
Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-121
SERIAL
I/F
COMMUNICATION
16
PROCESSOR
MODULE
SWTR—Switch Transmit and Receive
This bit is only valid in the receive route RAM and is ignored in the transmit route RAM. This
bit affects the operation of both the L1RXDA and L1TXDA pins. The SWTR bit is only set in
special situations where you prefer to receive data from a transmit pin and transmit data on
a receive pin. For instance, consider the situation where devices A and B are connected to
the same time-division multiplex, each with different time-slots. Normally, there is no
opportunity for stations A and B to communicate with each other directly over the
time-division multiplex since they both receive the same TDM receive data and transmit on
the same TDM transmit signal.
The SWTR option gives station B the opportunity to listen to transmissions from station A
and transmit data to station A. To do this, station B would set the SWTR bit in its receive
route RAM. For this entry, receive data is taken from the L1TXDA pin and data is transmitted
on the L1RXDA pin. If you only want to listen to station A transmissions and not transmit
data on L1RXDA, then you should clear the CSEL field in the corresponding transmit route
RAM entry to prevent transmission on the L1RXDA pin.
It is also possible for station B to transmit data to station A by setting the SWTR bit of the
entry in its receive route RAM. Data is transmitted on the L1RXDA pin rather than the
L1TXDA pin, according to the transmit route RAM. This configuration could, however, cause
collisions with other data on the L1RXDA pin unless care is taken to choose an available
(quiet) time-slot. If you only want to transmit on L1RXDA and not receive data on L1TXDA,
then you should clear the CSEL field in the receive route RAM to prevent reception of data
on L1TXDA.
0 = Normal operation of the L1TXDA and L1RXDA pins.
1 = Data is transmitted on the L1RXDA pin and is received from the L1TXDA pin for
the duration of this entry.
Figure 16-48. Using the SWTR Bit
Note: If the transmit and receive sections of the TDMA do not use a single clock source,
the SWTR feature can cause erratic results to occur.
RX
TX
STATION A
TDM TRANSMIT DATA
TDM RECEIVE DATA
STATION B