National Instruments AT-MIO-16D, Руководство пользователя

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Theory of Operation Chapter 3
AT-MIO-16D User Manual 3-14 © National Instruments Corporation
The 16-bit counters in the Am9513A can be diagrammed as shown in Figure 3-7.
SOURCE
GATE
OUT
COUNTER
Figure 3-7. 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 through 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 programmed 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. There are five gating modes supported by the Am9513A: no gating, level gating active
high, level gating active low, low-to-high edge gating, and 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. The 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 over. In many counter applications, the counter reloads from an internal register when it
reaches TC. In TC pulse output mode, the counter generates a pulse during the cycle that it reaches
TC and reloads. In TC toggle output mode, the counter output changes state after it reaches TC
and reloads. In addition, the counters can be configured for positive logic output or negative
(inverted) logic output for a total of four possible output signals generated for one timing mode.
The SOURCE, GATE, and OUT pins for Counters 1, 2, and 5 of the onboard Am9513A are
located on the AT-MIO-16D I/O connector. A rising edge signal on the STOP TRIG pin of the I/O
connector sets the flip-flop output signal connected to the GATE4 input of the Am9513A and can
be used as an additional gate input. The flip-flop output connected to GATE4 is cleared when the
sample counter reaches TC, when an overflow or overrun occurs, or when the A/D Clear Register
is written to.
The Am9513A SOURCE5 pin is connected to the AT-MIO-16D RTSI switch, which means that a
signal from the RTSI trigger bus can be used as a counting source for the Am9513A counters.