Lesson 6 Counters

1
Counters

• Topics
• Counter Overview
• Easy Counter VIs for
• Event Counting
• Pulse Generation
• Pulse Measurement
• Frequency Measurement
• Position Measurement

2
Counter Signals

• Counters accept and generate TTL signals

3
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

4
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

5
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

6
Different Counter Chips
Less

• 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
More
7
Counter Palette
8
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

9
Counter Applications

• Event 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

10
Event Counting
Simple Event Counting

• Active edges on source signal increment the count
• Active edge can either be rising or falling

Time Measurement

• Timebase has known frequency
• Time elapsed (Count) x (timebase period)

11
Event Counting

• 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

12
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

13
Pulse Generation

• Generates a TTL signal on counters out pin

14
Pulse Characteristics
Pulse Polarity
15
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

16
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

17
Pulse Measurement

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

18
Period 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)

19
Pulse Width Measurement

• 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)

20
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

21
Frequency Measurement
Your Signal
High Frequency
Low Frequency
Measure Pulse Width or Period
Measure Frequency
22
Low Frequency Measurement
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

Low Frequency
Measure Pulse Width or Period
23
Synchronization Error

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

24
Effect of Synchronization error

• You can measure a frequency of F with error of
  0.x
• FMAX FSOURCE - (FSOURCE/(10.x))
• If frequency you are measuring exceeds acceptable
  error use Measure Frequency VI

25
High Frequency Measurement
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
Measure Frequency
26
High Frequency Setup
Pulse Width Measurement
Single Pulse Generation
Gate
Out
Gate
Out
Count Register
Count Register
Known Pulse
Source
Source
Counter 1
Counter 0
Your Signal
Timebase

• 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)

27
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

28
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

29
How Does an Encoder Work?

• Shaft and disk rotate
• Code track either passes or blocks light to
  sensor
• Light sensor creates two pulse trains

Quadrature Encoder
30
Quadrature Encoder

• 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

Clockwise Rotation
Counter-Clockwise Rotation
31
Up/Down Line

• 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

32
DAQ-STC and Encoders

• 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

33
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