Measurement Computing DaqBoard 3000USB Series - Manual (Page 63)

Counter Input Modes 5
Tips for Making High-Speed Counter Measurements ( > 1 MHz ) …… 5-1
Debounce Module …… 5-1
Terms Applicable to Counter Modes…….5-5
Counter Options …… 5-5
Counter/Totalize Mode …… 5-6
Period Mode …… 5-8
Pulsewidth Mode …… 5-11
Timing Mode …… 5-13
Encoder Mode …… 5-15
Note: Each of the high-speed, 32-bit counter channels can be configured for counter,
period, pulse width, time between edges, or encoder modes.
Tips for Making High-Speed Counter Measurements ( > 1 MHz )
o Use coax or twisted-pair wire. Connect one side to Digital Common.
o If the frequency source is tolerant, parallel-terminate the coax (or twisted-pair) with a 50 ohm or
100 ohm resistor at the terminal block.
o The amplitude of the driving waveform should be as high as possible without violating the over-voltage
specification.
o To ensure adequate switching, waveforms should swing at least 0V to 5V and have a high slew rate.
Debounce
Each channel’s output can be debounced with 16 programmable debounce times from 500 ns to 25.5 ms.
The debounce circuitry eliminates switch-induced transients typically associated with electro-mechanical
devices including relays, proximity switches, and encoders.
From the following illustration we can see that there are two debounce modes, as well as a debounce
bypass. In addition, the signal from the buffer can be inverted before it enters the debounce circuitry. The
inverter is used to make the input rising-edge or falling-edge sensitive.
Edge selection is available with or without debounce. In this case the debounce time setting is ignored and
the input signal goes straight from the inverter [or inverter bypass] to the counter module.
There are 16 different debounce times. In either debounce mode, the debounce time selected determines
how fast the signal can change and still be recognized.
The two debounce modes are “trigger after stable” and “trigger before stable.” A discussion of the two
modes follows.
Debounce Model
DaqBoard/3000USB Series User’s Manual 887794 Counter Input Modes 5-1