SYNCHRONOUS SERIAL INTERFACE (SSI)
6 - 136
PORT C
MOTOROLA
The frame sync signal indicates the beginning of a new data frame. Each data frame is di-
vided into time slots; transmission or reception can occur in each time slot (rather than in
just the frame sync time slot as in normal mode). The frame rate dividers (controlled by DC4,
DC3, DC2, DC1, and DC0) control the number of time slots per frame from 2 to 32. Time-slot
assignment is totally under software control. Devices can transmit on multiple time slots, re-
ceive multiple time slots, and the time-slot assignment can be changed dynamically.
A simplified flowchart showing operation of the network mode is shown in Figure 6-76.
Two counters are used to track the current transmit and receive time slots. Slot counter
one (SLOTCT1) is used to track the transmit time slot; slot counter two (SLOTCT2) is
used for receive. When the transmitter is empty, it generates an interrupt; a test is then
made to see if it is the beginning of a frame. If it is the beginning of a frame, SLOTCT1 is
cleared to start counting the time slots. If it is not the beginning of a frame, SLOTCT1 is
incremented. The next test checks to see if the SSI should transmit during this time slot.
If it is time to transmit, data is written to the TX; otherwise, dummy data is written to the
TSR, which prevents a transmit underrun error from occurring and three-states the STD
pin. The DSP can then return to what it was doing before the interrupt and wait for the next
interrupt to occur. SLOTCT1 should reflect the data in the shift registers to coincide with
TFS. Software must recognize that the data being written to TX will be transmitted in time
slot SLOTCT1 plus one.
DSP56002 MASTER
TIME SLOT 1
STD
SRD
SCK
SC2
DSP56002 SLAVE 1
TIME SLOT 2
STD
SRD
SCK
SC2
DSP56002 SLAVE 2
TIME SLOT 3
STD
SRD
SCK
SC2
DSP56002 SLAVE 3
TIME SLOT 4
STD
SRD
SCK
SC2
MASTER TRANSMIT
MASTER RECEIVE
MASTER CLOCK
MASTER SYNC
Figure 6-75 Network Mode Example
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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