Programming instructions

Chapter 24 Generating a Square Pulse or Pulse Trains
LabVIEW Data Acquisition Basics Manual 24-22
© National Instruments Corporation
Knowing the Accuracy of Your Counters
When you generate a waveform, there can be an uncertainty of up to one
timebase period between the start signal and the first counted edge of the
timebase. This is due to the uncertainty in the exact relation of the start
signal, which the software calls or the gate signal supplies to the first edge
of the timebase, as shown in Figures 24-27.
Figure 24-27.
Uncertainty of One Timebase Period
8253/54
In addition to the above uncertainty, the 8253/54 chip has an additional
uncertainty when used in mode 0. Mode 0 generates a low pulse for a
chosen number of clock cycles, but a software delay is involved. This
delay is because with mode 0 the counter output is set low by a software
write to the mode setting. Afterward the count can be loaded and the
counter starts counting down. The time between setting the output to low
and loading the count is included in the output pulse. This time was found
to be 20 microseconds when tested on a 200 MHz Pentium computer.
phase 1 phase 2
uncertainty of
1 timebase period
output
timebase
starting
signal
1 timebase period