Intel 80C186XL, 80C188XL user manual 9.2.2Clock Sources, 9.2.3Counting Modes, Timer/Counter Unit

TIMER/COUNTER UNIT

9.2.2Clock Sources

The 16-bit Timer Count register increments once for each timer event. A timer event can be a low-to-high transition on a timer input pin (Timers 0 and 1), a pulse generated every fourth CPU clock (all timers) or a timeout of Timer 2 (Timers 0 and 1). Up to 65536 (216) events can be count- ed.

Timers 0 and 1 can be programmed to count low-to-high transitions on their input pins as timer events by setting the External (EXT) bit in their control registers. Transitions on the external pin are synchronized to the CPU clock before being presented to the timer circuitry. The timer counts transitions on this pin. The input signal must go low, then high, to cause the timer to increment. The maximum count-rate for the timers is ¼ the CPU clock rate (measured at CLKOUT) because the timers are serviced only once every four clocks.

All timers can use transitions of the CPU clock as timer events. For internal clocking, the timer increments every fourth CPU clock due to the counter element’s time-multiplexed servicing scheme. Timer 2 can use only the internal clock as a timer event.

Timers 0 and 1 can also use Timer 2 reaching its maximum count as a timer event. In this config- uration, Timer 0 or Timer 1 increments each time Timer 2 reaches its maximum count. See Table 9-1 for a summary of clock sources for Timers 0 and 1. Timer 2 must be initialized and running in order to increment values in other timer/counters.

		Table 9-1. Timer 0 and 1 Clock Sources
EXT	P		Clock Source
0	0		Timer clocked internally at ¼ CLKOUT frequency.
0	1		Timer clocked internally, prescaled by Timer 2.
1	X		Timer clocked externally at up to ¼ CLKOUT frequency.

9.2.3Counting Modes

All timers have a Timer Count register and a Maxcount Compare A register. Timers 0 and 1 also have access to a second Maxcount Compare B register. Whenever the contents of the Timer Count register equal the contents of the Maxcount Compare register, the count register resets to zero. The maximum count value will never be stored in the count register. This is because the counter element increments, compares and resets a timer in one clock cycle. Therefore, the max- imum value is never written back to the count register. The Maxcount Compare register can be written at any time during timer operation.

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