Chapter 3 Theory of Operation
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National Instruments Corporation 3-27 AT-MIO-16X User Manual
Figure 3-18. Counter Block Diagram
Each counter has a SOURCE input pin, a GATE input pin, and an output
pin labeled OUT. The Am9513A counters are numbered 1 throug h 5 ,
and their GATE, SOURCE, and OUT pins are labeled GATE N,
SOURCE N, and OUT N, where N is the counter number.
For counting operations, the counters can be programme d to use any of
the five internal timebases, any of the five GATE and five SOURCE
inputs to the Am9513A, and the output of the previous counter
(Counter 4 uses Counter 3 output, and so on). A counter can be
configured to count either falling or rising edges of the selected input.
The counter GATE input allows counter operation to be gated. Once a
counter is configured for an operation through software, a signal at the
GATE input can be used to start and stop counter operation. The five
gating modes available with the Am9513A are as follows:
• No gating
• Level gating active high
• Level gating active low
• Low-to-high edge gating
• High-to-low edge gating
A counter can also be active high level gated by a signal at GATE N+1
and GATE N-1, where N is the counter number.
The counter generates timing signals at its OUT output pin. Th e OUT
output pin can also be set to a high-impedance state or a
grounded-output state. The counters generate two types of output
signals during counter operation: terminal count pulse output and
terminal count toggle output. Terminal count is often referred to as TC.
A counter reaches TC when it counts up or down and rolls ove r. In many
SOURCE
GATE
OUT
Counter