I developed my first board based on the i.MX RT1064 processor. This board must be connected to another one, handling digital and analog inputs and outputs.
Now, I am working on validation of my hardware and I get a power up issue: the cpu seems not to reset well and I cannot have the MCUXpresso debugger connected to the target core through JTAG/SWD port.
I know about the i.MX RT1064 power up sequence and on my hardware I used quite the same power supply circuits on the MIMXRT1064-EVK.
My cpu board is +5V power supplied by my other external board. On this external board there are also some digital and analog devices which are supplied with a +3.3V generated locally from the same +5V power.
After some investigations I realized that all the inputs from those digital and analog devices directly connected to the cpu pins may keep VDD_HIGH_IN and DCDC_IN at about 2V (through the cpu internal clamp diodes, I think), even if the +5V/+3.3V DCDC on my cpu board is disabled. In that reset condition the cpu internal DCDC does not start properly, so the cpu core is not well powered and the JTAG/SWD connection is not possible.
Now my questions:
- With i.MX RT1064 cpu what is the correct way to handle self-powered inputs coming from the external world?
- Should also all the external 3,3V powered devices be switched off during the reset sequence?
- Alternatively (if I could not switch off the external power), should I decouple every self-powered input coming from the external world? I think this is made on the JTAG lines on the MIMXRT1064-EVK, right?
- Regarding to ADC inputs, I think they have internal clamp diodes too, right? Decoupling an analog input is harder than a digital one....
