History with solder mask colour is really unbelievable.
At this day I have additional tested 20 boards.
2 of 20 boards are bad at minus.
Below is attached layout of pcb.
I mentioned above about some test.
- At normal temp device is powered on, codec is initialized and device works.
- I put device into low temp chamber and wait some hours.
- Sound throw codec flows but volume control doesn't has affect.
It means that digital core of codec works fine and I2S (SPORT) also works fine.
Only I2C doesn't work.
Therefore I think we can reject power supply issues.
Power supply at low temp is 3.27...3.29 Volts.
Total capacitance at normal temp between 3.3V and GND is about 32 uF.
Total capacitance at low temp is about 30 uF.
And don't forget I2C is software implemented at GPIO because ADSP-BF533 has not I2C.
Today I soldered wires to I2C lines about 10 cm, and make hole in enclosure.
I tested 4 terminals with wires. Problem fades at 3 of them!
One of them was unstable. Some times it works some times not.
And I see with logic analyzer that codec doesn't give ACK.
Earlier I also prepare new firmware.
In my normal firmware I make request at fixed codec I2C address.
In new firmware I implement search of any valid I2C addresses from 0x00 to 0x7F.
In both firmwares it is done 10 times.
At normal temp on both firmware it finds codec at first attempt.
At low temp on both firmware it doesn't find codec, no ACK.
Maybe it's really ring on tracks?
SCL pin always works as output it has short rise and fall time, about 20 ns.
But maybe it's not real time. Because I have only 60 Mhz bandwidth oscilloscopes.
20 ns is close to bandwidth and real rise and fall time can be shorter and maybe problem is here?
Maybe soldered wires add capacitance and increase rise time and remove ringing at wires?
SDA pins works like "software open drain" - logical 0 output or input.
Rise time at SDA is about 350 ns. Fall time is about 20 ns.
I think SDA rise and fall time doesn't matter for I2C.
For next step I want to solder capacitor on SCL line to ground. About 20 pf.
