Analog Input Timing Signals

The USB-6453 samples the AI signals of every channel in the task once for every AI Sample Clock. A measurement acquisition consists of one or more samples.

You can specify an internal or external source for AI Sample Clock. You can also specify whether the measurement sample begins on the rising edge or falling edge of AI Sample Clock.

Using an Internal Source

One of the following internal signals can drive AI Sample Clock:

• Counter n Internal Output
• AI Sample Clock Timebase (divided down)
• A pulse initiated by host software
• Change Detection Event
• Counter n Sample Clock
• AO Sample Clock (ao/SampleClock)
• DI Sample Clock (di/SampleClock)
• DO Sample Clock (do/SampleClock)

A programmable internal counter divides down the sample clock timebase.

Several other internal signals can be routed to AI Sample Clock through internal routes.

Using an External Source

Use DIO <0..15> as the source of AI Sample Clock.

Routing AI Sample Clock Signal to an Output Terminal

You can route AI Sample Clock out to any DIO <0..15> terminal. This pulse is always active high.

Other Timing Requirements

The USB-6453 only acquires data during an acquisition. It ignores AI Sample Clock when a measurement acquisition is not in progress. During a measurement acquisition, you can cause the USB-6453 to ignore AI Sample Clock using the AI Pause Trigger signal.

A counter/timing engine on your device internally generates AI Sample Clock unless you select some external source. AI Start Trigger starts this counter and either software or hardware can stop it once a finite acquisition completes. When using the AI timing engine, you can also specify a configurable delay from AI Start Trigger to the first AI Sample Clock pulse. By default, this delay is set to four ticks of the AI Sample Clock Timebase signal.

The following figure shows the relationship of AI Sample Clock to AI Start Trigger.

You can route any of the following signals to be the AI Sample Clock Timebase (ai/SampleClockTimebase) signal:

• 100 MHz Timebase (default)
• 20 MHz Timebase
• 100 kHz Timebase
• DIO <0..15>

AI Sample Clock Timebase is not available as an output on the I/O connector. AI Sample Clock Timebase is divided down to provide one of the possible sources for AI Sample Clock. You can configure the polarity selection for AI Sample Clock Timebase as either rising or falling edge, except on 100 MHz Timebase or 20 MHz Timebase.

You can route AI Hold Complete Event out to any DIO <0..15>.

The polarity of AI Hold Complete Event is software-selectable, but it is typically configured so that a low-to-high leading edge can clock external AI multiplexers indicating when the input signal has been sampled and can be removed.

A measurement acquisition consists of one or more samples. If you do not use triggers, begin a measurement with a software command. Once the acquisition begins, configure the acquisition to stop:

• When a certain number of points are sampled (in finite mode)
• After a hardware reference trigger (in finite mode)
• With a software command (in continuous mode)

An acquisition that uses a start trigger (but not a reference trigger) is sometimes referred to as a post-triggered acquisition.

Retriggerable Analog Input

The AI Start Trigger is configurable as retriggerable. When the AI Start Trigger is configured as retriggerable, the timing engine generates the sample and convert clocks for the configured acquisition in response to each pulse on an AI Start Trigger signal.

The timing engine ignores the AI Start Trigger signal while the clock generation is in progress. After the clock generation is finished, the counter waits for another Start Trigger to begin another clock generation. The following figure shows a retriggerable analog input with three AI channels and four samples per trigger.

Reference triggers are not retriggerable.

Using a Digital Source

To use AI Start Trigger with a digital source, specify a source and an edge. The source can be any of the following signals:

• DIO <0..15>
• Counter n Internal Output

The source can also be one of several other internal signals on your DAQ device. Refer to Device Routing in MAX for more information.

You can also specify whether the measurement acquisition begins on the rising edge or falling edge of AI Start Trigger.

Routing AI Start Trigger to an Output Terminal

You can route AI Start Trigger out to any DIO <0..15> terminal. The output is an active high pulse.

The USB-6453 also uses AI Start Trigger to initiate pre-triggered DAQ operations. In most pre-triggered applications, a software trigger generates AI Start Trigger.

Refer to the AI Reference Trigger Signal section for a complete description of the use of AI Start Trigger and AI Reference Trigger in a pre-triggered DAQ operation.

To use a reference trigger, specify a buffer of finite size and a number of pre-trigger samples (samples that occur before the reference trigger). The number of post-trigger samples (samples that occur after the reference trigger) desired is the buffer size minus the number of pre-trigger samples.

Once the acquisition begins, the USB-6453 writes samples to the buffer. After USB-6453 captures the specified number of pre-trigger samples, it begins to look for the reference trigger condition. If the reference trigger condition occurs before the USB-6453 captures the specified number of pre-trigger samples, it ignores the condition.

If the buffer becomes full, the USB-6453 continuously discards the oldest samples in the buffer to make space for the next sample. This data can be accessed (with some limitations) before the USB-6453 discards it. Refer to Can a Pre-triggered Analog Acquisition be Continuous? for more information.

When the reference trigger occurs, the USB-6453 continues to write samples to the buffer until the buffer contains the number of post-trigger samples desired. The following figure shows the final buffer.

Using a Digital Source

To use AI Reference Trigger with a digital source, specify a source and an edge. The source can be any of the following signals:

• DIO <0..15>
• Change Detection Event
• Counter n Internal Output
• DI Reference Trigger (di/ReferenceTrigger)
• DO Start Trigger (do/StartTrigger)
• AO Start Trigger (ao/StartTrigger)

The source can also be one of several internal signals on the USB-6453. Refer to Device Routing in MAX for more information.

You can also specify whether the measurement acquisition stops on the rising edge or falling edge of AI Reference Trigger.

Routing AI Reference Trigger Signal to an Output Terminal

You can route AI Reference Trigger out to any DIO <0..15> terminal.

The internal sample clock pauses while the external trigger signal is active and resumes when the signal is inactive. You can program the active level of the pause trigger to be high or low, as shown in the following figure. In the figure, T represents the period, and A represents the unknown time between the clock pulse and the post-trigger.

Using a Digital Source

To use AI Pause Trigger, specify a source and a polarity. The source can be one of the following signals:

• DIO <0..15>
• Counter n Internal Output
• Counter n Gate
• AO Pause Trigger (ao/PauseTrigger)
• DO Pause Trigger (do/PauseTrigger)
• DI Pause Trigger (di/PauseTrigger)

The source can also be one of several other internal signals on your DAQ device. Refer to Device Routing in MAX for more information.

Routing AI Pause Trigger Signal to an Output Terminal

You can route AI Pause Trigger out to any DIO <0..15> terminal.