8254 Просмотр технического описания (PDF) - Intel

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8254

PROGRAMMABLE INTERVAL TIMER

Intel 

8254

respectively Both are normally referred to as one

unit and called just OL These latches normally ‘‘fol-

low’’ the CE but if a suitable Counter Latch Com-

mand is sent to the 8254 the latches ‘‘latch’’ the

present count until read by the CPU and then return

to ‘‘following’’ the CE One latch at a time is enabled

by the counter’s Control Logic to drive the internal

bus This is how the 16-bit Counter communicates

over the 8-bit internal bus Note that the CE itself

cannot be read whenever you read the count it is

the OL that is being read

Similarly there are two 8-bit registers called CRM

and CRL (for ‘‘Count Register’’) Both are normally

referred to as one unit and called just CR When a

new count is written to the Counter the count is

stored in the CR and later transferred to the CE The

Control Logic allows one register at a time to be

loaded from the internal bus Both bytes are trans-

ferred to the CE simultaneously CRM and CRL are

cleared when the Counter is programmed In this

way if the Counter has been programmed for one

byte counts (either most significant byte only or least

significant byte only) the other byte will be zero

Note that the CE cannot be written into whenever a

count is written it is written into the CR

The Control Logic is also shown in the diagram

CLK n GATE n and OUT n are all connected to the

outside world through the Control Logic

8254 SYSTEM INTERFACE

The 8254 is a component of the Intel Microcomputer

Systems and interfaces in the same manner as all

other peripherals of the family It is treated by the

system’s software as an array of peripheral I O

ports three are counters and the fourth is a control

register for MODE programming

Basically the select inputs A0 A1 connect to the A0

A1 address bus signals of the CPU The CS can be

derived directly from the address bus using a linear

select method Or it can be connected to the output

of a decoder such as an Intel 8205 for larger sys-

tems

OPERATIONAL DESCRIPTION

General

After power-up the state of the 8254 is undefined

The Mode count value and output of all Counters

are undefined

How each Counter operates is determined when it is

programmed Each Counter must be programmed

before it can be used Unused counters need not be

programmed

Programming the 8254

Counters are programmed by writing a Control Word

and then an initial count

The Control Words are written into the Control Word

Register which is selected when A1 A0 e 11 The

Control Word itself specifies which Counter is being

programmed

Figure 6 8254 System Interface

231164 – 6

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