Quote:
Originally Posted by Lemaru
Kludge had it working with some boards just by tying BR and VPA to ground on the A600 CPU side and to 5v on the accelerator side. There's more info on it over on the Exxoshost forum.
There's plenty of documentation online regarding 68000 bus arbitration though, and what is required, and the signals involved
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VPA isn't involved in the bus grant, but they are tied together on the A600 anyway, looks like just because they were too cheap to use another pull-up resistor. The A600 doesn't use VPA/VMA/E because Gayle generates these signals for the CIAs and turns them into normal DTACK bus cycles - I think this is probably because the 020 doesn't have VPA/VMA/E so that logic was needed for the 1200. Incidentally, without this feature it would have be much harder to defeat the on-board 68000 because the E clock output runs even when /BR is asserted, and would conflict with the accelerator's output. As it is these pins can just be ignored altogether (VPA should be pulled up on the accelerator end).
Something to perhaps be aware of is that the 68000 will carry on and complete the current bus cycle after /BR is pulled down (not sure what happens if it is already low when RESET/HALT are released), and it even asserts /BG a tick or two before it actually releases the bus. To avoid a potential conflict at the point of handover I think it would be sensible to have the accelerator held off until a short period of time after the motherboard 68000 has asserted its /BG output, just to be sure the bus has really been released.
Mike