Specifications

Counter/Timers
Mode 3 - Square Wave Generator
The most common use for Mode 3 is baud rate generation. The V40
Serial Control Unit requires that counter/timer 1 be configured in
Mode 3 to provide the serial transmit and receive clock. The only
difference between Mode 2 and Mode 3 is the duty cycle of the
counter/timer output. In Mode 2, the output pulses low for the last
count, while in Mode 3 the output is low for half of the programmed
count. All counter/timers support Mode 3 with the exception that
counter/timers 0 and 1 do not have a TCTL input. The TCTL input
for counter/timer 2 is available through connector J3 of the ZT 8832.
Refer to Figure 7-10 for examples of Mode 3 operation. The TOUT
signal remains high until the programmed number of counts is
decremented by half. TOUT then goes low for the remainder of the
count. The entire operation is periodic because it repeats
automatically. An initial count of N produces a square wave with a
period of N clock cycles.
The TCTL signal provides a means of suspending 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 counting to resume.
The operation of the Down Counter is not affected if a new count is
written while it is counting. The new count will be loaded into the
Down Counter at the end of the current half-cycle or after a TCTL
trigger.
7-22