8254 Просмотр технического описания (PDF) - Intel

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8254

PROGRAMMABLE INTERVAL TIMER

Intel 

8254

231164 – 3

Figure 3 Block Diagram Showing Data Bus Buffer and Read Write Logic Functions

READ WRITE LOGIC

The Read Write Logic accepts inputs from the sys-

tem bus and generates control signals for the other

functional blocks of the 8254 A1 and A0 select one

of the three counters or the Control Word Register

to be read from written into A ‘‘low’’ on the RD in-

put tells the 8254 that the CPU is reading one of the

counters A ‘‘low’’ on the WR input tells the 8254

that the CPU is writing either a Control Word or an

initial count Both RD and WR are qualified by CS

RD and WR are ignored unless the 8254 has been

selected by holding CS low

CONTROL WORD REGISTER

The Control Word Register (see Figure 4) is selected

by the Read Write Logic when A1 A0 e 11 If the

CPU then does a write operation to the 8254 the

data is stored in the Control Word Register and is

interpreted as a Control Word used to define the

operation of the Counters

The Control Word Register can only be written to

status information is available with the Read-Back

Command

COUNTER 0 COUNTER 1 COUNTER 2

These three functional blocks are identical in opera-

tion so only a single Counter will be described The

internal block diagram of a single counter is shown

in Figure 5

The Counters are fully independent Each Counter

may operate in a different Mode

The Control Word Register is shown in the figure it

is not part of the Counter itself but its contents de-

termine how the Counter operates

The status register shown in Figure 5 when

latched contains the current contents of the Control

Word Register and status of the output and null

count flag (See detailed explanation of the Read-

Back command )

The actual counter is labelled CE (for ‘‘Counting Ele-

ment’’) It is a 16-bit presettable synchronous down

counter

OLM and OLL are two 8-bit latches OL stands for

‘‘Output Latch’’ the subscripts M and L stand for

‘‘Most significant byte’’ and ‘‘Least significant byte’’

3

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