Hello, * On Fri, Mar 11, 2022 at 07:06:46PM +0100 Francesco Messineo wrote: > Hi, > > On Fri, Mar 11, 2022 at 6:13 PM Spiro Trikaliotis > <ml-cbmhackers_at_trikaliotis.net> wrote: > > > > Hello, > > > > I am trying to understand the sources of the 1571 and 1581 drives. > > > > In the original drive sources of the 1571 and 1581 (as found on > > zimmers.net), there is a macro WDTEST: > > > > WDTEST .macro > > .ife <*!.$03 > > nop > > .endif > > .endm > > > > I am not completely familiar with the syntax of the assembler used, but > > if I understand it correctly, it issues a NOP if bit 0 and bit 1 of the > > PC are both 0 - that is, the PC address is divisble by 4. > > wild guess: > The WD1770 in the Commodore 1571 (and I guess in the 1581 too, but > I've never seen one) has its chip select line > qualified with the wrong clock. Let me explain better: when a > peripheral device must be connected to the 6502's bus, it either has a > phi2 input According to the schematics, it is wired with PHI2: http://www.zimmers.net/anonftp/pub/cbm/schematics/drives/new/1581/1581-16.gif > Now, if one uses phi1 as "poor man's" inverted phi2, it turns out that > the chip select to the WD1770 might happen too early (phi2 not yet > high) and cause unpredictable > behaviour. Yes, this was part of my wild guess, too, that the timing is not correct. The WD177x was not desinged with the 65xx family in mind. > Since also the WD1770 need two address lines and they are supplied by > A0 and A1, it was probably wise to never have both A0 and A1 both low > just before a real access > to this device, since its chip select has the wrong timing and maybe > A0=A1=0 was the worst possible fake access to this device. Well, in fact, it filters out the case when there are A1=1, A0=0 before /CS going active. The next step after /CS going inactive would be A1=1, A0=1 (it might be the release of /CS that is critical?) I mean, A0=A1=1 is the DATA address. Here, a simple read let's the WD177x decide that the data was read, and the signaling is removed. That's why I believe this case might be the critical one. Regards, Spiro -- Spiro R. Trikaliotis https://spiro.trikaliotis.net/Received on 2022-03-12 11:03:47
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