8254 Просмотр технического описания (PDF) - Intel

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Компоненты Описание

производитель

8254

PROGRAMMABLE INTERVAL TIMER

Intel 

8254

Control Word Format

A1 A0 e 11 CS e 0 RD e 1 WR e 0

D7 D6 D5 D4 D3 D2 D1 D0

SC1 SC0 RW1 RW0 M2 M1 M0 BCD

SC Select Counter

SC1

SC0

M Mode

M2

M1

M0

0

0

Select Counter 0

0

0

0

Mode 0

0

1

Select Counter 1

0

0

1

Mode 1

1

0

Select Counter 2

X

1

0

Mode 2

1

1

Read-Back Command

(see Read Operations)

X

1

1

Mode 3

1

0

0

Mode 4

RW Read Write

RW1 RW0

0 0 Counter Latch Command (see Read

Operations)

0 1 Read Write least significant byte only

1 0 Read Write most significant byte only

1

0

1

Mode 5

BCD

0

1

Binary Counter 16-bits

Binary Coded Decimal (BCD) Counter

(4 Decades)

1 1 Read Write least significant byte first

then most significant byte

NOTE

Don’t care bits (X) should be 0 to insure compatibility with future Intel products

Figure 7 Control Word Format

By contrast initial counts are written into the Coun-

ters not the Control Word Register The A1 A0 in-

puts are used to select the Counter to be written

into The format of the initial count is determined by

the Control Word used

Write Operations

The programming procedure for the 8254 is very

flexible Only two conventions need to be remem-

bered

1) For each Counter the Control Word must be writ-

ten before the initial count is written

2) The initial count must follow the count format

specified in the Control Word (least significant

byte only most significant byte only or least sig-

nificant byte and then most significant byte)

Since the Control Word Register and the three

Counters have separate addresses (selected by the

A1 A0 inputs) and each Control Word specifies the

Counter it applies to (SC0 SC1 bits) no special in-

struction sequence is required Any programming

sequence that follows the conventions in Figure 7 is

acceptable

A new initial count may be written to a Counter at

any time without affecting the Counter’s pro-

grammed Mode in any way Counting will be affected

as described in the Mode definitions The new count

must follow the programmed count format

If a Counter is programmed to read write two-byte

counts the following precaution applies A program

must not transfer control between writing the first

and second byte to another routine which also writes

into that same Counter Otherwise the Counter will

be loaded with an incorrect count

6

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