We have a custom board with u-boot and Linux running. We run using the 5V-only option.
We have a Gold-Cap connected to the BATTERY pin, we use the application note from Freescale for this.
When we disconnect the external power supply by removing the power cord, all goes well.
When we disconnect the external power supply by unplugging the wall socket, the i.MX28 does not start anymore.
The difference is due to the capacitors in the power supply, it caused the 5VDC to slowly drop when being unplugged.
What we see is:
- when we enable power again, we see that the gold-cap is being drained.
- the 4V2 power supply is zero.
We can only restart the i.MX28 by removing power and discharging the Gold-Cap.
So we get a lock-up when the 5V is slowly dropping (1 sec.) and the i.MX28 seems to want to use the BATTERY as power supply.
We used u-boot (when it boots) to check the several registers but all seem to be OK. The HW_POWER_DCDC4P2 register is configured so it will use the DCDC_4P2 as the power source.
What can cause that the 4P2 power supply is not present after a slow drop of the 5V?
And how can we fix this without having to drain the gold-cap.
Thanks in advance.