The default state of OVR is 96MHz, 1.1V, so it is not reliable to start with a 0.9V external supply.
The recommended procedure for getting to 0.9V operation is:
- -Power on with external supply off
- -Change OVR to 24MHZ, 0.9V
- -Enable PMIC 0.9V output
- -Disable LDO
It is important not to load or pull down the 1.1V LDO. It is also important to understand that the power sequencing registers are only reset by a power on reset (POR). The reset pin has no effect on these registers.
If you want to transition back to 96MHz, 1.1V, it is recommended to re-enable the internal LDO before changing OVR:
- -Enable LDO
- -Change OVR to 96MHz, 1.1V
- -Adjust PMIC to 1.1V
- -Disable LDO