aduncan@rhea.trl.oz.au (Allan Duncan) (01/23/90)
This may have been discussed before, but I didn't see it if it did.
How is the async running of the 2630 done? Is it the same kludge as the
Lucas board, with potential metastates and truncated instruction cycles?
Allan Duncan ACSnet aduncan@rhea.trl.oz
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Telecom Research Labs, PO Box 249, Clayton, Victoria, 3168, Australia.daveh@cbmvax.commodore.com (Dave Haynie) (01/24/90)
in article <931@trlluna.trl.oz>, aduncan@rhea.trl.oz.au (Allan Duncan) says: > How is the async running of the 2630 done? Is it the same kludge as the > Lucas board, with potential metastates and truncated instruction cycles? Well, any fast board talking to the A2000, asynchronous or not, is going to run into wait states. But a good design should never have the potential of going metastable. The A2630 design is reasonably simple. A fast PAL decodes any on-board access as soon as a cycle starts; on-board access can be for the FPU, the memory, or daughterboard. If no on-board activity is called for, a state machine for 68000 cycles is started. This machine clocks out AS* and DS* equivalents on the proper 7MHz edges, and one in the proper part of the cycle, it samples the DTACK* line, first on the falling and then the rising edge of the 7MHz clock. Once DTACK* is indicated, the next falling edge latches the data, for reads, and also asserts the on-board DSACK1* to terminate the 68030 cycle. So what's actually seen is the 68030 trailing just a bit behind the 68000 cycle. Fortunately, at 25MHz, it's fast enough to end its lagging cycle and start a new one without necessarily adding a 68000 wait state, though the start-up synchronization delay possible can occasionally add a 68000 wait state. This synchronizer is pretty special purpose, based on specific knowledge of what's on the A2630. The nice feature of this design is that it's 68030-speed independent; the only limiting condition is the speed of the data latches between the 68030 and 68000 busses. > Allan Duncan ACSnet aduncan@rhea.trl.oz -- Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Too much of everything is just enough