What is feedback?
Depending on the device that is used to implement a sequential logic
design, there are different ways of specifying the values of the flip-flops.
A person can specify the output on the pin, the output of the flip-flop
Q, or the feedback value. By specifying .fb we are telling ABEL that, if
the device offers a feedback wire from the flip-flop back into the circuit
that is the value we want. We could just as easily used 'Q' like A := A.Q,
or even A := A to take the value off of the pin. But we need to stick to
some convention because if we are not consistent throughout our ABEL file
things can get messy. For example, if we say something like A := A.Q one
place and t hen somewhere else refer to the .fb value of A we have a problem.
When we map our logic to a specific device we might not be able to use
the two different flip-flop outputs because of the limitations of the device.
For this reason, stick to using .fb when what you want is a current value
in the flip-flop. Even if the device you map to does not have a feedback
wire, ABEL will make the necessary adjustments to the resulting equations
and use the correct wires and most importantly will be consistent.
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