Counters Topics• Counter Overview• Easy Counter VIs for:

– Event Counting– Pulse Generation– Pulse Measurement– Frequency Measurement

• Position Measurement

Counter Signals

+0.8 V

0 Vlow

+5.0 V

+2.0 V

high

indeterminate

Maximum Rise/Fall Time = 50ns

Minimum Pulse Width = 10ns

Counters accept and generate TTL signals

Parts of a Counter

• Count Register– Stores the current count

• Source– Input signal that changes the current count– Active edge (rising or falling) of input signal changes the count

– Choose if count increments or decrements on an active edge

• Gate– Input signal that controls when counting occurs– Counting can occur when gate is high, low, or between various combinations

of rising and falling edges

• Out– Output signal used to generate pulses

Gate

Source

Out

Count Register

Counter Pins• Counter gate and source are PFI pins

– PFI stands for Programmable Function Input– Allows use of one pin for multiple applications

For Example– Use pin 3 as digital trigger for analog input and counter gate

Counter Terminology

• Terminal Count– Term for the last count before a counter reaches 0

– When counter reaches max count it starts over at 0

• Resolution– The size of the counter register specified in bits

– Counter register size = 2(resolution) - 1

– Typical resolutions - 16, 24, 32 bit

• Timebase– Internal signal that can be routed to the source

– Common timebases - 100kHz, 20MHz

Different Counter Chips

• 8253 (16-bit)– Used on Lab and 1200 Series Devices

• Am9513 (16-bit)– Used on PC-TIO-10 and Legacy

Devices (i.e. AT-MIO-16F)

• DAQ-STC (24-bit)– Used on E-Series Devices– Created by NI

• NI-TIO (32-bit)– Used on 660x Devices– Created by NI

Features

Less

More

Counter Palette• Easy VIs

– Built out of Intermediate VIs

+ Easy to use

- Less flexible

• Intermediate VIs– Built out of Advanced VIs

+ Very Flexible

• Advanced VIs– Building blocks for other levels

Easy VIs

Intermediate VIs

Advanced VIs

Easy VIs

• Perform basic counter operations• Not suitable for more advanced applications• Compatible with Am9513 and DAQ-STC

Count Events or Time

Generate Delayed Pulse

Generate Pulse Train

Measure Frequency

Measure Pulse Width or Period

Counter ApplicationsEvent Counting

• Simple Event Counting• Time Measurement

Pulse Generation• Single Pulse Generation• Pulse Train Generation

Pulse Measurement• Period Measurement• Pulse Width Measurement

Frequency Measurement

Position Measurement

Event Counting

• Active edges on source signal increment the count

– Active edge can either be rising or falling

Gate

Source

Out

Count Register

Your Signal

Gate

Source

Out

Count Register

Timebase

Simple Event Counting

Time Measurement

• Timebase has known frequency– Time elapsed = (Count) x

(timebase period)

Event Counting

0 1 2 3 0TC-1 TC

Counter Armed

Source

Count

• Count will increment for each rising edge on source• You can change active edge to falling• Counter will roll over when it reaches terminal count

– Terminal count = 2(Counter resolution) - 1

Count Events or Time VI• Source Edge

– Chooses active edge of source signal (rising or falling)

• Event Source/Timebase– Chooses source signal

• Counter– Chooses counter to address

• Start/Restart– Set to TRUE to start/restart counter

• Stop– Set to TRUE to stop counter

• Count– Returns value stored in count register

• Seconds Since Start– Time since counter started

Pulse Generation

• Generates a TTL signal on counter’s out pin

Single Pulse Pulse Train

Gate

Source

Out

Count Register

Single Pulse or Pulse Train

Timebase

Pulse Characteristics

Pulse Polarity

Delay Width

High Polarity Low Polarity

Delay Width

Pulse Period = Delay + Width

Pulse Frequency =1

Pulse Period

Duty Cycle =Width

Pulse Period

Generate Delayed Pulse VI• Timebase Source

– Internal or External

• Counter– Chooses counter to

address

• Pulse Polarity– High or Low

• Pulse Delay & Width– Seconds if using internal timebase– Cycles if using external timebase

• Actual Delay & Width– May differ from desired values

because hardware has limited resolution

Generate Pulse Train VI• Pulse Polarity

– High or Low

• Counter– Chooses counter to

address

• Number of Pulses– Set to 0 for continuous– Set to -1 to stop

• Frequency (Hz)– 1/(Pulse Period)

• Duty Cycle– (Width)/(Pulse Period)

• Actual Parameters – May differ from desired values

because hardware has limited resolution

Pulse Measurement

• Use a timebase with a known frequency to measure characteristics of a unknown signal

Gate

Source

Out

Count Register

Your Signal

Timebase

Pulse Period

Period Measurement

Width

Pulse Width Measurement

• Count will increment for each rising edge on source– Counting can either start and end on rising or falling edges

• Period of Gate = (Count) x (1/source frequency)

Period Measurement

0 1 2 3 4 4

Armed

Source

Count 4

Gate

Yes No

Pulse Width Measurement

0 1 2 2 2 2

Armed

Source

Count 2

Gate

Yes No

• Count will increment for each rising edge on source– Counting can start on either rising or falling edge

• Width of Gate = (Count) x (1/source frequency)

Measure Pulse Width or Period VI• Counter

– Chooses counter to address

• Type of Measurement– Measure high pulse width– Measure low pulse width– Measure period (rising edge

to rising edge)– Measure period (falling edge

to falling edge)

• Timebase– Routed to source– DAQ-STC has choice of

100kHz or 20Mhz

• Pulse Width/Period (s)– Returns value in seconds

• Valid– False if counter rolls over

Frequency Measurement

High FrequencyLow Frequency

Measure Pulse Width

or PeriodMeasure Frequency

Your Signal

Low Frequency Measurement

Low Frequency

Measure Pulse Width

or Period

Your Signal

Measure period and take the inverse• Frequency = 1/Period

Pros• Only uses one counter• Good at low frequencies

Cons• Can see large error at high frequencies

due to a phenomenon called synchronization error

Synchronization Error

• Gate period is exactly four source cycles• Measurement could be off by +/- 1

GATE

SOURCE

Miss both edges 0 1 2 3 3

0 1 2 3 4 Miss one, catch one

1 2 3 4 5 Catch both edges

Effect of Synchronization error

• You can measure a frequency of F with error of 0.xFMAX = FSOURCE - (FSOURCE/(1+0.x))

• If frequency you are measuring exceeds acceptable error use Measure Frequency VI

High Frequency Measurement

High Frequency

Measure Frequency

Your Signal

Still Period Measurement except:• Gate is known

– Pulse from another counter

• Source is your signal

Pro• Works well at high frequencies

Con• Uses two counters

High Frequency Setup

Gate

Source

Out

Count Register

Your Signal

Gate

Source

Out

Count Register Known Pulse

Timebase

Pulse Width MeasurementSingle Pulse Generation

• Frequency = (count of Counter 1)/(pulse width of Counter 0 Out)• Synchronization error still exists in count of Counter 1

– Error = (+/-1)/(period of Counter 0 Out)

Counter 1Counter 0

Measure Frequency VI• Counter

– Chooses counter to address

• Gate Width (s)– Desired length of pulse used

to gate the signal– The lower the signal

frequency, the longer the width must be

• Frequency (Hz)– Returns value in Hertz

• Valid– False if counter rolls over

NOTE: You must connect Gate of chosen counter to Out of other counter

Position Measurement• With a transducer called a quadrature

encoder you can measure position• DAQ Signal Accessory has a

quadrature encoder• NI-TIO is only counter chip that

directly supports quadrature encoders– Possible to measure quadrature encoder

with Am9513 and DAQ-STC

How Does an Encoder Work?

Light SourceLight Sensor

Shaft

Rotating Disk

Code Track

Channel B

Channel A

• Shaft and disk rotate• Code track either

passes or blocks light to sensor

• Light sensor creates two pulse trains

Quadrature Encoder

Quadrature Encoder

900 phase difference

Channel B

Channel A

900 phase difference

Channel B

Channel A

Clockwise Rotation

Counter-Clockwise Rotation

• Quadrature Encoders produce two pulse trains 90 degrees out of phase

• Clockwise rotation– Channel A leads Channel B

• Counter-Clockwise rotation– Channel B leads Channel A

• DAQ-STC counters also have an up/down line– DIO6 is up/down for counter 0– DIO7 is up/down for counter 1– TTL High = Count up– TTL Low = Count down

Up/Down Line

Gate

Source

Out

Count RegisterU/D

• Count on the falling edge of Channel A– Clockwise - B is high so count increments

– Counter-Clockwise - B is low so count decrements

• Channel B is hardwired to DIO6 on the DAQ Signal Accessory

DAQ-STC and Encoders

Gate

Source

Out

Count Register

Channel ASimple Event Counting

U/DChannel B

Channel B

Channel A

Clockwise Rotation

Channel B

Channel A

Counter-Clockwise Rotation

Chapter 6 Summary• Counters accept and generate TTL signals• The main components of a counter are the source, gate,

out, and count register• National Instruments devices could have one of four

different counter chips• E-Series devices use the DAQ-STC chip• The Easy VIs can be used to perform event counting, pulse

generation, pulse measurement, and frequency measurement

• A quadrature encoder is a transducer that converts rotary motion into two pulse trains which are out of phase by 90 degrees