Floppy controller questions

[Philip Pemberton]

In message <m1E7h9n-000IyXC at p850ug1>
          ard at p850ug1.demon.co.uk (Tony Duell) wrote:

> Also be aware that many HD-capcable 5.25" drives turn at 360 rpm all the 
> time. Which means the data rate if you put a DD (or for that matter an 
> SD) disk in them is 6/5 times what you'd get with that disk in a DD drive 
> (which turns at 300 rpm). You may well need to provide a suitable clock 
> for 300 kbps, for example.

Ick. Time to find a 24MHz crystal oscillator :-/
Divide by 3 gives 8MHz, div24 for 1MHz, div48 for 500kHz, div96 for 250kHz.
And none of those are powers of two. Yay. I think I'll just stick to 8MHz and
1M/500k write clocks.

> I always thought they were invented to make my job more painful than it 
> should be ;-)

Only if you're trying to reverse engineer something :)
I hate ASICs too, unless they're fully documented down to gate level in some
way. The CBM 1541 is a great example... Version 1 was all TTL, v2 had an ASIC
to do the GCR coding, v3 had an ASIC that did the GCR *and* analogue
interfacing IIRC. Ick.

I still need to deal with the "only 16uS to grab a byte of data" issue.
That's going to be a major pain when the CPU is running at 2MHz... Thankfully
IDE/ATA is fairly speed independent.
I suppose I could use interrupt mode, and hijack the INT (or maybe the NMI)
vector whenever the FDD is being accessed.

Isn't learning fun?

> data separators. And I've seen a board of logic (the original IBM 
> controller, for example). But I've never seen a real 765 linked to an 

I really should try and get a copy of the IBM PC Technical Reference manuals
at some point.

> Unless you are using the original Sony 3.5" drives, which turn at 600 
> rpm, and thus use twice the data rate you might expect, 5.25" drives and 
> 3.5£ drives use the same data rates for the same modes.

All new-ish PC-type floppy drives here. I don't think I've got any of the
original Sony drives - most of mine are MFD920s, with one or two MFD520s (I
prefer the 520s - they're jumperable for DS0 and ideal for RiscPCs). Some of
the Citizen drives that Acorn used are pretty well made, too - I've never
seen one that didn't work properly. Well, except for the one that some oik
(not me) shoved a mini-CD into.

> HD disks (1.2Mbyte, 1.44Mbyte respectively) in their approriate drives 
> use the same data rates as 8". SD and DD disks are half of those rates 
> _apart from the 360rpm issue I mentioned above).

Easy way to solve the 360rpm issue: Find the SPEED_SEL pin on the motor
controller, lift the pin then wire it to the edge connector.

I

> FWIW, 3" are the same as those too (there never was a HD 3" disk AFAIK).

Heh. Amstrad 3" "indestructable" floppies. Those bring back some memories...
Mostly dealing with disintegrating drive belts.

-- 
Phil.                              | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem at philpem.me.uk              | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/          | Panasonic CF-25 Mk.2 Toughbook
... I'm spending a year dead for Tax Purposes