National Instruments AT-DIO-32F, Руководство пользователя

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Chapter 3 Theory of Operation
© National Instruments Corporation 3-3 AT-DIO-32F User Manual
If a port is configured as a single-buffered write port, the data written to that port is latched into
the port and driven on the corresponding digital I/O lines. Reading the port returns the data that
is currently driven by the port. Write ports configured for double-buffering are often used for
pattern generation. A double-buffered write port consists of two buffers: the write buffer and
the output buffer. Data written to the port is loaded into the write buffer. When a handshaking
request, REQ1 or REQ2, is received on the I/O connector for the double-buffered group, the
contents of the write buffer are loaded into the output buffer. Data loaded in the output buffer is
driven on the digital I/O lines.
Onboard Counters
The AT-DIO-32F includes three onboard counters, useful for pattern generation and periodic
data acquisition. Counter 1 can be programmed to generate group 1 handshaking requests on the
REQ1 line. Likewise, counter 2 can be programmed to generate group 2 handshaking requests
on the REQ2 line. Counter 3 can alter the counting rate of the other two counters and can also be
programmed to generate periodic interrupts.
The clock that runs the counters, BRDCLK, connects to a 10 MHz clock source: either an
onboard clock, OSC, or to the RTSI clock line, RTSICLK. With BRDCLK connected to both
OSC and RTSICLK, the onboard clock source can drive both the counters and the RTSI clock
line. See RTSI Bus Clock Selection in Chapter 2, Configuration and Installation, for information
about connecting the BRDCLK signal.
The signal you select for BRDCLK is slowed by a factor of 5, creating a 2 MHz clock signal that
operates Counter 3. The output of Counter 3 forms a time base for Counters 1 and 2, but
Counters 1 and 2 can also run directly from the BRDCLK clock, if so directed in CFG3 register.
Figure 3-2 shows the clock routing scheme.