6 Sep
2010
6 Sep
'10
7:26 p.m.
On 06/09/10 20:55, Chuck Guzis wrote:
If it's got a "capture" mode on a timer,
then jitter and latency are
of little concern--the CPU need only come around often enough to read
the capture register and stash it away. Some DSPs even include a DMA
mode for capture events, so the CPU is more or less out of the loop.
Depends if the capture mode has auto-retrigger. Some PICs implement
capture/compare by storing the timer value and throwing an interrupt on
an external event. The timer is left running -- IIRC there isn't an
option to reset it after the C/C event.
What kind of hard drive MFM resolution are you going
for on the DF?
The CW samples floppies at 56MHz; similar resolution on an ordinary
ST506 hard drive would seem to require a clock of 560MHz, which is
high, even for an FPGA.
Quartus reckons the ACQCLK will go to 200MHz internal. External
oscillator is a 20MHz crystal, which gets PLL'd up accordingly.
On a standard floppy, the rate I've been using most often is 40MHz.
80MHz is most likely going to be the default in the 'release' microcode.
As for HDDs.. decoding a 5Mbps stream with a good margin would require
about 10 clocks per bit. 50MHz. I'd sample at 100MHz or 200MHz just for
a bit of additional margin.
20Mbit ESDI will most likely be a "design a controller and implement in
an FPGA" exercise -- timing-acquisition might not work particularly well
for that kind of bitrate.
Synthesizing a commodity floppy controller is of
little interest to
me, as much of what I deal with is very far from "standard" 3740-type
floppies. Group code, weird address marks, hard sectoring, unique
CRCs (or even integer checksums) are more the rule. Besides, I think
that the SMSC floppy controller is still available (in TQFP).
Point taken. But the SMSC floppy controller is a bitch to find, and if
memory serves it has issues with precompensation (if it's anything like
modern controllers, it just plain doesn't do precomp). Totally worthless
for anything vaguely resembling reliable R/W work.
--
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/