Communications Processor (CP)
4-16
MC68302 USER’S MANUAL
MOTOROLA
The monitor channel is used to transfer data between layer-1 devices and the control unit
(i.e., the M68000 core). The command/indication channel is used to control activation/deac-
tivation procedures or for the switching of test loops by the control unit.
The IMP supports all five channels of the GCI channel 0. The following table shows where
each channel can be routed. The two B channels can be concatenated and routed to the
same SCC channel.
The GCI interface supports the CCITT I.460 recommendation for data rate adaptation. The
GCI interface can access each bit of the B channel as an 8-kbps channel. The mask register
(SIMASK) for the B channels specifies which bits are supported by the GCI interface. The
receiver will receive only the bits that are enabled by SIMASK; the transmitter will transmit
only the bits that are enabled by SIMASK and will not drive the L1TXD pin otherwise (L1TXD
in GCI mode is an open-drain output).
The IMP supports contention detection on the D channel. When the IMP has data to transmit
on the D channel, it checks bit 4 of the SCIT C/I channel 2. The physical layer device mon-
itors the physical layer bus for activity on the D channel and indicates with this bit that the
channel is free. If a collision is detected on the D channel, the physical layer device sets bit
4 of C/I channel 2 to logic high. The IMP then aborts its transmission and retransmits the
frame when this bit is asserted again. This procedure is handled automatically for the first
two buffers of a frame. The L1GR line may also be used for access to the S interface D chan-
nel. This signal is checked by the IMP, and the physical layer device should indicate that the
S interface D channel is free by asserting L1GR.
In the deactivated state, the clock pulse is disabled, and the data line is a logic one. The
layer-1 device activates the IMP by enabling the clock pulses and by an indication in the
channel 0 C/I channel. The IMP will then report to the M68000 core by a maskable interrupt
that a valid indication is in the SMC2 receive buffer descriptor.
When the M68000 core activates the line, it sets SETZ in the serial interface mode (SIMO-
DE) register, causing the data output from L1TXD to become a logic zero. Code 0 (com-
mand timing TIM) will be transmitted on channel 0 C/I channel to the layer-1 device until the
SETZ is reset. The physical layer device will resume transmitting the clock pulses and will
give an indication in the channel 0 C/I channel. The M68000 core should reset SETZ to en-
able data output.
4.4.3 PCM Highway Mode
In PCM highway mode, one, two, or all three SCCs can be multiplexed together to support
various time-division multiplexed interfaces. PCM highway supports the standard T1 and
CEPT interfaces as well as user-defined interfaces. In this mode, the NMSI1 pins have new
names and functions (see Table 4-2).
GCI Channel 0
Serial Controllers
D
B1
B2
M
C/I
SCC1, SCC2, SCC3
SCC1, SCC2, SCC3
SCC1, SCC2, SCC3
SMC1
SMC2