P83C524 datasheet(23/76 Pages) PHILIPS

23 / 76 page

1997 Dec 15

Philips Semiconductors

Product speciﬁcation

8-bit microcontrollers

P83C524; P80C528; P83C528

11 TIMERS/COUNTERS

The P83C528 contains three 16-bit timer/counters, Timer

0, Timer 1 and Timer 2, and one 8-bit timer, the Watchdog

Timer T3. Timer 0, Timer 1 and Timer 2 may be

programmed to carry out the following functions:

• measure time intervals and pulse durations

• count events

• generate interrupt requests.

11.1

Timer 0 and Timer 1

Timers 0 and 1 each have a control bit in TMOD SFR that

selects the timer or counter function of the corresponding

timer. In the timer function, the register is incremented

every machine cycle. Thus, one can think of it as counting

machine cycles. Since a machine cycle consists of 12

oscillator periods, the count rate is 1

⁄12 of the oscillator

frequency.

In the counter function, the register is incremented in

response to a HIGH-to-LOW transition at the

corresponding external input pin, T0 or T1. In this function,

the external input is sampled during S5P2 of every

machine cycle. When the samples show a HIGH in one

cycle and a LOW in the next cycle, the counter is

incremented. Thus, it takes two machine cycles (24

oscillator periods) to recognize a HIGH-to-LOW transition.

There are no restrictions on the duty cycle of the external

input signal, but to ensure that a given level is sampled at

least once before it changes, it should be held for at least

one full machine cycle.

Timer 0 and Timer 1 can be programmed independently to

operate in one of four modes:

Mode 0 8-bit timer/counter with divide-by-32 prescaler

Mode 1 16-bit timer/counter

Mode 2 8-bit timer/counter with automatic reload

Mode 3 Timer 0: one 8-bit timer/counter and one 8-bit

timer. Timer 1: stopped.

When Timer 0 is in Mode 3, Timer 1 can be programmed

to operate in Modes 0, 1 or 2 but cannot set an interrupt

request flag and generate an interrupt. However, the

overflow from Timer 1 can be used to pulse the Serial Port

transmission-rate generator. With a 16 MHz crystal, the

counting frequency of these timer/counters is as follows:

• in the timer function, the timer is incremented at a

frequency of 1.33 MHz (oscillator frequency divided by

12).

• in the counter function, the frequency handling range for

external inputs is 0 Hz to 0.66 MHz.

Both internal and external inputs can be gated to the timer

by a second external source for directly measuring pulse

duration.

The timers are started and stopped under software control.

Each one sets its interrupt request flag when it overflows

from all logic 1's to all logic 0's (respectively, the automatic

reload value), with the exception of Mode 3 as previously

described.

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