Intel 80C188XL, 80C186XL user manual Chapter TIMER/COUNTER Unit

CHAPTER 9

TIMER/COUNTER UNIT

The Timer/Counter Unit can be used in many applications. Some of these applications include a real-time clock, a square-wave generator and a digital one-shot. All of these can be implemented in a system design. A real-time clock can be used to update time-dependent memory variables. A square-wave generator can be used to provide a system clock tick for peripheral devices. (See “Timer/Counter Unit Application Examples” on page 9-17 for code examples that configure the Timer/Counter Unit for these applications.)

9.1FUNCTIONAL OVERVIEW

The Timer/Counter Unit is composed of three independent 16-bit timers (see Figure 9-1). The op- eration of these timers is independent of the CPU. The internal Timer/Counter Unit can be mod- eled as a single counter element, time-multiplexed to three register banks. The register banks are dual-ported between the counter element and the CPU. During a given bus cycle, the counter el- ement and CPU can both access the register banks; these accesses are synchronized.

The Timer/Counter Unit is serviced over four clock periods, one timer during each clock, with an idle clock at the end (see Figure 9-2). No connection exists between the counter element’s se- quencing through timer register banks and the Bus Interface Unit’s sequencing through T-states. Timer operation and bus interface operation are asynchronous. This time-multiplexed scheme re- sults in a delay of 2½ to 6½ CLKOUT periods from timer input to timer output.

Each timer keeps its own running count and has a user-defined maximum count value. Timers 0 and 1 can use one maximum count value (single maximum count mode) or two alternating max- imum count values (dual maximum count mode). Timer 2 can use only one maximum count val- ue. The control register for each timer determines the counting mode to be used. When a timer is serviced, its present count value is incremented and compared to the maximum count for that tim- er. If these two values match, the count value resets to zero. The timers can be configured either to stop after a single cycle or to run continuously.

Timers 0 and 1 are functionally identical. Figure 9-3 illustrates their operation. Each has a latched, synchronized input pin and a single output pin. Each timer can be clocked internally or externally. Internally, the timer can either increment at ¼ CLKOUT frequency or be prescaled by Timer 2. A timer that is prescaled by Timer 2 increments when Timer 2 reaches its maximum count value.

9-1