Specifications

Counter/Timers
Mode 2 - Rate Generator
The output of a counter/timer in Mode 2 is high until the programmed
count reaches one, then pulses low for a single count before returning
to a high value and repeating the operation.
Counter/timer 2 fully supports Mode 2 operation with TCTL available
through connector J3 of the ZT 8832. Counter/timers 0 and 1 support
Mode 2 with the exception of the TCTL input. The output of
counter/timer 0 is connected to INT1 of the interrupt controller, which
makes it the first choice for generating periodic interrupts. The output
of counter/timer 1 can be connected to INT2 of the interrupt controller
by programming the OPCN V40 configuration register. Counter/timer
1 should be used to generate periodic interrupts only when the V40
serial controller is not needed.
Figure 7-9 includes examples of Mode 2 operation. TOUT remains
high until the programmed number of counts is decremented to one.
At this point, TOUT transitions low for only one clock pulse. As
TOUT becomes high, the Down Counter is reloaded automatically
and the process is repeated. The entire cycle is periodic, with a period
of N clock cycles, for an initial count of N.
The TCTL signal provides a means of stopping the Down Counter for
counter/timer 2 only. If TCTL is high, the operation continues as
defined above. If TCTL is lowered during the count operation, the
Down Counter stops and the TOUT signal is immediately set high. A
TCTL trigger causes the count to be transferred from the Count Latch
to the Down Counter, and operation to be resumed.
Programming the Count register with a new count while the Down
Counter is counting does not affect the current counting sequence.
The new count will be loaded automatically after the current cycle is
completed, or on the occurrence of a TCTL trigger.
7-20