1、Lesson 5Timing an FPGA VITiming Express VIsImplementing Loop Execution RatesCreating Delays Between EventsMeasuring Time Between EventsBenchmarking Loop PeriodsA. Timing Express VIsLoop Timer Express VIImplementing Loop RatesWait Express VICreating Delays Between EventsTick Count Express VIBenchmark

2、ingB. Implementing Loop Execution RatesUse the Loop Timer Express VIWaits the value you specify in Count between loop iterationsControls the period of a loopPlace the Loop Timer Express VI inside a loopPerform I/O operations at a specific frequencyControl loop execution rateConfigure Loop TimerCount

3、er UnitsTicks clock cyclesSec microsecondsmSec millisecondsSize of Internal Counter32 Bit16 Bit8 BitSize of Internal Counter determines the maximum time a timer can track.To save space on the FPGA, use the smallest Size of Internal Counter possible.Loop Timer Express VI FunctionalityFirst iterationR

4、ecords current time as initial timeImmediately executes the code in the next frame without any delaySecond iterationAdds Count to initial time and waits until Count has elapsed from the initial recorded timeExecutes the code in the next frame after Count has elapsedSubsequent iterationsLoop Timer co

5、ntinues to increment the time record it initiated upon the first callDoes not reset each timeLoop Timer Express VI CaveatIf an execution instance is missed, such as when the logic in the loop takes longer to execute than the specified Count:Loop Timer returns immediately and establishes a new refere

6、nce time stamp for subsequent callsNo delayImplementing Loop Execution RatesWatch out for rolloverSize of Internal Counter determines the maximum time the timer can trackSize of Internal CounterTickssms32-bit1 to 4,294,967,296 ticks1 s to 71 minutes1 ms to 49 days16-bit1 to 65,536 ticks1 s to 65 ms1

7、 ms to 65 seconds8-bit1 to 256 ticks1 s to 256 us1 ms to 256 msTick PeriodTick period is based on the frequency of the FPGA clockIf using a 40MHz FPGA clock, the tick period is 25nsMust use ticks for nanosecond timingSize of Internal CounterTickssms32-bit1 to 4,294,967,296 ticks1 s to 71 minutes1 ms

8、 to 49 days16-bit1 to 65,536 ticks1 s to 65 ms1 ms to 65 seconds8-bit1 to 256 ticks1 s to 256 us1 ms to 256 mscRIO Module Loop Execution RatesSome cRIO modules have configurable timingDo not need to use Loop Timer Express VI to time loop rateExampleNI 9233You can configure the data rate at which the

9、 NI 9233 module acquires and returns dataStatic/Programmatic ConfigurationC-Series Module Properties dialog boxFPGA I/O Property NodeStatic Configuration of NI 9233 Data RateProgrammatic Configuration of NI 9233 Data RatecRIO Module Loop Execution RatesLoop Timer Express VI is not neededWhile Loop r

10、ate is determined by NI 9233 data rateIf other code in the loop executes at a slower rate, the loop rate will be limited by the other codeLoop Conditional TerminalFalse constant is acceptableFPGA logic is often meant to run indefinitely on the FPGAControls and application logic is still acceptableWh

11、ile Loop Iteration Terminal MaximumThe iteration terminal count will keep outputting 2,147,483,647 once it reaches this valueIf you need a counter to rollover when it reaches its maximum value, you should implement your own counterExercise 5-1: While Loop TimingUse the Loop Timer Express VI to time

12、a While Loop running on an FPGA VI.Exercise 5-1: While Loop Timing If the code in the second frame of the Sequence structure takes longer to execute than the value of AI Sample Time control, how long would each iteration of the While loop take?C. Creating Delays Between EventsUse Wait Express VI to

13、create a delay between eventsExamplesCreate delay between a trigger and subsequent outputCreate a series of delaysWait Express VI ConfigurationCounter UnitsSize of Internal CounterDetermines maximum possible wait timeComparing Loop Timer and Wait VIsCode structure is the sameLoop Timer does not wait

14、 the first time it is calledWait Express VI waits every iteration of the While LoopD. Measuring Time Between EventsTick Count Express VIReturns the value of a free-running FPGA counter in the units specifiedUse Tick Count Express VI to measure time between eventsExamplesMeasure the time between edge

15、s on a digital signalDetermine the execution time of a section of codeMeasuring Time Between EventsCalculate the difference between the results of two Tick Count Express VIs to determine the execution timeSubtract one from the result of the calculation to compensate for the execution time of the Tic

16、k Count Express VIE. Benchmarking Loop PeriodsUse Tick Count Express VI to measure loop periodsCalculate the time differenceRemove the benchmarking code laterBenchmarking code executes in parallelDoes not add additional time to the loop periodTick Count RolloverWatch out for rolloverIf the code you

17、are benchmarking takes more than the maximum value Tick Count can handle, the elapsed time will be much lower than expected due to rolloverSize of Internal CounterTickssms32-bit1 to 4,294,967,296 ticks1 s to 71 minutes1 ms to 49 days16-bit1 to 65,536 ticks1 s to 65 ms1 ms to 65 seconds8-bit1 to 256

18、ticks1 s to 256 us1 ms to 256 msExercise 5-2: While Loop BenchmarkingBenchmark the loop period of a While Loop containing code.Exercise 5-2: While Loop Benchmarking If you configured the Tick Count Express VI to use Counter Units of s or ms instead of Ticks, would the benchmarking results in this ex

19、ercise still be reasonable? If the code you want to benchmark executes within nanoseconds, what Counter Unit should you use?SummaryMatching QuizLoop Timer Express VIWait Express VITick Count Express VIUse to create delays between eventsUse to benchmark execution speedsUse to control loop execution r

20、ateSummaryMatching Quiz AnswersLoop Timer Express VIWait Express VITick Count Express VIUse to create delays between eventsUse to benchmark execution speedsUse to control loop execution rateSummaryQuizWhat is a potential drawback of using an 8-bit counter instead of a 32-bit counter?Decreased FPGA resources used32-bit clocks are slowerMaximum time the timer can track is not large enough for the applicationSummaryQuiz AnswerWhat is a potential drawback of using an 8-bit counter instead of a 32-bit counter?Decrea