CASE STUDY : Tight waveform synchronisation using PXI

Designing a EEPROM programmer to characterise devices

In this case study we will show how PXI technology is used to solve a problem which requires precise timing controls, the challenges that this introduced and how we solved them.
Our client's requirement was to characterise their EEPROM devices, looking specifically at how fast the devices were so that they could guarantee timing characteristics to their customers. All devices come off the same production line, but some will perform better than others.
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A PXI platform was ideal for this application as the PXI backplane allows for synchronisation and triggering between different PXI cards. The requirement called for I/O control of more than 30 address and data lines to the unit under test. We selected the National Instruments 6552 Digital Waveform Generator to generate programming waveforms and also read in data from the unit under test. The application required 2 such cards and so the system would have to synchronise the input/output of both cards.

The timing requirements of this application were very constrained. In order to correctly chacterise a device, we had to guarantee that the required critical timings were met. Read on to find out how we solved this problem.

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System design

This diagram shows the major system components. The PXI chassis containing the 6552 Waveforem Generator. The output signals from this are fed into a custom designed PCB.

​The PCB had a 15 layers and was designed in-house by our PCB expert. The signals are all buffered and the layout of the PCB was designed such that the outputs of the buffers are as close as possible to the device under test. The final part of the track would act like an antenna so this minimises signal loss and noise. Amfax has extensive experience in programming devices using digital waveforms from both PXI equipment and FPGA

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Uncertaintly

The timing between these two times has to be less than 75 nano seconds for a pass. The system had to therefore set the time to as close as possible to 75 nano seconds as possible. The PXI 6552 card has a slew of +/- 1.8ns between all of the output channels, the card is controlled via an on-board FPGA which updates the outputs every 10 ns, it is not able to update the outputs simultaneously hence the skew.
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The PXI 6552 card can individually delay the output of channels by one epoch/256. The timing epoch is 10ns so this gives adjustment granularity of 0.039 nano seconds.
By understanding the timing diagrams, it is possible to correct this error. In the first diagram you can see what happens if you simply count 8 cycles between the events on the PXI 6552 The grey boxes show the time when a desired output may actually occur. The OE line is set at T0, but with a worst case slew this occurs 1.8ns before T0. The data becomes valid from the device at 75 ns, but the card will not read the data until 80ns. This has not guaranteed the timing requirements.
In the second diagram this has been corrected. The system delays the input waveform by 6.8ns so as long as the data becomes valid before 75ns, the system will detect this correclty. As the PXI 6552 epoch is 10ns, 5ns is added to the measured time. Correcting for this skew has introduced some wastage, the final diagram shows that if the data becomes valid in the 73.2ns to 75ns region, the PXI 6552 would not be able to detect it.

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Re-triggering

One test required checking the idle time between two write cycles. When a write cycle was completed, the device would signal it is busy and then it is ready for new input. Each device might have had a sighlty different write cycle time and so the event was considered to be asynchronous.
We made use of the powerful programming features of the PXI 6552 to overcome this problem. The PXI 6552 allows you to load up multiple waveform patterns to the card's on board memory. You can also write a pseudo-code script which includes triggering information. The Amfax PCB took the very weak output signal of the device, boosted its power through a buffer into a trigger line of the PXI 6552. The script loaded on the card was configured to wait for this signal and then output the second waveform.