We noticed that VDDIO of i.MX28 steps up and down during bootlet.
When 5V is turned on to supply VDD5V, VDDIO becomes 3.1V which is reset value.
Then somehow VDDIO steps up to ca. 3.4V even though the target value is set to 3300mV. (ddi_power_SetVddio(3300, 3150);)
After that, VDDIO steps down to 3.3V for a brief moment and then steps up again.
When bootlet is completed and Kernel is loaded, then VDDIO finally settles to 3.3V.
The following image is the behavior of the EVK (Rev.D).
(Power on -> 3.1V -> 3.4V -> 3.3V(instantaneous) -> 3.4V -> 3.3V(stable))
When we changed the target value to 3150mV (ddi_power_SetVddio(3150, 3000);), VDDIO still became 3250mV.
We reviewed the source codes of the bootlet but could not find a part which is causing the behavior.
Does anyone know what is causing the 100mV offset during bootlet and how to prevent it?
We are aware that the output accuracy is 3%. But since VDDIO becomes 3.3V in the end, we don't like the 100mV step-up behavior.
For your reference, we also find the following behaviors.
1. When VDD5V is not supplied and instead 3.3V is supplied to DCDC_BATT on the EVK, VDDIO acted similar way (initial voltage was not 3.1V but 3.15V.)
2. Our custom board, which is VDD5V source only configuration, acted same way as the EVK.
Thank you in advance,