Intel 80C186XL, 80C188XL Programming the TIMER/COUNTER Unit, Timer/Counter Unit Output Modes

TIMER/COUNTER UNIT

When configured for internal clocking, the Timer/Counter Unit uses the input pins either to en- able timer counting or to retrigger the associated timer. Externally, a timer increments on low-to- high transitions on its input pin (up to ¼ CLKOUT frequency).

Timers 0 and 1 each have a single output pin. Timer output can be either a single pulse, indicating the end of a timing cycle, or a variable duty cycle wave. These two output options correspond to single maximum count mode and dual maximum count mode, respectively (Figure 9-4). Inter- rupts can be generated at the end of every timing cycle.

Timer 2 has no input or output pins and can be operated only in single maximum count mode (Figure 9-4). It can be used as a free-running clock and as a prescaler to Timers 0 and 1. Timer 2 can be clocked only internally, at ¼ CLKOUT frequency. Timer 2 can also generate interrupts at the end of every timing cycle.

Maxcount A Maxcount B

Dual Maximum

Count Mode

	One CPU
Maxcount A	Clock

Single Maximum

Count Mode

A1296-0A

Figure 9-4. Timer/Counter Unit Output Modes

9.2PROGRAMMING THE TIMER/COUNTER UNIT

Each timer has three registers: a Timer Control register (Figure 9-5 and Figure 9-6), a Timer Count register (Figure 9-7) and a Timer Maxcount Compare register (Figure 9-8). Timers 0 and 1 also have access to an additional Maxcount Compare register. The Timer Control register con- trols timer operation. The Timer Count register holds the current timer count value, and the Max- count Compare register holds the maximum timer count value.

9-6