Sorry missed the [PIC]: tag!

I shall be starting a project soon that has an outline specification for a
user interface of up to 21 keys.  This in itself is no problem but I have
now been asked to consider replacing 6 of the keys (Up/Down function x 3)
with 3 rotary encoders (probably low cost detented & contacting type).  1 to
3 more keys could also be replaced by using the momentary contact built into
some types of encoders.

I have not used panel type encoders before and although they seem simple
enough to use singly, I am not sure about using three together with up to 15
keys in a matrix.  The target micro at the moment is the 16F874/7, running
at either 16 or 20Mhz.  The I/O is going to be in heavy demand for other
functions, so the number used for the user interface needs to be kept as low
as possible. I cannot afford to dedicate 2 I/O for each encoder.  I also
don't want to increase the chip count by expanding the I/O, although I will
have to consider it if necessary.

Providing the scanning is carried out quickly enough (likely to be interrupt
driven via a timer) I am assuming that I can include the encoder outputs in
the same matrix as the keys. I know I need dedicated routines for detecting
direction but is there anything fundamentally wrong with this approach?  I
am trying to think about all the possible problems that could occur like the
user operating more than one at a time or slippage due to too fast rotation
etc.

Before I build a prototype to test the idea does anybody have any experience
(links?) they can pass on of using multiple encoders and keys together with
minimal hardware and software overhead?

Many thanks

David J Binnington

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