MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 410

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

MC68302 Applications
D.8.8 Using Transparent Mode with PCM Highway Mode
In PCM highway mode, the physical interface controls the multiplexing of data from one, two,
or all three SCCs onto the NMSI1 pins. In PCM highway mode, the NMSI1 pins take on the
names and functions shown in Table D-4.
Notice that there is only one clock signal for PCM highway. This clock functions as both a
receive and transmit clock. Thus, if receive data needs to be clocked into the MC68302 at a
different time or speed than transmit data is being clocked out, then PCM highway is not an
appropriate interface and NMSI should be used instead. Also, L1CLK is an input only. To
drive it from the MC68302, a timer or baud rate generator output pin must be externally con-
nected to the L1CLK pin.
The two sync signals are L1SY0 and L1SY1. They are also inputs to the MC68302. Togeth-
er, they select one of three PCM channels to which data is routed or select no channel (see
Table D-5).
A PCM channel can be routed to any SCC, as selected in the SIMODE register. This routing
ability gives one extra layer of indirection; thus, hardware which must generate the L1SY1
and L1SY0 signals externally does not have to be modified if a change in the SCC data rout-
ing is required.
Two different modes exist for using the L1SY1 and L1SY0 pins: one-clock-prior mode and
envelope mode. In one-clock-prior mode, the sync signals go active for a single clock period
prior to an 8-bit time slot. In envelope mode, the sync signals go active on the first bit of the
time slot and stay active for the entire time slot. Thus, envelope mode is more general since
it allows time slots to be between one and N bits long. Figure D-29 illustrates the operation
of envelope mode. The three PCM channels defined in the figure show some of the flexibility
available in the PCM mode.
D-60
L1RXD
L1TXD
L1CLK
L1SY0
L1SY1
RTS1-RTS3
Table D-4. PCM Highway Pin Names and Functions
Name
Table D-5. PCM Highway Channel Selection
L1SY1
0
0
1
1
Receive Data
Transmit Data
Receive and Transmit Clock
Sync Signal 0
Sync Signal 1
Three RTS Signals
with LlSY0 and L1SY1
MC68302 USER’S MANUAL
L1SY0
0
1
0
1
Function
No Channel Selected
PCM Channel 1 Selected
PCM Channel 2 Selected
PCM Channel 3 Selected
Selected Channel
Input/Output
Outputs
Output
Input
Input
Input
Input
MOTOROLA