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Chapter 2 Configuration and Installation
© National Instruments Corporation 2-21 Lab-NB User Manual
8253 digital input specifications (referenced to DGND):
V
IH
input logic high voltage 2.2 V min
V
IL
input logic low voltage 0.8 V max
Input load current ±10 µA max
8253 digital output specifications (referenced to DGND):
V
OH
output logic high voltage 2.4 V min
V
OL
output logic low voltage 0.45 V max
I
OH
output source current, at V
OH
400 µA max
I
OL
output sink current, at V
OL
2.2 mA max
General-Purpose Timing Signal Connections
The general-purpose timing signals include the GATE, CLK, and OUT signals for the three
8253(B) counters. The 8253 Counter/Timers can be used for general-purpose applications such
as pulse and square wave generation; event counting; and pulse-width, time-lapse, and frequency
measurement. For these applications, CLK and GATE signals are sent to the counters, and the
counters are programmed for various operations. The single exception is counter B0, which has
an internal 2-MHz clock.
The 8253 Counter/Timer is described briefly in Chapter 3, Theory of Operation
. For detailed
programming information, consult Appendix C, AMD 8253 Data Sheet.
Pulse and square wave generation are performed by programming a counter to generate a timing
signal at its OUT output pin.
Event counting is performed by programming a counter to count rising or falling edges applied
to any of the 8253 CLK inputs. The counter value can then be read to determine the number of
edges that have occurred. Counter operation can be gated on and off during event counting.
Figure 2-16 shows connections for a typical event-counting operation where a switch is used to
gate the counter on and off.