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External oscillator voltage required when running at 5V

Question asked by Evan Slatyer on Nov 16, 2014
Latest reply on Nov 16, 2014 by Evan Slatyer

Hello everyone,


Hopefully this is a straightforward question. Is the EXTAL clock input pin on the KE02_40 MCU just a standard digital input, or does it have special characteristics? The datasheet doesn't seem to indicate anything unusual about it.


I'm planning to use a KE02_40 MCU in a project, running at 5V. I'd like to clock it from a Silicon Labs Si501 oscillator, which will be running at 3.3V. If the EXTAL pin is treated as a standard digital input then it requires at least 3.25V to register a high input, and the Si501 will only guarantee that its "high" voltage is over 2.97V. Therefore I'll probably either have to replace the Si501 with a crystal or insert a high-speed level converter in there; neither option is very attractive. If the EXTAL pin does have special characteristics (eg. on the STM32 chips, clock inputs definitely don't behave the same as GPIO pins) then it may well be fine.


Any advice is much appreciated.