I met a problem when I power on my custom board. The ADuC7021 on it can't reset correctly sometimes:
Here is aduc part of schematic.
Net +3.3V in the schematic connects to a PC power module. And I found that if I turn on the switch of the module, this problem occurs occasionally. But if I plug borad's the power lines to the module while the module is already working, this problem NEVER comes.
I don't know how to deal with it, anybody have ideas
Sounds somehow the part gets latched up if you switch on the power. What filters do you have in the power-supply line? See recommendations in data-sheet please!
For entering the Download-Mod you must pull down the BM pin while resetting the part.
So it is the power-supply...
I use a filter composes of a ferrite bead and some caps (10uF and 0.1uF).
In fact, I suspect that the MCU somehow enters the download mode, for the level of MRST pin never goes low during the power up process when the problem comes.
I'm afraid some ripples on BM and RST cause 7021 enters the download-mode wrongly. I saw those on my oscilloscope when I turned on the switch.