MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 411

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

The PCM highway interface has three RTS signals. One of these signals is asserted when
an SCC wants to transmit over the PCM Highway just like in NMSI mode), and stays contin-
uously asserted until the entire frame is transmitted (regardless of how many time slots the
transmission takes). Which RTS signal asserts depends on which SCC is transmitting; there
is one RTS signal for each SCC. Notice, however, that there is no CTS signal, so there is
nothing to hold off the transfer. If the RTS signals are not needed, they can be ignored or
reassigned as parallel l/O lines.
What other signals are missing from PCM mode? First, there is no CD signal for the receiver.
The receiver is enabled whenever the ENR bit is set. However, you could say there is a CD
(sync) of sorts that is implemented with the L1SY1 and L1SY0 pins. Two pins are used since
not only is the timing important, but also the selection of the PCM channel as well.
The way transparent mode works with a PCM highway interface is very similar to the oper-
ation of the gated clocks example discussed previously. Whether or not a time slot environ-
ment is present, PCM mode gives greater control over what intervals transparent data can
be transmitted and received. However, in PCM mode, the clocks are gated by the physical
interface on the MC68302 as opposed to external hardware.
MOTOROLA
8 BITS
8 BITS
Figure D-29. Routing Channels in PCM Envelope Mode
PCM CHANNEL 1 IS THE BASIC CASE OF AN 8-BIT
TIME SLOT. IT CAN BE ROUTED TO ANY SCC.
8 BITS
8 BITS
MC68302 USER’S MANUAL
8 BITS
8 BITS
PCM CHANNEL 2 IS COMPRISED OF TWO 8-BIT
TIME SLOTS. IT CAN BE ROUTED TO ANY SCC.
10 BITS
10 BITS
PCM CHANNEL 3 IS 10 BITS, AND
IT CAN BE ROUTED TO ANY SCC.
CLOCK NOT TO SCALE
MC68302 Applications
L1TXD
L1RXD
L1CLK
L1SY0
L1SY1
D-61