Disk hardware emulation, was Re: Grandfather system RTE6/VM?

From: Dwight K. Elvey <dwight.elvey_at_amd.com>
Date: Thu Dec 11 16:26:09 2003

>From: "Eric Smith" <eric_at_brouhaha.com>
>
>I wrote:
>>> Without decoding the data, it is proposed to sample the data at 50 Mbps
>>> or more.
>
>Andy wrote:
>> Why so fast? You only need sample at 2x the bandwidth.
>
>2x is the theoretical minimum; sampling at 2x isn't always adequate
>in practice due to various real-world limitations.
>
>The minimum write data pulse width spec for the WD1000 controller
>is 60 ns. If we were sampling and reconstructing a sine wave with
>a 120 ns period, 16.7 MHz sampling would be adequate. But that

Hi
 Even for a sine wave, the brick wall filter would be difficult
to create. You need a little more room.

>wouldn't yield good results for square waves, and there wouldn't be
>any margins. In practice, I suspect that 33.3 MHz sampling would be
>barely adequate provided that no other problems arise. Operating at
>50 MHz seems advisable to have reasonable margins.

 The thing to consider is that we are not sampling a "square" wave.
It is a variable pulse width digital signal. The only kind of
processors I know of that could handle this from a port is some
of the latest DSP chips. Using a PC, even one like our ( AMD )
latest Opteron wouldn't do well since it is so highly pipelined
and unpredictable in execution speed at this short a time
interval.
 There are a number of DSP evaluation boards out there that places
like ADI and TI sell that could be made to handle this task.
Of course, a simple pulse width counter on the front end could
simplify the sampling process.
>
>
Received on Thu Dec 11 2003 - 16:26:09 GMT