Datasheet

Programmer’s Model of Message Storage
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
Freescale Semiconductor 137
12.12 Programmer’s Model of Message Storage
This section details the organization of the receive and transmit message buffers and the associated
control registers. For reasons of programmer interface simplification, 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 defined for the transmit
buffers.
12.12.1 Message Buffer Outline
Figure 12-12 shows the common 13-byte data structure of receive and transmit buffers for extended
identifiers. The mapping of standard identifiers into the IDR registers is shown in Figure 12-13. All bits of
the 13-byte data structure are undefined out of reset.
NOTE
The foreground receive buffer can be read anytime but cannot be written.
The transmit buffers can be read or written anytime.
Addr
(1)
Register Name
$05b0 IDENTIFIER REGISTER 0
$05b1 IDENTIFIER REGISTER 1
$05b2 IDENTIFIER REGISTER 2
$05b3 IDENTIFIER REGISTER 3
$05b4 DATA SEGMENT REGISTER 0
$05b5 DATA SEGMENT REGISTER 1
$05b6 DATA SEGMENT REGISTER 2
$05b7 DATA SEGMENT REGISTER 3
$05b8 DATA SEGMENT REGISTER 4
$05b9 DATA SEGMENT REGISTER 5
$05bA DATA SEGMENT REGISTER 6
$05bB DATA SEGMENT REGISTER 7
$05bC DATA LENGTH REGISTER
$05bD TRANSMIT BUFFER PRIORITY REGISTER
(2)
$05bE UNUSED
$05bF UNUSED
1. Where b equals the following:
b=4 for receive buffer
b=5 for transmit buffer 0
b=6 for transmit buffer 1
b=7 for transmit buffer 2
2. Not applicable for receive buffers
Figure 12-11. Message Buffer Organization