11Mhz vs 22Mhz
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You need some extenal reference. Suggestions have been comm-lines or power lines.

Think about timing restrictions.

Consider:

You probably have EEProm or RTC chips, internal or external.

EEProm: There is a write cycle time. Allow for some variance and time how long until the subsequent write is accepted, divid the time into two classes of elapsed time {11Mhz, 22Mhz}. This is better because it requires no additional pin use.

RTC: DUH... Count an elapsed second. Counter values reveals which clock speed. No extra pins required.

If you want to consume an extra pin: RTCs often have a square wave output as on the DS1307. Its programmable for up to 1 second period... choose fast period and time it at an 8051 input pin.

-Jay C. Box

PS: Why do it he hard way?