Use cases

• Measure and indicate the total processing time in milliseconds per iteration of a process loop
• Especially useful to benchmark non-deterministic loops

Features

• Negligible CPU effort, i.e., a noninvasive measurement
• Easy to code:
  • “Tick Count” reports the value of a millisecond counter on each call
  • Use a feedback node to remember the value of the previous call
  • Subtract the previous value from the current value to measure the elapsed time

Keep in mind

• The “Tick Count (ms)” function has a minimum resolution of 1 millisecond
• The reference time is officially unspecified, therefore, the value itself returned by “Tick Count” should be interpreted with caution; however, experiments with the Academic RIO Device suggest that the timer resets to zero when the product powers up
• The timer is based on a 32-bit unsigned integer which wraps back to zero after reaching the value 2^32 minus 1, therefore, the measurement is erroneous once every 2^32 milliseconds or about every 50 days of continuous operation

LabVIEW block diagram elements

Locate these elements with "Quick Drop" (press Ctrl+Space and start typing the name); click on an icon to see more sample code that uses that element:

Example code

• Connect your Academic RIO Device to your PC using USBLAN, Ethernet, or Wi-Fi. NOTE: Not all Academic RIO Devices have Ethernet and Wi-Fi connectivity options.
• Download and unpack the rt_measure-loop-time.zip (for use with NI myRIO 1900) or the NIELVISIII-rt_measure-loop-time.zip (for use with NI ELVIS III) archive, and then double-click the ".lvproj" file to open the project. NOTE: This project was written for a NI myRIO 1900 or NI ELVIS III connected by USBLAN at IP address 172.22.11.2. If you are using a different IP address or another Academic RIO Device (Example: NI myRIO 1950 or NI RIO Control Module) do the following:
  • If using the NI myRIO 1950 or NI RIO Control Module start with the NI myRIO 1900 Archive.
  • Different IP address: Right-click on the "NI myRIO 1900" Device, choose "Properties", and then enter the new IP address
  • Different device:
    • Right-click on the top of the project hierarchy, select "New Targets and Devices", keep the "Existing target or device" option, and then find and select your particular device
    • Select all of the components under the "NI myRIO 1900" device: click the first one and then shift+click the last one
    • Drag the selected components to the new device
    • Right-click the "NI myRIO 1900" device and select "Remove from project"
• Run “RT Main” that contains a conventional while-loop (non-deterministic) and a timed loop (deterministic) running in parallel, each set for a nominal 100-millisecond loop time
• Observe the variation in the non-deterministic loop time compared to the deterministic loop

Expected results

Developer walk-through

Outline

• Review overall structure
  • “Tick Count (ms)” function returns the value of a built-in 32-bit unsigned integer counter that increments every millisecond; initializes to zero when Academic RIO Device powers up
  • Feedback node and subtraction operation measures the number of elapsed milliseconds since the last time the “Tick Count” function was called, i.e., since the previous loop iteration
  • Measurement technique embedded in a timed loop set for 100-ms loop time
  • Same measurement embedded in a conventional loop with “Wait Until Next ms Multiple” to define the 100-ms loop time
  • Local variable used to stop both loops
• Configure the timed loop
• Stop both loops from a single front-panel “Stop” button
  • “Stop” button resides in timed loop
  • Local variable in conventional loop reads the “Stop” button value
  • “Stop” button mechanical action must be changed from default “Latch When Released” to “Switch When Pressed” because latching action is not compatible with local variables
  • Click “Stop” to stop both loops
  • Unstick the “Stop” button with the “F” constant after both loops have stopped, otherwise the button remains pressed

Related content

Tags:

• loop time
• process loop
• benchmarking
• deterministic loop
• non-deterministic loop
• stopping parallel loops

Targets:

• RT

LabVIEW VIs and block diagram elements:

• Timed Loop
• Tick Count (ms)
• Feedback Node
• Wait Until Next ms Multiple
• Local Variable