7 Jan
2011
7 Jan
'11
11:28 a.m.
PALs are simple two-level programmable logic. It
shouldn't be hard to
create a truth table from the real PALs by adapting them to a PROM
reader. From there you can recreate PAL equations from the truth
table.
The rpoblem comes when the PAL is not simple combinatorial logic, but
implements sequiential functions too.
There are 2 main ways this can happen. Firslty, there are registered PALs
(often with an 'R' in the part number) which hae intenral D-tpye
flip-flops. The D inputs come from the programmable logic, the Q outputs
are availabe on external pins _and are fed back to the logic internally_.
So you can make state machines, etc.
The second way is that some 'normal' PALs have feedback terms. That is
the outputs of the progamamble logic array not only goes ot output pins
but also back into the logic array. You can make SR flip-flops vry easily
this way.
Wth sequential logic, you can't just try all the inputs, record the
outputs nad reduce the logic equations...
Iunderstnd there are some early-vrsion PAL equations for this device
avaialble. Has anyone looked at them to see what htey do? Is it purely
combinatorial (e.g. address decoers), or are there sequential functions too?
-tony