8254 Просмотр технического описания (PDF) - Intel
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8254 Datasheet PDF : 21 Pages
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8254
Signal
Status
Modes
Low
Or Going
Low
Rising
High
0
Disables
Counting
Enables
Counting
1
1) Initiates
Counting
2) Resets Output
after Next
Clock
2
1) Disables
Counting
Initiates
2) Sets Output
Counting
Immediately
High
Enables
Counting
3
1) Disables
Counting
Initiates
2) Sets Output
Counting
Immediately
High
Enables
Counting
4
Disables
Counting
Enables
Counting
5
Initiates
Counting
Figure 21 Gate Pin Operations Summary
Mode
0
1
2
3
4
5
Min
Count
1
1
2
2
1
1
Max
Count
0
0
0
0
0
0
NOTE
0 is equivalent to 216 for binary counting and 104 for
BCD counting
Figure 22 Minimum and Maximum Initial Counts
Operation Common to All Modes
PROGRAMMING
When a Control Word is written to a Counter all
Control Logic is immediately reset and OUT goes to
a known initial state no CLK pulses are required for
this
GATE
The GATE input is always sampled on the rising
edge of CLK In Modes 0 2 3 and 4 the GATE input
is level sensitive and the logic level is sampled on
the rising edge of CLK In Modes 1 2 3 and 5 the
GATE input is rising-edge sensitive In these Modes
a rising edge of GATE (trigger) sets an edge-sensi-
tive flip-flop in the Counter This flip-flop is then sam-
pled on the next rising edge of CLK the flip-flop is
reset immediately after it is sampled In this way a
trigger will be detected no matter when it occurs a
high logic level does not have to be maintained until
the next rising edge of CLK Note that in Modes 2
and 3 the GATE input is both edge- and level-sensi-
tive In Modes 2 and 3 if a CLK source other than
the system clock is used GATE should be pulsed
immediately following WR of a new count value
COUNTER
New counts are loaded and Counters are decre-
mented on the falling edge of CLK
The largest possible initial count is 0 this is equiva-
lent to 216 for binary counting and 104 for BCD
counting
The Counter does not stop when it reaches zero In
Modes 0 1 4 and 5 the Counter ‘‘wraps around’’ to
the highest count either FFFF hex for binary count-
ing or 9999 for BCD counting and continues count-
ing Modes 2 and 3 are periodic the Counter reloads
itself with the initial count and continues counting
from there
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