Date   : Fri, 17 Dec 2010 14:59:05 +1100
From   : awilliams@... (Alan Williams)
Subject: Harddisc fakery using a microcontroller - back on

I have been vaguely thinking about a 1MhzE bus interfaced SD card
interface or about an emulator for the whole host adaptor ACB4000/4070
chain.

Since single chip micros don't tend to make good bus interfaced
peripherals I was wondering if anybody had any experience with
AT94K10AL-25AJC.  Its from the Atmel FP Slic family and is a FPGA with
an AVR core and can come in a 84 pin PLCC.  I can get them for about
AUD30 but they also need their config ram for another $12, and I have to
buy 10 of each so its looking a bit pricy to start with.

The chip looks very nice for this sort of thing but the licensing for
the dev tools is some what draconian. Its free for a very short time,
after which I think its about $1200 a year.

Just wondering if anybody knew any cute ways around that....

Also on the use of SD or any Flash memory for that matter as a hard disc
replacement, its harder than might be expected.

What becomes immediately obvious when you start reading the data sheets
is that these sorts of memory cards do not have single sector erase and
being flash memory require data to be erased before it can be rewritten.
The number of sectors that have to be erased in one go varies with the
capacity of the device but is in the order of 8, 16, 32 (x512 bytes)
etc.

This makes writing one sector quite a time consuming job and it also
requires a big (in 8 bit terms) ram buffer in reach of the micro
controller to backup all the other sectors that have not changed.

This starts to suggest that a new filing system should be written that
understands the underlying media, but since I am interested in this sort
of adaptor for use with a Filestore rather than a BBC that's not all
that possible.

Back to the ACB4000 to IDE emulation I think.

Alan