> Ok, I'll admit it, I never really understood the need for diodes. If
> I'm controlling the scan, how do they help? Assume that I'm driving
> columns and reading rows. If I drive column A, then read in all the
> rows, I know that any high line must be from a column A key, right?

Take a chessboard, the scan is pulling the letters-side selectively low
(first A, then B, etc), you read the numbers-side, with pull-ups. Now
press keys A1, B1, B2. Pulling A low will pull 1 low dircetly, but also
2 (via A1, B1, B2). Now you can't detect whether A2 is pressed or not.
When diodes are used the one at B1 would prevent this.

> > For debouncing: just take care that you scan slower than the longest
> > devounce time (50ms is a good upper limit if the switches do not
> > document this). So with a 20 Hz scan frequency you can forget about
> > bouncing.
> 
> Doesn't this assume that the keypresses will always happen directly
> after a scan,

No, but it assumes that bouncing takes at most one scan interval. Think
like this: the scan before the counce will read 'not pressed', the one
after it will read 'pressed'. There can be at most one during the
bounce, which will read either 'pressed' or 'not pressed', but who
cares?


Wouter van Ooijen

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