SYNCHRONOUS SERIAL INTERFACE (SSI)
MOTOROLA
PORT C
6 - 87
6.4.2.1
SSI Control Register A (CRA)
CRA is one of two 16-bit read/write control registers used to direct the operation of the
SSI. The CRA controls the SSI clock generator bit and frame sync rates, word length, and
number of words per frame for the serial data. The high-order bits of CRA are read as ze-
ros by the DSP CPU. The CRA control bits are described in the following paragraphs.
6.4.2.1.1
CRA Prescale Modulus Select (PM7–PM0) Bits 0–7
The PM0–PM7 bits specify the divide ratio of the prescale divider in the SSI clock generator.
A divide ratio from 1 to 256 (PM=0 to $FF) may be selected. The bit clock output is available
at the transmit clock (SCK) and/or the receive clock (SC0) pins of the DSP. The bit clock
output is also available internally for use as the bit clock to shift the transmit and receive
shift registers. Careful choice of the crystal oscillator frequency and the prescaler modulus
will allow the industry-standard codec master clock frequencies of 2.048 MHz, 1.544 MHz,
and 1.536 MHz to be generated. Hardware and software reset clear PM0–PM7.
6.4.2.1.2
CRA Frame Rate Divider Control (DC4–DC0) Bits 8–12
The DC4–DC0 bits control the divide ratio for the programmable frame rate dividers used
to generate the frame clocks (see Figure 6-43). In network mode, this ratio may be inter-
preted as the number of words per frame minus one. In normal mode, this ratio deter-
mines the word transfer rate. The divide ratio may range from 1 to 32 (DC=00000 to
11111) for normal mode and 2 to 32 (DC=00001 to 11111) for network mode.
A divide ratio of one (DC=00000) in network mode is a special case (see 6.4.7.4). In nor-
mal mode, a divide ratio of one (DC=00000) provides continuous periodic data word trans-
fers. A bit-length sync (FSL1=1, FSL0=0) must be used in this case. Hardware and soft-
ware reset clear DC4–DC0.
6.4.2.1.3
CRA Word Length Control (WL0, WL1) Bits 13 and 14
The WL1 and WL0 bits are used to select the length of the data words being transferred via the
SSI. Word lengths of 8, 12, 16, or 24 bits may be selected according to Table 6-10.
Table 6-10 Number of Bits/Word
WL1
WL0
Number of Bits/Word
0
0
8
0
1
12
1
0
16
1
1
24
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For More Information On This Product,
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