Introduction
Clock Slowdown & Halt Restart (CMOS boards only)
For power conservation, the ZT 88CT08A and ZT 88CT09A contain
two features to slow down or stop processor execution program-
matically. These are the clock slowdown and halt with interrupt
restart features, provided by a special Harris Semiconductor 82C85
clock chip that replaces the 82C84A normally shipped on the
ZT 8809A board.
Clock slowdown divides the existing clock frequency by 256,
allowing a selection between 5 MHz and 19.5 kHz for the
ZT 88CT08A,
and
between
8 MHz
and
31.25 kHz
for
the
ZT 88CT09A. Writing to a bit at the printer port allows programmatic
control of the slowdown feature. If the printer requires this bit for
proper control (most do not), a jumper selection is also provided to
allow hardware selection of this feature.
Interrupt restart allows the processor to stop processing via a software
halt instruction and remain dormant until a time interval has passed or
until the processor is needed by an external device. This conserves the
ZT 88CT08A/88CT09A power until processing is actually needed. To
use this feature, the software programs the on-board 8259A interrupt
controller to enable the proper interrupt level. This interrupt may be
from the on-board timers with their independent oscillator or from an
external source. The program then halts the microprocessor with the
HLT instruction until the interrupt arrives to restart it.
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