User`s guide

PCI-2513 User's Guide Functional Details
18
When reading synchronously, all counters are set to zero at the start of an acquisition. When reading
asynchronously, counters may be cleared on each read, count up continually, or count until the 16 bit or 32 bit
limit has been reached. See counter mode descriptions below.
Figure 5. Typical PCI-2513 counter channel
Mapped channels
A mapped channel is one of four counter input signals that can get multiplexed into a counter module. The
mapped channel can participate with the counter's input signal by gating the counter, latching the counter, and
so on. The four possible choices for the mapped channel are the four counter input signals (post-debounce).
A mapped channel can be used to:
 gate the counter
 decrement the counter
 latch to current count to the count register
Usually, all counter outputs are latched at the beginning of each scan within the acquisition. However, you can
use a second channel—known as the mapped channel—to latch the counter output.
Counter modes
A counter can be asynchronously read with or without clear on read. The asynchronous read-signals strobe
when the lower 16-bits of the counter are read by software. The software can read the counter's high 16-bits
some time later after reading the lower 16-bits. The full 32-bit result reflects the timing of the first
asynchronous read strobe.
Totalize mode
The Totalize mode allows basic use of a 32-bit counter. While in this mode, the channel's input can only
increment the counter upward. When used as a 16-bit counter (counter low), one channel can be scanned at the
12 MHz rate. When used as a 32-bit counter (counter high), two sample times are used to return the full 32-bit
result. Therefore a 32-bit counter can only be sampled at a 6 MHz maximum rate. If you only want the upper 16
bits of a 32-bit counter, then you can acquire that upper word at the 12 MHz rate.
The counter counts up and does not clear on every new sample. However, it does clear at the start of a new scan
command.
The counter rolls over on the 16-bit (counter low) boundary, or on the 32-bit (counter high) boundary.
Clear on read mode
The counter counts up and is cleared after each read. By default, the counter counts up and only clears the
counter at the start of a new scan command. The final value of the counter —the value just before it was
cleared—is latched and returned to the PCI-2513.