[msandford]

On the next page the author talks about ways to bulletproof this by avoiding latches and to use hysteresis to ensure that the debouncing is perfect. Not perfect in the sense that it always gets the state right -- it might register a button press when none is there if there was absolutely horrific EMI, or it might not register a button press that is too short -- but perfect in the sense that there are no undefined states. That's the point of hysteresis; you can't fake it out. If it's in the no-mans-land between decision points, it defaults to whatever it was previously.

You're right that any long chain of latches could end up metastable and screwed up. But this technique doesn't use latches, it uses multiple reads on an analog input which is interpreted digitally (but not an ADC) to preclude the possibility of any kind of undesired behavior.

[schoen]

The "reads on an analog input which is interpreted digitally" are effectively a one-bit ADC or comparator, aren't they?

[msandford]

Yes, but typically when you're talking about an ADC you're talking multiple bits. These days an 8 bit ADC is "low resolution" and 10-12 bits are nearly free (considering they're built into tons and tons of microcontrollers).

The point was more that it was a single bit, not a 10 bit ADC that's used to make determinations about switches. From a theoretical perspective the idea of a 1 bit ADC makes sense. From an engineering perspective, it doesn't. Since I'm an engineer that's why I said what I did.