Chapter 10 Freescale’s Scalable Controller Area Network (S12MSCANV3)
MC9S12XDP512 Data Sheet, Rev. 2.17
Freescale Semiconductor
445
10.3.3
Programmer’s Model of Message Storage
The following section details the organization of the receive and transmit message buffers and the
associated control registers.
To simplify the programmer interface, the receive and transmit message buffers have the same outline.
Each message buffer allocates 16 bytes in the memory map containing a 13 byte data structure.
An additional transmit buffer priority register (TBPR) is dened for the transmit buffers. Within the last
two bytes of this memory map, the MSCAN stores a special 16-bit time stamp, which is sampled from an
internal timer after successful transmission or reception of a message. This feature is only available for
The time stamp register is written by the MSCAN. The CPU can only read these registers.
Table 10-23. CANIDMR4–CANIDMR7 Register Field Descriptions
Field
Description
7:0
AM[7:0]
Acceptance Mask Bits — If a particular bit in this register is cleared, this indicates that the corresponding bit in
the identier acceptance register must be the same as its identier bit before a match is detected. The message
is accepted if all such bits match. If a bit is set, it indicates that the state of the corresponding bit in the identier
acceptance register does not affect whether or not the message is accepted.
0 Match corresponding acceptance code register and identier bits
1 Ignore corresponding acceptance code register bit
Table 10-24. Message Buffer Organization
Offset
Address
Register
Access
0x00X0
Identier Register 0
0x00X1
Identier Register 1
0x00X2
Identier Register 2
0x00X3
Identier Register 3
0x00X4
Data Segment Register 0
0x00X5
Data Segment Register 1
0x00X6
Data Segment Register 2
0x00X7
Data Segment Register 3
0x00X8
Data Segment Register 4
0x00X9
Data Segment Register 5
0x00XA
Data Segment Register 6
0x00XB
Data Segment Register 7
0x00XC
Data Length Register
0x00XD
Transmit Buffer Priority Register1
0x00XE
Time Stamp Register (High Byte)2
0x00XF
Time Stamp Register (Low Byte)3