmit buffer, the CAN x TFLG register is read
and this value is written into the CAN x TBSEL register. Suppose that all three transmit buf-
fers are available. The value read from CAN x TFLG is therefore %00000111. When writing
this value back to CAN x TSEL, the transmit buffer 0 is selected as the foreground transmit
buffer because the lowest-numbered bit set to 1 is at bit position 0. Reading back this value
of CAN x TSEL results in %00000001, because only the lowest-numbered bit position set to 1
is presented. This mechanism eases the application software selection of the next available
transmit buffer. This situation is shown in the following instruction sequence:
ldaa CAN1TFLG ; value read is %00000111

staa CAN1TBSEL ; value written is %00000111

ldaa CAN1TBSEL ; value read is %00000001

MSCAN I DENTIFIER A CCEPTANCE C ONTROL R EGISTER (CAN X IDAC)
This register provides for identifier acceptance control as shown in Figure 13.28. The
IDHITx indicators are always related to the message in the foreground receive buffer (RxFG).
When a message gets shifted into the foreground buffer of the receiver FIFO, the indicators are
updated as well.

7 6 5 4 3 2 1 0

Reset value
= 0x00

0 0 IDAM1 IDAM0 0 IDHIT2 IDHIT1 IDHIT0

IDAM1 , IDAM0: identifier acceptance mode
00 = two 32-bit acceptance filters.
01 = four 16-bit acceptance filters.
10 = eight 8-bit acceptance filters.
11 = filter closed
IDHIT2 , IDHIT0: identifier acceptance hit indicator (read only)
000 = filter 0 hit.
001 = filter 1 hit.
010 = filter 2 hit.
011 = filter 3 hit.
100 = filter 4 hit.
101 = filter 5 hit.
110 = filter 6 hit.
111 = filter 7 hit.

Figure 13.28 ■ MSCAN identifier acceptance control register (CAN x IDAC, x 5 0, 1, 2, 3, or 4)