22 May
2013
22 May
'13
6:56 a.m.
One further item...
Most of the late generation Z80s will over clock if the clock drive has
good edges and
levels. So a 6mhz (especially the CMOS versions) will hit and exceed 10mhz.
Even the old (ca1980) NMOS parts will easily run to 5 and as high as 7mhz.
Allison
-------- Original Message --------
Subject: Re: HD floppies on a ZX Spectrum
Date: Wed, 22 May 2013 09:33:09 -0400
From: allison <ajp166 at verizon.net>
To: General Discussion: On-Topic Posts Only <cctech at classiccmp.org>
On 05/22/2013 07:28 AM, Vince Mulhollon wrote:
On Tue, May 21, 2013 at 4:39 PM, Chuck Guzis
<cclist at sydex.com> wrote:
Actually you have 16us to complete the
transfer. It can be done even
with 8080
but the software loop must assume the in or out has a implied wait (stall)
to keep the loop tight enough to keep up...
The problem is two fold, one if the data ready/request never happens
(open door/no media)
the system is hung.
The other is if your using DRAM you are not refreshing in that state so
a hang or long delay
(like waiting a full revolution for the sector) will likely crash the
system on ram errors.
Interrupts during that time will be pending till the wait is released.
I have done it with 3mhz 8085 easily and 4mhz Z80 this way.
A few years back, Herb Johnson opened a
discussion of this on his web
site. The conclusion was that a 4 MHz Z80 was indeed fast enough to do
this with programmed I/O and DD 8" disks (500KHz data rate) and that it was
*barely* possible to do at 2 MHz Z80 set up correctly and not at all with a
2 MHz 8080.
There are hardware issues also relating to your selection of FDC. The
general consensus and experience of the N8VEM CP/M board builders is that
programmed IO with a 8 MHz and up Z80 and a modern 1.44 MHz floppy and a
9266 FDC works perfectly. I assembled a system this winter and that worked
perfectly. Obtaining 8+ MHz chips for modern Z80 designs is non trivial
and involves lots of ebay and searching.
Someday I will remove my CPU canned osc, locate a TTL output function
generator, and figure out exactly what the minimum is. The disk io v3
board has headers and software support for a 8 inch floppy and if I can
locate one I could do pretty extensive testing of different disk
technologies. This type of test with a live system, lots of different
drive types, and a ttl output function generator in place of CPU clock
would certainly prove once and for all what is required for what drive
technology.
The minimum is:
Assumes a FDC read or write will pull /wait until the FDC
clears it with a data request.
Read or write a byte,
Index to zero byte count
index ram pointer
get next byte in the Acc
jump to start if not zero
Allison