I am designing a board where the datasheet suggests a power up sequence. Basically, the VDD_PLL should be turned on first, after 100us, the VAA should be turned on, followed by the internal supply and then the VDD_IO. Current I am using the AMS1117 as my voltage regulator. I know I would have to chnage this to some other power up sequencing IC if i need to meet these requirements. My question is what happens if I dont meet the power up sequence? Is it merely a safet mechanism or will the board not work. Can I ignore the specfications in the datasheet?
Do not ignore the power up sequences specified by chip manufacturers. They specify these because that is what it takes to ensure that the device powers up correctly and will run reliably.
If you ignore sequencing you can cause internal chip latch-up if voltage rails and reset/power good signals come in the wrong order. If you are lucky the latch-up can be cleared via a full power cycle. Unlucky and there could be a catastrophic amount of current flowing through a path on the chip that can cause fusing and permanent damage.
Improper sequencing of PLLs or OSC or CLK inputs with respect to power rails or other control signals can lead to on chip meta-stability problems that will prevent the internals of the chip from working.