Question:

For the F2803x/2802x power on reset, if a customer uses an external 1.8v regulator for core voltage and an external oscillator what is the proper sequence and timing?  


Answer:

Section 6.9 of the data manual deals with Power Sequencing requirements.

The device powers up using the internal OSC1 and you can switch to other clocks for more accurate timing via S/W. The chip has an internal power up reset (POR) block that will hold reset low for around 600us after power up. This is sufficient to start the chip up properly.

The XRS pin is an open drain so you can hold it low for longer if you desire. Just make sure you don't actively drive the pin high (open drain/collector). If you don't want to drive the pin at all you should still put an RC circuit on it so system noise doesn't trip the pin during run time.

You will also want to allow enough time for your external clock source to become stable before switching to it. Also, don't apply power to any pins before powering the device. Other than that there are no requirements.