Noise on JB (Arty A7)

[pablo]

Hello all, 

I'm trying to communicate with external DAC through SPI but DAC does not respond correctly to my commands. For verify my frames are ok I've verify them with my digital discovery and I notice that CS signal and DATA signals are noisy. I'm using JB PMOD on Arty A7 board. Then I realize that there are a switching converter near JB, then I've change my communications to JB, and noise has disappear...  does anyone have notice the same than me? or maybe I have this connector broken? Thanks!

[xc6lx45]

Hi,

AFAIK, PMOD ports have a 200 ohms series resistor. Did you maybe switch to an unprotected FPGA output (no added resistor) when the noise went away? This wasn't clear from reading your post.

[pablo]

Sorry!, I've change my communications to JA, and noise dissapear. there is a mistake in my original post. Thanks!

[xc6lx45]

Hi,

have a look at the schematic, page 1:

https://reference.digilentinc.com/_media/arty:arty_sch.pdf

apparently, JA and JD do use 200 ohms resistors, but JB and JC don't.

I suspect you're observing not "additive" noise (like, from a switching converter) but signal integrity issues, caused by the signal itself (say, reflections on a long, unterminated wire). There are several reasons why a series resistor will make it look better.