Timing and Synchronization in NI LabVIEW

Overview
Timing is critical to all test, control, and design applications and must be considered as a key consideration in your system. Timing and synchronization techniques relate events to time when coordinated actions need to be performed. For software to perform these coordinated actions, programs must be synchronized based on time. NI LabVIEW includes timing structures that you can use to synchronize your programs in your system.
LabVIEW Timing Principles—Nanosecond Engine and NI-TimeSync
LabVIEW uses a software component called the Nanosecond Engine to keep track of time in your program. The engine runs in the background and interacts with the operating system to manage time. There are several functions and structures in LabVIEW that use this engine to keep track of time, such as the Wait function and the Timed Loop structure. The Nanosecond Engine can use the local real-time clock (RTC) or it can be driven by an external reference clock through the NI Timing and Synchronization Architecture (NI-TimeSync) (Figure 1).

Figure 1. LabVIEW nanosecond timing mechanisms work with NI-TimeSync to provide clocks for applications.
LabVIEW 2010 introduces a new clock in NI-TimeSync. The IEEE1588 plug-in in NI-TimeSync 1.1 provides a synchronized reference clock with up to 1 ms accuracy. You can configure multiple instruments on a network to use the same IEEE 1588 reference clock, allowing multiple platforms to be synchronized on a standard Ethernet network. You can also configure devices to use the software 1588 precision time protocol through the NI Measurement & Automation Explorer (MAX) tool (Figure 2).

Figure 2. Configuring the device's time synchronization source from MAX [page]

LabVIEW Timing Structure - Timed Loop
A timed loop is a loop structure that executes when a configurable timing source generates an event. It can use a variety of timing sources (detailed in the following tutorials). If you develop applications for multi-rate processing, precise timing and synchronization, loop execution feedback, dynamically changing timing characteristics, or multiple execution priorities, you can use a timed loop. In addition to the strict timing characteristics of the timed loop, the timed structure can also be used to allocate processor resources for multi-core programming. With the timed loop, you can specify multiple timing properties including period, priority, deadline, offset, and delay. Combining these properties with a rich set of timing sources, you can create complex applications no matter what timing method is required.

Figure 3. LabVIEW Timed Loop structure using timing constants to execute code

Timing Sources for Timed Loops
Timing sources control the execution of timed structures. You can select from three types of timing sources: internal timing source, software trigger, or external timing source (Figure 4).

Figure 4. Timing sources for a Timed Loop structure: built-in timing sources, software-triggered sources, and external sources
Built-in timing sources
The built-in timing sources keep track of time using a nanosecond engine. You can configure a timed loop on a real-time (RT) target using either a 1 kHz clock or a 1 MHz clock. You can use a timed structure with millisecond resolution using a 1 kHz clock. All LabVIEW platforms that can run timed structures support the 1 kHz timing source. Targets that support the 1 MHz timing source can provide timed structures with microsecond resolution. You can also configure a Timed Loop structure to use these built-in timing sources as an absolute time reference to start the loop execution with a timestamp. For example, you can configure a Timed Loop to start at a certain time each day.
Another built-in timing source is the Synchronous Scan Engine. It synchronizes a timed structure with the NI Scan Engine. With this timing source, a timed structure executes at the end of each scan. The loop execution period corresponds to the Scan Period (µS) setting, which you configure on the NI Scan Engine page.
Software-triggered timing sources
You can create a software-triggered timing source to trigger a timed structure based on a software event. The Create Timing Source VI creates a software-triggered timing source. The Fire Software-Triggered Timing Source VI programmatically triggers a timed loop controlled by a software-triggered timing source. You can use a software-triggered timing source as a real-time-compatible event responder or to notify a consumer loop in a producer-consumer model when new data is available.
External Timing Sources
You can create external timing sources to control timed structures with NI-DAQmx 7.2 or later. Use the DAQmx Create Timing Source VI to programmatically select an external timing source. You can also use a variety of NI-DAQmx timing sources, including frequency, digital edge counters, digital change detection, and task signal sources to control timed structures. Use the DAQmx Data Acquisition VIs to create the following types of NI-DAQmx timing sources to control timed structures. Frequency
—Creates a timing source that executes a timed structure at a certain frequency.
Digital Edge Trigger—Creates a timed structure that executes on the rising or falling edge of a digital signal.
Digital Change Detection—Creates a timed structure that executes on the rising or falling edge of one or more digital lines.
Task Signal—Creates a timed structure that can be triggered to execute using a signal.
Timing and Synchronization in LabVIEW
LabVIEW provides the necessary timing and synchronization capabilities for your system using the language's built-in timing structures, nanosecond timing engine, and timed loop structures.

If you are not familiar with synchronization, you can visit Implementing Synchronization to learn more. Or view case studies in the industry.

To learn more about LabVIEW, visit ni.com/labview/