Re: [hatari-users] Floppy in Falcon mode

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On 28 Dec 2011 at 0:30, Thomas Huth wrote:
> > >
> > > So it seems that the FDC delays are twice as expected in Falcon
> > > mode. This is enough to explain all my trouble.
> > 
> > I barely use the Falcon mode, so I never noticed this.
> > 
> > In fact, the Falcon Ajax FDC uses a 16 MHz clock, while other ST
> > models use a 8 MHz, and a stated in FDC_DelayToCpuCycles, FDC_Freq is
> > expected to be the same as CPU_Freq (ie 8 MHz) for correct timings
> > calculation.
> > 
> > So, on Falcon converting an FDC delay from microsec to cpu cycles 
> > returned twice the expected value. This is now corrected, I added a 
> > special case for Falcon to correct the cpu cycles.
> Great that this problem is fixed. But what about TT mode? Didn't the TT
> have an Ajax chip, too? If yes, which frequency did it use?
I think early TTs had a WD1772 (max 8MHz, only good for DD) and later TTs had 
an Ajax chip.  I have some documentation (the TT MCU RevB Functional 
Description) that says that 0xff860e controls the frequency of the input clock 
to the Ajax/1772 (either 8MHz or 16Mhz).  I assume that TOS 3.06 sets the 
frequency, probably based on the DIP switch setting, but I've never checked 

If anyone needs a copy of that MCU doc, let me know and I'll email it (I think 
I got it from the Atari documentation site).

Roger Burrows

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