AXIe Timing and Triggering

By Greg Hill, Agilent Technologies/AXIe Consortium

Test and measurement applications often have unique timing and sequencing requirements. AXIe’s timing and trigger interfaces can help address them. Here are typical examples. 

Instrument Sequencing
One basic requirement is simple sequencing of instruments performing a stimulus-response test. The system typically applies a stimulus to the device-under-test and sequentially measures the response at multiple outputs. For example, the system might consist of a digital-to-analog converter (DAC) source, a digital voltmeter (DVM), and an analog switch or multiplexer (MUX). The test may be accomplished by sequential programming of each action with the application program ensuring the proper sequence.

Test time may be shortened dramatically by preprogramming the devices and using cross triggering to advance the sequence. Each instrument then sends a trigger signal to its successor when it completes a task. The AXIe trigger interface with its 12 bussed trigger pairs is well suited for this situation. Separate pairs may be used for the DAC -> DVM trigger, the DVM -> MUX trigger, the MUX -> DMM trigger, and the DMM -> DAC trigger. 

Simultaneous Triggering
Another common requirement is the need to sample multiple signals simultaneously. With the AXIe trigger bus pairs, the skew between signals is just a few nanoseconds, based on the signal propagation delay across the backplane.

To reduce skew even further, the AXIe system module may drive the sample trigger onto the SYNC pairs. They are fanned out on the backplane to all modules with a skew of no more than 100 picoseconds. An alternative is to drive the STRIG pairs to selected slots. The STRIG backplane propagation skew is less than 20 picoseconds. 

Time Stamping
A similar requirement is event time stamping across modules.  Here the modules are acquiring data continuously. Each uses the incoming trigger event as a marker for later data correlation and analysis. As with simultaneous triggering, the critical parameter is the module-to-module trigger skew. 

Time Base Synchronization
Applications may require that instruments operate from a common time base. The CLK100 signal provides it, operating at 100 MHz with less than 100 picoseconds of module-to-module skew. Some applications may further require that the time bases be synchronized. The system module may assert a synchronous (to CLK100) pulse on the SYNC pairs to mark a particular edge as the time origin reference.

Trigger Routing
In many applications requiring low-skew triggering, the trigger signal must originate from an instrument module. Since the low-skew SYNC and STRIG signals are driven from the system slot, there must be a mechanism to route triggers originating from instrument slots to it. Such routing is a required feature of every AXIe system module.

Conclusion
Clearly AXIe provides powerful, flexible timing and triggering resources. They are well suited to the unique needs of test and measurement and can serve a wide variety of applications.

Greg Hill is a member of the AXIe Technical Committee and the editor of AXIe 1.0: Base Architecture Specification. You can reach him at greg_hill@agilent.com.