I am in the process of making a custom carrier card for MicroZed. I have a question on the proper shutdown sequencing order, the MicroZed Carrier Design Guide reads on pg. 17 "To enable power to the MicroZed, PWR_EN must be pulled high. PWR_EN is tied to JX1 pin 5 and is pulled up to Vin on the MicroZed. To shut down power to the MicroZed, PWR_EN and VCCIO_EN should be pulled low. VCCIO_EN should be pulled low first to maintain proper shutdown sequencing. See the Power Architecture and Sequencing Diagram for more detail."
From the MicroZed Breakout Carrier Card schematics I can see how this is readily implemented with an RC delay circuit going into an NMOS transistor for the PWR_EN signal while the VCCIO_EN has no such delay capacitor. (Sheet 3 of 5). My problem is that this circuit relies on a "5V_OFF" signal. This signal is driven to 5V when the switch is turned off. In my application however, the 5V supply will turn on with the rest of the system (an autonomous vehicle of sorts that I'm building), and then turn off when the overall system is powered down. So in other words, there will be no extra 5V line that receives power (5V_OFF) once the main 5V line is powered down.
Is there an example on how to implement proper shut down sequencing if all of the power to the board is cut? Likewise, I suppose I should ask the question then of what would happen with the MicroZed and the Breakout Carrier card, being powered off of USB, if the USB cable is suddenly unplugged? (Switch still turned "on"). That would seemingly prevent the proper shutdown sequencing since the "5V_OFF" signal would not be driven high.
Thanks for your time and help