Zilog Z16C35 - Manual (Page 46)

ISCC
User Manual
UM011002-0808
40
3.6 CLOCK SELECTION
The ISCC can select several clock sources for internal and external use. Write Register 11
is the Clock Mode Control register for both the receive and transmit clocks. It determines
the type of signal on the /SYNC and /RTxC pins and the direction of the /TRxC pin.
The ISCC may be programmed to select one of several sources to provide the receive and
receive clocks.
The source of the receive clock is controlled by bits D6 and D5 of WR11. The receive
clock may be programmed to come from the /RTxC pin, the /TRxC pin, the output of the
baud rate generator, or the receive output of the DPLL.
The source of the transmit clock is controlled by bits D4 and D3 of WR11. The transmit
clock may be programmed to come from the /RTxC pin, the /TRxC pin, the output of the
baud rate generator, or the transmit output of the DPLL.
Ordinarily the /TRxC pin is an input, but it becomes an output if this pin has not been
selected as the source for the transmitter or the receiver, and bit D2 of WR11 is set to “1.”
The selection of the signal provided on the /TRxC output pin is controlled by bits D1 and
D0 of WR11. The /TRxC pin may be programmed to provide the output of the crystal
oscillator, the output of the baud rate generator, the receive output of the DPLL or the
actual transmit clock. If the output of the crystal oscillator is selected, but the crystal oscil-
lator has not been enabled, the /TRxC pin will be driven HIGH. The option of placing the
transmit clock signal on the /TRxC pin when it is an output allows access to the transmit
output of the DPLL.
Figure 3-9 shows a simplified schematic diagram of the circuitry used in the clock multi-
plexing. It shows the inputs to the multiplexer section, as well as the various signal inver-
sions that occur in the paths to the outputs.
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