AT89LP428 datasheet(41/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

11. Enhanced Timer 0 and Timer 1 with PWM

The AT89LP428/828 has two 16-bit Timer/Counters, Timer 0 and Timer 1, with the

following features:

• Two 16-bit timer/counters with 16-bit reload registers

• Two independent 8-bit precision PWM outputs with 8-bit prescalers

•UART or SPI baud rate generation using Timer 1

•Output pin toggle on timer overflow

• Split timer mode allows for three separate timers (two 8-bit, one 16-bit)

•Gated modes allow timers to run/halt based on an external input

Timer 0 and Timer 1 have similar modes of operation. As timers, they increase every clock cycle

by default. Thus, the registers count clock cycles. Since a clock cycle consists of one oscillator

period, the count rate is equal to the oscillator frequency. The timer rate can be prescaled by a

value between 1 and 16 using the Timer Prescaler (see Table 6-2 on page 23). Both Timers

share the same prescaler.

As counters, the timer registers are incremented in response to a 1-to-0 transition at the corre-

sponding input pins, T0 or T1. The external input is sampled every clock cycle. When the

samples show a high in one cycle and a low in the next cycle, the count is incremented. The new

count value appears in the register during the cycle following the one in which the transition was

detected. Since 2 clock cycles are required to recognize a 1-to-0 transition, the maximum count

rate is 1/2 of the oscillator frequency. There are no restrictions on the duty cycle of the input sig-

nal, but it should be held for at least one full clock cycle to ensure that a given level is sampled at

least once before it changes.

Furthermore, the Timer or Counter functions for Timer 0 and Timer 1 have four operating modes:

variable width timer, 16-bit auto-reload timer, 8-bit auto-reload timer, and split timer. The control

bits C/T in the Special Function Register TMOD select the Timer or Counter function. The bit

pairs (M1, M0) in TMOD select the operating modes.

Table 11-1.

Timer 0/1 Register Summary

Name

Address

Purpose

Bit-Addressable

TCON

88H

Control

TMOD

89H

Mode

TL0

8AH

Timer 0 low-byte

TL1

8BH

Timer 1 low-byte

TH0

8CH

Timer 0 high-byte

TH1

8DH

Timer 1 high-byte

TCONB

91H

Mode

RL0

92H

Timer 0 reload low-byte

RL1

93H

Timer 1 reload low-byte

RH0

94H

Timer 0 reload high-byte

RH1

95H

Timer 1 reload high-byte

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