Apply the following sequence on the VIM:
- VIM disconnected from everything.
- Connect the HDMI to a powered monitor.
- Power on the VIM.
- Let the VIM boot.
- Either power off the VIM with the remote control in Android (you then get a blue LED) or disconnect the power cord, but let the HDMI cable connected to the powered display.
After this sequence, there is 0.5 V on VCC3.3V because the HDMI_PW supply from the display flows through HDMI_SDA/SCL into the S905X, which is mostly off (except AO_5V on USBOTG_B_VBUS if the power cord is still there), so there are residual voltages flowing inside the S905X towards several pins, including VCC3.3V.
This residual voltage is an issue for extension boards connected on the J9 header if they use this VCC3.3V. We had an issue because of this using the PCA9555 and the TLV320AIC3104 because the power-on reset of the PCA requires VCC < 0.2 V, so a power cycle with the HDMI connected means a residual 0.5 V on VCC3.3V and no reset of the PCA, which then retains its previous on state, so the TLV is not properly initialized if its reset pin is controlled by the PCA, which may make the TLV break the I²C, so it’s not even possible to manually tell the PCA to reset the TLV by software.
This residual voltage may be an issue for other components as well, all the more it is also present on all the pins pulled up to VCC3.3V, and there are also lower residual voltages on other pins.
Using a local 3.3 V supply on the extension board is not always a possible solution either because of the residual voltages on the other pins, which might exceed the absolute maximum ratings of some components when off or during the power-on sequence.
Do you have any plan of completely isolating HDMI_PW from the rest of the VIM when AO_5V or PWR_EN is off?