Clock And Control Module - RCA COSMAC Operator's Manual

CLOCK AND CONTROL MODULE
The four control panel switches are interfaced to the
Microkit system on this card. Depression of the RESET
button causes the internal clock to be gated off. re-
moves any pending DMA Out request. turns off load. and
transmitsa CLEAR pulse to the system. The clear pulse
will reset the COSMAC CPU.* Depression of RUN PROGRAM
causes the clock to turn on and a DMA Out request to be
issued. This will cause the COSMAC CPU to begin instruc-
tion execution at address 0001. assuming it was previously
reset. Depression of RUN UTILITY initiates program ex-
ecution in the same manner. except that AIS is forced
true. starting execution at 8001. if previously reset.
Also on this card is a free-running Pierce crystal
oscillator. with a crystal tuned to 1.95 MHz. This
internal clock can be gated off by an external clock
inhibit as well as by CLEAR. If a user wishes to run
at some other frequency. there is provision for an
external clock.
Depression of RUN UTILITY (or AIS true) deselects the low
half of memory. Otherwise. the upper half of memory is
deselected.
Additionally. this card uses a special state and a sync
to deduce when the COSMAC is running. i.e .• in the in-
struction fetch-execute mode. The RUN indicator is
illuminated under these conditions. On the other hand,
in the reset state. when executing the Idle instruction,
or during DMA service or interrupt response. the RUN
indicator will be off.
*
See COSMAC Microprocessor Manual for details.
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