Lower than ~8V supply voltage causes malfunction (like a noise as a small sine wave)

[SidsMarvelousRobot]

Hi ardyesp. first of all Thanks a lot for this firmware
As I said, I confronted this issue, I assumed it should be a problem with negative voltage rail because that's the only part of voltage regulation which is controlled by STM32.
By reading the DSO138 manual. I saw VGEN should be around 0.8V . I measured mine it was ~0.64V ( On official firmware it was 0.83V and there was no problem).
So in "io.ino " line 48: pwmWrite(VGEN, 700); I changed PWM value To 900. problem solved and VGEN is now 0.83V.