On Jun 5, 2012, at 12:19 PM, Thomas Giesel wrote: >> btw, at the end, isnt it all NANDs ? or NORs ? > > How exactly it's done in the 82S100 is written in its datasheet: It has > (up to) 48 product terms which are AND combinations of any of the > inputs and inverted inputs. > > Any of these 48 product terms can be combined to 8 sums, which are OR > combinations of the product terms. > > The final result can be inverted (which is done with the CASRAM output > obviously, which re-uses most of the terms of the other outputs). It's the classic AND-OR-INVERT (AOI) logic, also called "sum of products". http://en.wikipedia.org/wiki/AND-OR-Invert > For my PLA clone (which should arrive in the next few days from > Donald...) I wrote the terms in exactly this form. Which makes no > difference as the CPLD synthesizer/fitter optimizes it anyway, even if > it is written in other forms - but it feels more original ;) Heh. For those that don't know, a CPLD/FPGA implements a lookup table using SRAM or flash. So instead of actual logic, you just select one of the pre-coded outputs, based on the input. This is exactly like a truth table as used by people. So however you specify the equations, it gets synthesized to a lookup table in the end. The PLA was the early version of this, with a ROM mask selecting individual gates to implement the equations. But determining the mask was more low-level, like assembly coding. You check off a term on graph paper and blow that bit in ROM to manually implement your equations. -Nate Message was sent through the cbm-hackers mailing listReceived on 2012-06-05 20:00:33
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