SYNCHRONOUS SERIAL INTERFACE (SSI)
6 - 140
PORT C
MOTOROLA
6.4.7.3.1
Network Mode Transmit
When TE is set, the transmitter will be enabled only after detection of a new data frame
sync. This procedure allows the SSI to synchronize to the network timing.
Normal startup sequence for transmission in the first time slot is to write the data to be
transmitted to TX, which clears the TDE flag. Then set TE and TIE to enable the transmit-
ter on the next frame sync and to enable transmit interrupts.
Alternatively, the DSP programmer may decide not to transmit in the first time slot by writ-
ing any data to the time slot register (TSR). This will clear the TDE flag just as if data were
going to be transmitted, but the STD pin will remain in the high-impedance state for the
first time slot. The programmer then sets TE and TIE.
When the frame sync is detected (or generated), the first data word will be transferred from
TX to the transmit shift register and will be shifted out (transmitted). TX being empty will
cause TDE to be set, which will cause a transmitter interrupt. Software can poll TDE or use
interrupts to reload the TX register with new data for the next time slot. Software can also
write to TSR to prevent transmitting in the next time slot. Failing to reload TX (or writing to
the TSR) before the transmit shift register is finished shifting (empty) will cause a transmitter
underrun. The TUE error bit will be set, causing the previous data to be retransmitted.
The operation of clearing TE and setting it again will disable the transmitter after comple-
tion of transmission of the current data word until the beginning of the next frame sync pe-
riod. During that time, the STD pin will be three-stated. When it is time to disable the trans-
mitter, TE should be cleared after TDE is set to ensure that all pending data is transmitted.
The optional output flags are updated every time slot regardless of TE.
To summarize, the network mode transmitter generates interrupts every time slot and re-
quires the DSP program to respond to each time slot. These responses can be:
1. Write data register with data to enable transmission in the next time slot
2. Write the time slot register to disable transmission in the next time slot
3. Do nothing – transmit underrun will occur the at beginning of the next time slot,
and the previous data will be transmitted
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
.
..