I don't think this is a Blackfin problem, per se, but since I'm connected to a BF518, I'll ask here. I have a SSM2603 codec connected via SPORT to the DSP, and everything seems OK except LINE OUT. The noise on LINE OUT is awful. I've seen this on two of our proto boards, so I don't think it's an individual component problem. I'm wondering if anyone has seen a problem like this, or has any suggestions.
- Headphone out and Mic In seem to work fine. We have done a sidetone through the DSP test (echo MIC input data back to HP output) so we know that the SPORT is sending & receiving data OK. Generating a triangle wave works fine on the HP out, but the LINE out looks terrible, with 50 mV pulses of noise that swamp low-level output signals. The DAC is working because the HP output looks fine - only LINE out is affected.
- LINE IN looks OK. By wrapping the HP output back to LINE IN and looking at the received SPORT data, everything looks reasonable.
- The noise I'm seeing are pulses about 50 mV by 500 nsec wide, first high, then low. It almost looks like 100 Mbit Ethernet data. It's easiest to see this in AC coupled mode on the 'scope. The pattern is irregular, but is more regular on the left channel, which I have filled with 0 while the right channel has the triangle wave.
- I've looked for some sort of digital correlation to the noise pulses, without success thus far. The SPORT clock runs at 3.072 MHz, which is faster than these pulses. I've looked at the switcher for the 3.3V and DSP core - no correlation.
- The noise seems to originate in the DAC. If I enable only bypass (disable DAC output to the mixer), the noise is gone. If I disable power to the DAC or output, the noise is gone.
Next on my list is to feed LINE IN with something like an iPod and look at the output in bypass mode. But the iPod is at home today...
This smells like a power supply problem to me, but I don't see anything even close to the noise I'm getting. What really confuses me about it is why the LINE out and HP out behave so differently.