MOTOROLA
Chapter 21. Serial Interface
21-7
Part V. The Communications Processor Module
routing, and the second 64 are for transmit routing. The entries deÞne the number of bits or
bytes to be routed to and from the SCCs or SMCs and also control external strobes.
The amount of SI RAM available for time-slot programming depends on the conÞguration
of the SIGMR; see Section 21.2.4.1, ÒSI Global Mode Register (SIGMR).Ó Using all 64
entries of the Rx or Tx SI RAM, TDMa can support a maximum frame length of 8,192 bits.
Using both TDMs divides the SI RAM equally for each channel. Enabling on-the-ßy
changes also divides the SI RAM to allow for routing workspace. Using both TDMs with
dynamic changes still leaves 16 SI RAM entries per channel for extensive time-slot
programming. See Section 21.2.3, ÒSI RAM.Ó
The SI supports two testing modesÑecho and loopback. The echo mode provides a return
signal from the physical interface by retransmitting the signal it receives. The physical
interface echo mode differs from the individual SCC echo mode in that it operates on the
entire TDM signal rather than on an individual SCC channel. Loopback mode causes the
physical interface to receive the same signal it is sending. Checking both the entire SI and
the internal channel routes, the SI loopback mode does more than the individual SCC
loopback. Programming echo and loopback modes are programmed in SIMODE[SDMx];
see Section 21.2.4.2, ÒSI Mode Register (SIMODE).Ó Loopback mode can also be
programmed on a time-slot basis in an individual SI RAM entry; see Section 21.2.3.7,
ÒProgramming the SI RAM.Ó Note that loopback operation requires that the receive and
transmit sections of the TDM use common clock and synchronization signals.
The maximum external serial clock that may be an input to the TSA is SYNCCLK/2.5. If
an SCC or SMC is operating with the NMSI, the serial clock rate may be slightly faster at
a value not to exceed SYNCCLK/2.
Note that a sync pulse received during TSA frame routing is ignored. However, when
programmed for a one-clock delay between the sync and start-of-frame pulses, the TSA can
accept the last bit of a frame overlapping the sync pulse of the next frame.
21.2.1 TSA Signals
The TSA signals for TDMa and TDMb are shown in Table 21-1.
Table 21-1. TSA Signals
Signal
Description
L1RSYNC
x
/L1TSYNC
x
Receive/transmit synchronization signals. Input to the MPC860.
L1RCLK
x
/L1TCLK
x
Receive/transmit clocks. Input to the MPC860.
L1RXD
x
Receive data. Input to the MPC860.
L1TXD
x
Transmit data. Open-drain output of the MPC860.
L1CLKO
x
Clock output (optional). Output from the MPC860. Needed only for clocking external
devices in GCI mode.
L1RQ
x
/L1GR
x
IDL request (output) and grant (input) signals. Used if D-channel arbitration is required.
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