P89C738ABP datasheet(19/64 Pages) PHILIPS

19 / 64 page

1998 Apr 07

Philips Semiconductors

Product speciﬁcation

8-bit Flash microcontrollers

P89C738; P89C739

10 TIMERS/COUNTERS

The P89C738 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.

10.1

Timer 0 and Timer 1

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

⁄12 × oscillator frequency)

• In the counter function, the frequency handling range for

external inputs is 0 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.

10.1.1

Timer/Counter Mode Control Register (TMOD)

Table 9

Timer/Counter Mode Control Register (SFR address 89H)

Table 10 Description of TMOD bits for Timer 1 and Timer 0

Timer 0: bit TMOD.0 to TMOD.3; Timer 1: bit TMOD.4 to TMOD.7; n = 0, 1.

76543210

GATE

C/TM1

GATE

C/TM1

BIT

SYMBOL

DESCRIPTION

7 and 3

GATE

Gating control. When set Timer/counter ‘n’ is enabled only when INTn pin is HIGH and

control bit TRn (TR1 or TR0) is set. When cleared Timer n is enabled whenever TRn

control bit is set.

6 and 2

C/T

Timer or Counter Selector. Cleared for Timer operation; input from internal system

clock. Set for Counter operation; input from pin Tn (T1 or T0).

5 and 1

Timer 0, Timer 1 mode select; see Table 11.

4 and 0

P89C738ABP Datasheet(PDF) 19 Page - NXP Semiconductors

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