Intel 80C186XL, User Manual

Page: 243 / 405

TIMER/COUNTER UNIT
9-14
When the EXT bit is clear and the RTG bit is set, every low-to-high transition on the timer input
pin causes the Count register to reset to zero. After the timer is enabled, counting begins only after
the first low-to-high transition on the input pin. If another low-to-high transition occurs before
the end of the timer cycle, the timer count resets to zero and the timer cycle begins again. In dual
maximum count mode, the Register In Use (RIU) bit does not clear when a low-to-high transition
occurs. For example, if the timer retriggers while Maxcount Compare B is in use, the timer resets
to zero and counts to maximum count B before the RIU bit clears. In dual maximum count
mode, the timer retriggering extends the use of the current Maxcount Compare register.
9.2.4 Pulsed and Variable Duty Cycle Output
Timers 0 and 1 each have an output pin that can perform two functions. First, the output can be a
single pulse, indicating the end of a timing cycle (single maximum count mode). Second, the out-
put can be a level, indicating the Maxcount Compare register currently in use (dual maximum
count mode). The output occurs one clock after the counter element services the timer when the
maximum count is reached (see Figure 9-9).
With external clocking, the time between a transition on a timer input and the corresponding tran-
sition of the timer output varies from 2½ to 6½ clocks. This delay occurs due to the time-multi-
plexed servicing scheme of the Timer/Counter Unit. The exact timing depends on when the input
occurs relative to the counter element’s servicing of the timer. Figure 9-2 on page 9-3 shows the
two extremes in timer output delay. Timer 0 demonstrates the best possible case, where the input
occurs immediately before the timer is serviced. Timer 1 demonstrates the worst possible case,
where the input is latched, but the setup time is not met and the input is not recognized until the
counter element services the timer again.
In single maximum count mode, the timer output pin goes low for one CPU clock period (see Fig-
ure 9-4 on page 9-6). This occurs when the count value equals the Maxcount Compare A value.
If programmed to run continuously, the timer generates periodic pulses.
Table 9-2. Timer Retriggering
EXT RTG Timer Operation
0 0 Timer counts internal events, if input pin remains high.
0 1 Timer counts internal events; count resets to zero on every low-to-high transition on
the input pin.
1 X Timer input acts as clock source.