Praline: spectrum inverts randomly when changing sample rate

[mossmann]

What type of issue is this?

permanent - occurring repeatedly

What issue are you facing?

Spectrum inverts randomly when changing sample rate since 99f96d2

What are the steps to reproduce this?

TX or RX with GNU Radio and gr-osmosdr. Adjust sample rate with slider.

Can you provide any logs? (output, errors, etc.)

No response

[mossmann]

I've reverted #1610, but I would like this bug fixed so we can merge it again.

[mossmann]

This problem can be partially mitigated by holding CAPTURE_EN high permanently as @mndza has done in a recent update to: generic-sgpio-intf

However, the problem still happens occasionally when the decimation ratio is changed. I've added a test branch to demonstrate this: praline-test-1629

To test, install firmware from the praline-test-1629 branch and use the following GNU Radio flowgraph (which requires gr-osmosdr):

decim_test.py

Tune to a frequency with a distinctly asymmetric spectrum. Use the samp_rate slider to rapidly change the sample rate. When switching from a lower decimation ratio to a higher decimation ratio (e.g. when changing from 2.6 Msps to 2.5 Msps), the spectrum sometimes (maybe 1% of the time) inverts. The condition is cleared when returning to the lower decimation ratio.

maximum sample rate for each decimation ratio: 1.25 Msps, 2.5 Msps, 5 Msps, 10 Msps, 20 Msps

[mossmann]

I'm not entirely certain that the problem when increasing the decimation ratio is really the same as the original problem in the description, but the symptom is similar. The problem with decimation happens even with #1610 reverted.