Dear Moto-san,
The AD9228 Power On Reset circuit (POR) will retrigger when the supply goes below about 1.2V (typical value). This POR threshold depends on process and temperature and on the transient characteristics of the supply. If there is a short supply glitch then the POR trigger threshold is lower than 1.2V. However, the POR always re-triggers before any flip-flop “state” is lost, resetting the SPI to the default state.
I do not have a time duration number for you.
This is all the information I have. Does this give you what you need?
Thanks,
Doug
