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36.3 DSP Function Descriptors (FDs)
Similar in structure to SCC buffer descriptors, a function descriptor (FD) speciÞes the DSP
function and contains function-speciÞc parameters. Prepared in external memory, a group
of FDs can be chained together to form a circular queue of programmable length. There are
two such FD chainsÑone for the transmitter and one for the receiver. (FD chains are
logically equivalent to the BD tables for SCCs.) Figure 36-2 shows the FD chain structure.
Figure 36-2. DSP Function Descriptor (FD) Chain Structure
A function descriptor (FD) consists of eight 16-bit entries. The Þrst entry contains status
and control bits including the function opcode. The remaining seven entries contain the
functionÕs parameter packet. Figure 36-3 shows the general structure of an FD.
0
1
Offset + 0x0
S
Ñ
Offset + 0x2
¥ ¥ ¥
¥ ¥ ¥
¥ ¥ ¥
Offset + 0xE
Figure 36-3. Function Descriptor (FD) Structure
Table 36-2 describes the status and control bits. All of the library functions use the stop,
wrap, and interrupt bits. The use of the remaining control bits, apart from the opcode,
depends on the particular function. The parameter packets are described with the individual
functions.
MOTOROLA
Dual-Port RAM
FD Base (Rx Chain)
FD Base (Tx Chain)
Input, Output, and
CoefÞcient Buffers
2
3
4
5
W
I
X
IALL
Chapter 36. Digital Signal Processing
Part V. The Communications Processor Module
System Memory
RxFD Chain
TxFD Chain
6
7
8
9
10
INDEX
PC
Ñ
Ñ
Parameter 1
¥ ¥ ¥
¥ ¥ ¥
¥ ¥ ¥
Parameter 7
11
12
13
14
15
OPCODE
36-3
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