Measurement Computing

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USB-CTR Series

Features

The following example measures the time between the rising

edge on a counter input (

CxIN) and the falling edge on the

counter gate (

CxGT). The counter read operation returns zeroes

until one complete time measurement has been taken. Then,

the time in ticks is latched by the device until the next time

measurement has been completed. Rising edges on the counter

input channel clear the counter and falling edges on the gate

input latch the output of the counter at that time.

Example of a counter input channel in timing mode

The data returned is interpreted as time measured in ticks. This

data represents the number of tick size intervals counted during

the timing measurement.

Tick size: The tick size (or timing resolution) is a fundamental

unit of time derived from the period of the 96 MHz system clock.

Four counter channel tick sizes are available – 20.83 ns, 208.3

ns, 2083.3 ns, and 20833.3 ns.

Debounce Filters

USB-CTR Series devices have debounce circuitry which elimi-

nates switch-induced transients that are typically associated

with electromechanical devices including relays, proximity

switches, and encoders.

All debounce filter options are software selectable. You can select

a debounce time, debounce mode, and rising-edge or falling-edge

sensitivity. Each channel can be debounced with 16 program-

mable debounce times in the range of 500 ns to 25.5 ms.

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.

Debounce filter modes – trigger after stable and trigger before stable

– and a debounce bypass

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.

The two debounce filter modes are trigger after stable and trigger

before stable. In either mode, the selected debounce time deter-

mines how fast the signal can change and still be recognized.

Trigger After Stable Mode

In trigger after stable mode, the output of the debounce mod-

ule does not change state until a period of stability has been

achieved. The input has an edge, and then must be stable for a

period of time equal to the debounce time.

Trigger Before Stable Mode

In trigger before stable mode, the output of the debounce module

immediately changes state, but does not change state again until

a period of stability has passed. Use this mode to detect glitches.

Debounce Filter Mode Comparisons

The following diagram shows how the two modes interpret the

same input signal, which exhibits glitches. Notice that the trig-

ger before stable mode recognizes more glitches than the trigger

after stable mode. Use the bypass option to achieve maximum

glitch recognition.

Debounce Time

Debounce Time

Debounce Time

Input

Trigger Before Stable

Trigger After Stable

Bypass

Example of two debounce filter modes interpreting the same signal

Trigger after stable mode behaves more like a traditional debounce

function: rejecting glitches and only passing state transitions

after a required period of stability. Trigger after stable mode is

used with electromechanical devices like encoders and mechani-

cal switches to reject switch bounce and disturbances due to a

vibrating encoder that is not otherwise moving.

Digital I/O

USB-CTR Series devices can connect up to eight digital I/O

lines. The digital I/O connectors can detect the state of any

TTL-level input.

Pull-Up/Down Jumper

The digital port has 47 kΩ resistors that you can configure as

pull-up or pull-down with internal jumper.

Unconnected inputs are pulled low by default to 0 V through

47 kΩ resistors. The pull-up/down voltage is common to all of

these resistors.