Digital I/O Hardware Handshaking Using GX5280/GX5290 Series

Overview Often, testing devices with digital interfaces requires the ability to wait for the device to respond to specific pattern sequences sent to it.  This is especially true for devices that incorporate bus transfers that use hardware handshaking to synchronize the data transfer.  The GX5280/GX5290 digital instruments family of cards  provide sixteen external event inputs that can be used in support of this handshaking process.  This article explains how to use the external events to perform synchronous Pause/Resume operations with the device under test (DUT). External Events The GX5280 and GX5290 digital IO instruments (DIO) provide sixteen external input signals that can be used to pause and trigger (resume) the digital pattern sequences.  This ability to pause, and then resume pattern sequencing based on hardware signaling is essential for performing closed-loop handshaking of data transfers.  Figure 1 below provides a functional block diagram of the external event control.

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Figure 1 - External Events Block Diagram

At-Speed Hardware Handshaking

What's The Catch?

• Events are level sensitive, and are not generated by rising or falling edges

• The pause is not immediate.  It takes a minimum of 16 vectors to effect a pause.  The maximum run-on depends on the pattern rate.  Reasons for variations in the vector run-on are described below:
  
  • There is an approximately 350 nS propagation delay for pause events to be available to the pattern sequencing logic.
  • There is a 16-state stimulus/response pipeline that must be considered when using DIO loop-back for pause control.
  • Synchronous External Event Pause/Trig requires only 1 vector to initiate the pause.
  • Asynchronous External Event Pause/Trig requires a minimum of 2 vectors to initiate the pause.
  • A maximum of 8 vectors can be used for initiating a pause.  The DIO pipeline will retain pause event states which could initiate a pause immediately after a continue trigger is received if the pause signal is applied across too many vectors.
  • At high data rates, ground bounce can cause false or intermittent Pause/Trigger operations as the bounce could exceed the vector period the instrument is running at.
  The approximate 350 nS propagation delay and 16-state pipeline will delay pausing the DIO pattern from a minimum of 16, to a maximum of approximately 50 vector states.  The patterns immediately following the pause signal should be programmed with static states so as not to vary the logic states presented to the DUT while waiting for the handshake acknowledge.  The length of the run-on depends on the data rate of the DIO, for reason explained below.
  
  
  
  Figure 2 - Low Data Rate Pause Timing
  
  
  In Figure 2 above and Figure 3 below, the propagation delay (Tpd) is the same duration (~350 ns).  Figure 3 has been stretched to better show the timing detail.  You can see in Figure 2 that Tpd is less than 1 vector period, so its effect is contained in the first clock cycle after the pause signal.  The pattern clock is slow enough that the effect of Tpd is hidden (contained) in the vector where the pause trigger is generated (blue clock cycle).  So the pause run-on is affected only by the instrument pipeline, and the DIO pauses at 16 vectors after the pause event (red clock cycle).
  
  
  
  Figure 3 - High Data Rate Pause Timing
  
  
  In Figure 3, the vector clock is running much faster, and the same Tpd duration extends across multiple pattern clocks.  During Tpd, pattern clocks (yellow clock cycles) have no effect in moving the pause trigger state through the instrument pipeline.  So, in effect, the pause run-on is extended by the number of pattern clocks contained in the duration of Tpd - five clock cycles in the example.  The vector in which the pause occurs is 21 vectors after the pause event (red clock cycle) - 16 vectors for the pipeline + 5 vectors for Tpd.  If the DIO is running at 100 MHz (10 nS) period, an additional 35 clocks could occur during Tpd (350 nS / 10 nS period) - give or take.  At high data rates, and due to minor variations in Tpd delays, there is some ambiguity concerning the exact vector where the pause will take effect, so the pipeline and Tpd run-on should be characterized for the data rate at which the test pattern will be run.
  
  Examples of this are shown in Figure 4:1 - 4:6 below.  Each image is a screen capture of the GtDio panel running the same test pattern.  In each case, the pause signal was programmed at vector 5000.  The only difference between the screen captures is the data rate at which the pattern was executed.  Reviewing the highlighted fields of Figure 4:1 shows the pattern clock is running at 1 MHz.  The period, 1 uS, is well above the Tpd value, so the pause takes effect at vector 5016.  Subsequent figures show different pattern clocks, up to 100 MHz, and the resulting pause vector for each.
  
  
  
  Figure 4 - Pause Vector at 1 MHz - 100MHz Data Rate
  
  

  Conclusion While the GX5280 and GX5290 Digital IO instruments provide pattern control using programmable registers and/or software commands, utilizing unused DIO resources can extend the functionality of the DIO instruments to support multiple pause/resume points across the entire depth of the UUT test pattern, or within multiple pattern sets that can be pre-loaded into the 64M vector DIO pattern memory.

  GX5290 Series Learn More 32 input / output channels, dynamically configurable on a per channel basis Dynamically Controlled High Speed Digital I/O PXI Card

  
  
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