Measurement Computing PCI-2515, Руководство пользователя

Страница: 21 / 34

PCI-2515 User's Guide Functional Details
21
A mapped channel can be used to:
ƒ gate the counter
ƒ decrement the counter
ƒ 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 Error! Reference source not found. 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 modes 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-2515.
Stop at the top mode
The counter stops at the top of its count. The top of the count is FFFF hex (65,535) for the 16-bit mode, and
FFFFFFFF hex (4,294,967,295) for the 32-bit mode.
32-bit or 16-bit
Sets the counter type to either 16-bits or 32-bits . The type of counter only matters if the counter is using the
stop at the top mode—otherwise, this option is ignored.
Latch on map
Sets the signal on the mapped counter input to latch the count.
By default, the start of scan signal—a signal internal to the PCI-2515 pulses once every scan period to indicate
the start of a scan group—latches the count, so the count is updated each time a scan is started.
Gating "on" mode
Sets the gating option to "on" for the mapped channel, enabling the mapped channel to gate the counter.