Digital Discovery logic waveforms very glitchy

[davidday47]

Hi,

Not sure why SPI signal SI that comes from a chip so glitchy. TFEMPTY that is a TTL logic output also glitchy. It seem TXAOUT and TXBOUT causing it, they are +5V/-5V signals. Not sure how to handle those +5V/-5V swing signals.

Best regards

David

[JColvin]

Hi @davidday47,

Are you applying these +/-5 V signals to the Digital Discovery itself? When configured as inputs the Digital Discovery can tolerate +5 V inputs, though it is not designed to handle any negative voltages with respect to ground (I'm guessing that you have a shared ground line between the Digital Discovery itself and whatever DUT you have).

I'm not sure how you are drawing the conclusion that the TXAOUT and TXBOUT are causing the glitches on the other lines; very few of the pulse are (visually as far as I can tell) lined up with TFEMPTY and none of those three are lined up with the small positive pulse that occurs just over one clock cycles worth of the ~17 MHz ACLK.

Do you have a picture of the physical setup that you can share?

Thanks,
JColvin

[davidday47]

Hi JColvin,

You are right, TXAOUT and TXBOUT are not the issue. The chip has 3 GND pins and connect to DD2 3 GND pins.

Here is the bench set up, 800MHzx8. TXAOUT/TXBOUT unconnected, trigger on falling edge nCS. SI signal has a lot of glitches, sometimes TFEMPTY signal also glitched. I think more GND connections to the board will be better but not sure.

[attila]

Hi @davidday47

Use twisted wires with GND, see:

 

 

[davidday47]

Hi attilia,

I have connected all GND leads but still noise, so crosstalk is most like cause.

Do you know if Digilent sells compatible headers and twisted pairs? Or this one from digikey

https://www.digikey.com/en/products/detail/pactech/PC-US2-EXT2-06-K/16396808

 

Best regards,

 

David

[JColvin]

Hi @davidday47,

Digilent does sell some twisted pairs intended for the high speed usage of the Digital Discovery: https://digilent.com/shop/digital-discovery-high-speed-adapter-and-logic-probes/; each signal wire has a 100 Ohm resistor embedded in it. Alternatively if you don't need the adapter or as many sets, there is a set of 4 twisted pairs here: https://digilent.com/shop/high-speed-logic-probes/.

The link you provided has the right pitch spacing, but seems to just be a USB extension (VBUS, Ground, DP, and DM) and not twisted pairs, so I do not think it will offer any direct benefit (aside from offering some physical distance between different wires by virtue of the external cable casing).

Thanks,
JColvin

[davidday47]

Hi JColvin,

 

Ordered the Digilent twisted pairs per link, will arrive in 3-5 business days. Looking forward to eliminate those glitches.

Thank you guys  for the expertise.

 

Best regards,

 

David

[davidday47]

Hi JColvin,

 

I made some twisted pairs as shown on Digilent site and glitches are gone.

Best regards,

 

David