On Sun, 2006-02-05 at 05:16 +0100, Oliver Bandel wrote:

> But I'm not clear about how to write this function "f",
> because it needs mutual recursion...

No it doesn't, not even with feedback, because your
system is CLOCKED.

A simple model is: you have a collection of
chips with inputs and outputs. In phase one,
calculate the ouputs from the inputs.

In phase 2, you have a circuit board, representing
the wiring, this is a function that copies the outputs
of all the chip to their inputs, following the
wiring diagram. That's where the feedback comes in.

If you want to know how to make a subcircuit
do the feedback *internally* on one clock,
there is a simple answer and a hard one.

The simple answer is YOU CANT. Circuits are
monolithic.

The hard answer is .. you can have multiple
clocks, synchronised by a master clock. 
The implementation is simple .. a clock is
just a counter that writes TRUE when it is zero
and FALSE otherwise.

Then you invent a thing called a LATCH, which
is like wire, except it holds the output
stable and ignores the input UNLESS a special
ENABLE input is True. That's connected to your counter.

Basically .. you're asking hard questions about how
to design synchronous circuits. Don't expect a clocked
circuit to just be a simple composition of functions.
The functions get very complicated -- otherwise circuit
designers wouldn't have a job :)

-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net