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Post by relayer on Dec 24, 2008 14:49:19 GMT -5
I'm referring to Programmable Array Logic and Gate Array Logic chips.
I'm salvaging parts off of old circuit boards and coming across many chips whose numbers start off with PAL and GAL. After finding out what these are, I'm wondering if they serve any useful purpose these days, with programmable microprocessors in abundance. I don't even know if they generally are re-programmable. The ones I'm saving may not serve any useful purpose anymore.
I was just wondering if anybody had experience with these old style chips.
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Post by Unseen on Dec 24, 2008 19:20:23 GMT -5
I'm salvaging parts off of old circuit boards and coming across many chips whose numbers start off with PAL and GAL. After finding out what these are, I'm wondering if they serve any useful purpose these days, with programmable microprocessors in abundance. I don't even know if they generally are re-programmable. The ones I'm saving may not serve any useful purpose anymore. Usually PALs are one-time-programmable and GALs re-programmable. They serve a completely different purpose than a microprocessor, you can think of them as reprogrammable logic chips - while it is possible to do the same with the I/O pins of a microcontroller a MCU would take ages compared to a PAL/GAL. Just as a simple comparison: If you run an AVR 8-bit MCU at 20MHz and program it to just copy the inputs of one port to the outputs of another the lowest delay you can achieve is 2 clock cycles, 100 nanoseconds. The slowest GALs still on the market can do it in 25 nanoseconds and that delay stays constant when the logic function to be calculated gets more complicated (unless you need to do multiple passes through the matrix...). Their architecture is rather old though, so there are lots of limits to the structure of the internal logic, for example only up to 8 outputs and only a single clock-input for all 8 flip-flops in a GAL16V8. You'll also need a special programmer (search for GALBlast, IIRC it's relatively easy to build) and an equation compiler that converts a logic description (some allow you to start with a schematic) to a so-called fuse map that tells the programmer which bits to program in the chip - lots of choices here. Even with their limitations and their hunger for power (40-150mA depending on the version) GALs are still a quite nice way to get a single-chip address decoder for an 8-bit system that would require a few 74xx series chips if implemented without programmable logic. It's also much simpler to remove bugs from such a decoder - just reprogramming instead of rewiring.
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