G400D RTC current

I finally installed some of our custom boards on customers boats. While all seems more or less working great, I discover that RTC was not properly saved on POR. After some investigations, it appears that the VDDBU rail of the SOM draws current when the SOM is off resulting of a voltage about 1.2V on the rail.

Using a G400HDR doesn’t show the issue. So my custom board has something wrong. Between VDDBU and the CR2032 there’s juste à schottky diode so to my opinion the problem is not at this place.

My feeling is that I am using an LTC2952 to perform power supply management and that it causes some troubles by letting the SOM in a kind of undertimined state.

I will try to fix the state of the SOM before switching it off using PowerState…

If anyone have some other idea, any help would be appreciate…

@ leforban - Can we have a much larger circuit diagram, I can’t read it.

@ Mr. John Smith - Sure. I’ll do it right now.


Just dumb question but the board was stuffed correctly right? Nothing in backwards and the like.

As far as I can see no, all seems OK. The only things that do not work as expected are:

Some IO are supposed to be pullup during boot procedure are at low state and vice et versa
The vddbu vbat pin that consumes a lot of current when the SOM is not power supplied.

@ leforban - I’ve heard that the initial state of the pins at start up is undetermined.

Nop, states are defined in the SAM9X35 datasheet, but for some reasons the device does not work as expected on my custom board. Probably due to the circuitry used to power supply the SOM.

I want to sove this for the next run of PCB. Does anyone know if a voltage applied on VBAT RTC pin on the SAM9X35 can be propagated on other IO’s through vddbu rail? If yes, may be some of pulldown resistors on these IO’s could consume…

Here’s the schematics for the power supply management.
On the left is the main power suuply (battery between 9V and 35V).
A PTC (F1) fuse to protect against over current
A diode to protect against polarities inversion.
Two resistors as a voltage divider (R11 and R25) to check when the voltage is too low and start flushing SD card.
IC4 is a 5V switch regulator. It allwos to supply 2 supercaps (5Farad)
This then goes to the PSM (power supply management) module, which allows to switch on/off the board using BP_Power (a button). The PSM is connect to two field effect transistors FET1 and FET2. PSM and FET1 and FET2 allows to power on the board as soon as the battery is connected. This then power supply a linear regulator (3.3V to supply the G400D SOM).

Do you see anything wrong? anything that could be done simpler? Could this cause the problem on RTC?