Kontron PCI-AOB16/12, Руководство пользователя

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Introduction 6-3 PCI-AOB8/12 (16/12) User’s Guide
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Introduction
These cards contain a type 8254 programmable counter/timer that allows you to implement such
functions as a Real Time Clock, Event Counter, Digital One-Shot, Programmable Rate Generator,
Binary Rate Multiplier, Complex Wave Generator and/or a Motor Controller. The 8254 consists of
three, 16-bit, presettable, down counters. Each counter can be programmed to any count
between 1 or 2 and 65,535 in binary format depending on the mode chosen.
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Operational Modes
The 8254 modes of operation are described in the following paragraphs to familiarize you with
the versatility and power of this device. For those interested in more detailed information, a full
description of the 8254 programmable interval timer can be found in the Intel (or equivalent
manufacturers') data sheets. The following conventions apply for use in describing operation of
the 8254:
Clock: A positive pulse into the counter's clock input
Trigger: A rising edge input to the counter's gate input
Counter Loading: Programming a binary count into the counter
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Mode 0: Pulse on Terminal Count
After the counter is loaded, the output is set low and will remain low until the counter
decrements to zero. The output then goes high and remains high until a new count is loaded into
the counter. A trigger enables the counter to start decrementing.
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Mode 1: Retriggerable One-Shot
The output goes low on the clock pulse following a trigger to begin the one-shot pulse and goes
high when the counter reaches zero. Additional triggers result in reloading the count and
starting the cycle over. If a trigger occurs before the counter decrements to zero, a new count is
loaded. This forms a retriggerable one-shot. In mode 1, a low output pulse is provided with a
period equal to the counter count-down time.