參數(shù)資料
型號(hào)： P8XC557E8
廠商： NXP Semiconductors N.V.
元件分類： 8位微控制器
英文描述： 8 BIT MICROCONTROLLER
中文描述： 8位微控制器
文件頁(yè)數(shù)： 25/84頁(yè)
文件大?。?/td> 322K
代理商： P8XC557E8
1999 Mar 12
25
Philips Semiconductors
Product specification
8-bit microcontroller
P8xC557E8
12 TIMERS/COUNTERS
The P8xC557E8 contains,
Three 16-bit timer/event counters:
Timer 0, Timer 1 and Timer T2
One 8-bit timer, T3.
12.1
Timer 0 and Timer 1
Timer 0 and Timer 1 may be programmed to carry out the
following functions:
Measure time intervals and pulse durations
Count events
Generate interrupt requests.
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
×
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. 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 or 8-bit counter each with divide-by-32
prescaler.
Mode 1 16-bit time-interval or event counter.
Mode 2 8-bit time-interval or event counter with automatic
reload upon overflow.
Mode 3 Timer 0: one 8-bit time-interval or event counter
and one 8-bit time-interval counter.
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 or generate an interrupt. However, the
overflow from Timer 1 can be used to pulse the serial port
baud rate generator. With a 16 MHz crystal, the counting
frequency of these timers/counters is as follows:
In the timer function, the timer is incremented at a
frequency of 1.33 MHz (
1
12
×
the system clock
frequency)
When programmed for external inputs: 0 to 660 kHz
(
1
24
×
the system clock frequency).
Both internal and external inputs can be gated to the
counter by a second external source for directly measuring
pulse durations. When configured as a counter, the
register is incremented on every falling edge on the
corresponding input pin T0 or T1. The earliest moment, the
incremented register value can be read is during the
second machine cycle following the machine cycle within
which the incrementing pulse occurred.
The counters are started and stopped under software
control. Each one sets its interrupt request flag when it
overflows from all HIGHs to all LOWs (or automatic reload
value), with the exception of Mode 3 as previously
described.
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