External oscillator voltage required when running at 5V

[evanslatyer]

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.

Cheers,

Evan

[mjbcswitzerland · Accepted solution]

Hi Evan

It looks like you have studied both data sheets and the KE02's is quite clear that the Vin is valid for all digital inputs (which is what the oscillator input will be when not operating in its crystal mode).

You didn't specify what speed the oscillator will be running at (presumably somewhere between 4MHz and 20MHz) but you may be able to insert a series resistor in the line with a weak pull-up to 5V at the KE02's EXTAL input to increase the voltage at that point; eg. 470R in series and 3k3 pull-up. If you have a short distance and little input capacitance (<= 10pF) this should still give a good signal at the input even at 20MHz.

Regards

Mark

[evanslatyer]

Hi Mark,

Thanks for the reply. I wasn't really sure whether the oscillator input would be classed as a normal digital input, and you've clarified that for me. Adding a pullup resistor is an interesting idea; I'll see if the Si501 can handle that.

The oscillator will probably be 12MHz since I'm using those elsewhere in the project.

Cheers,

Evan