8254 データシートの表示（PDF） - Intel

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8254

PROGRAMMABLE INTERVAL TIMER

Intel 

8254

Mode Definitions

The following are defined for use in describing the

operation of the 8254

CLK Pulse

a rising edge then a falling edge in

that order of a Counter’s CLK in-

put

Trigger

a rising edge of a Counter’s GATE

input

Counter loading the transfer of a count from the CR

to the CE (refer to the ‘‘Functional

Description’’)

MODE 0 INTERRUPT ON TERMINAL COUNT

Mode 0 is typically used for event counting After the

Control Word is written OUT is initially low and will

remain low until the Counter reaches zero OUT then

goes high and remains high until a new count or a

new Mode 0 Control Word is written into the Coun-

ter

GATE e 1 enables counting GATE e 0 disables

counting GATE has no effect on OUT

After the Control Word and initial count are written to

a Counter the initial count will be loaded on the next

CLK pulse This CLK pulse does not decrement the

count so for an initial count of N OUT does not go

high until N a 1 CLK pulses after the initial count is

written

If a new count is written to the Counter it will be

loaded on the next CLK pulse and counting will con-

tinue from the new count If a two-byte count is writ-

ten the following happens

1) Writing the first byte disables counting OUT is set

low immediately (no clock pulse required)

2) Writing the second byte allows the new count to

be loaded on the next CLK pulse

This allows the counting sequence to be synchroniz-

ed by software Again OUT does not go high until

Na1 CLK pulses after the new count of N is written

If an initial count is written while GATE e 0 it will

still be loaded on the next CLK pulse When GATE

goes high OUT will go high N CLK pulses later no

CLK pulse is needed to load the Counter as this has

already been done

MODE 1 HARDWARE RETRIGGERABLE

ONE-SHOT

OUT will be initially high OUT will go low on the CLK

pulse following a trigger to begin the one-shot pulse

and will remain low until the Counter reaches zero

10

OUT will then go high and remain high until the CLK

pulse after the next trigger

After writing the Control Word and initial count the

Counter is armed A trigger results in loading the

Counter and setting OUT low on the next CLK pulse

thus starting the one-shot pulse An initial count of N

will result in a one-shot pulse N CLK cycles in dura-

tion The one-shot is retriggerable hence OUT will

remain low for N CLK pulses after any trigger The

one-shot pulse can be repeated without rewriting the

same count into the counter GATE has no effect on

OUT

If a new count is written to the Counter during a one-

shot pulse the current one-shot is not affected un-

less the counter is retriggered In that case the

Counter is loaded with the new count and the one-

shot pulse continues until the new count expires

MODE 2 RATE GENERATOR

This Mode functions like a divide-by-N counter It is

typically used to generate a Real Time Clock inter-

rupt OUT will initially be high When the initial count

has decremented to 1 OUT goes low for one CLK

pulse OUT then goes high again the Counter re-

loads the initial count and the process is repeated

Mode 2 is periodic the same sequence is repeated

indefinitely For an initial count of N the sequence

repeats every N CLK cycles

GATE e 1 enables counting GATE e 0 disables

counting If GATE goes low during an output pulse

OUT is set high immediately A trigger reloads the

Counter with the initial count on the next CLK pulse

OUT goes low N CLK pulses after the trigger Thus

the GATE input can be used to synchronize the

Counter

After writing a Control Word and initial count the

Counter will be loaded on the next CLK pulse OUT

goes low N CLK Pulses after the initial count is writ-

ten This allows the Counter to be synchronized by

software also

Writing a new count while counting does not affect

the current counting sequence If a trigger is re-

ceived after writing a new count but before the end

of the current period the Counter will be loaded with

the new count on the next CLK pulse and counting

will continue from the new count Otherwise the

new count will be loaded at the end of the current

counting cycle In mode 2 a COUNT of 1 is illegal

MODE 3 SQUARE WAVE MODE

Mode 3 is typically used for Baud rate generation

Mode 3 is similar to Mode 2 except for the duty cycle

of OUT OUT will initially be high When half the

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