Motorola DSP56156 Manual page 238

clears all CRA bits. SSIx reset and STOP reset do not affect the CRA bits. CRA is memory
mapped to X:$FFD0 for SSI0 and X:$FFD8 for SSI1. The CRA control bits are described
in the following paragraphs.
15 14 13
PSR WL1 WL0
7 6 5
PM7 PM6 PM5
8.11.1 CRA Prescale Modulus Select (PM0...PM7) Bits 0-7
The Prescale Modulus Select bits specify the divide ratio of the prescale divider in the SSI
clock generator. This prescaler is used only in internal clock mode to divide the internal
clock of the DSP. A divide ratio from 1 to 256 (PM=$00 to $FF) may be selected.The bit
clock output is available at SCK. The bit clock output is also used internally as the bit
clock to shift the transmit and receive shift registers. Careful choice of the crystal oscilla-
tor frequency and the prescaler modulus will allow the telecommunication industry stan-
dard codec master clock frequencies of 2.048 MHz, 1.544 MHz, and 1.536 MHz to be
generated. For example, a 24.576 MHz clock frequency may be used to generate the
standard 2.048 MHz and 1.536 MHz rates, and a 24.704 MHz clock frequency may be
used to generate the standard 1.544 MHz rate. Table 8-2 gives examples of PM0-PM7
values that can be used in order to generate different bit clocks:
The bit clock on the SSI can be calculated from the Fosc value using the following equa-
tion:
SCK = Fosc ÷ (4 x(7PSR+1) x(PM+1))
8.11.2 CRA Frame Rate Divider Control (DC0...DC4) Bits 8-12
The Frame Rate Divider Control bits control the divide ratio for the programmable frame
rate dividers. It operates on the word clock. In network mode this ratio may be interpreted
as the number of words per frame.
In normal mode, this ratio determines 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.
MOTOROLA SYNCHRONOUS SERIAL INTERFACE (SSI0 and SSI1)
SSI CONTROL REGISTER A (CRA)
12 11 10
DC4 DC3 DC2 DC1
4 3 2
PM4 PM3 PM2 PM1
9 8
DC0
READ-WRITE
SSIx CONTROL
REGISTER A (CRA);
1 0
ADDRESS $FFD0 — SSI0
ADDRESS $FFD8 — SSI1
PM0
8 - 13