counters
DESCRIPTION
Counters. Topics Counter Overview Easy Counter VIs for: Event Counting Pulse Generation Pulse Measurement Frequency Measurement Position Measurement. Maximum Rise/Fall Time = 50ns. +5.0 V. high. +2.0 V. indeterminate. +0.8 V. low. 0 V. Minimum Pulse Width = 10ns. - PowerPoint PPT PresentationTRANSCRIPT
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