USB Universal Floppy Disk controller
pkoning at equallogic.com
Tue Mar 15 08:49:18 CST 2005
>>>>> "Philip" == Philip Pemberton <philpem at dsl.pipex.com> writes:
Philip> In message <1110833470.10973.51.camel at weka.localdomain> Jules
Philip> Richardson <julesrichardsonuk at yahoo.co.uk> wrote:
>> I forsee four goals to make it useful:
>> o Cheap
Philip> CPLDs tend to cost between Â£10 and Â£20 each in 1-off, but you
Philip> can stuff nearly all of the logic into just one of them. You
Philip> could even make the board in-circuit reprogrammable if you
Philip> wanted, but I'd be tempted to use an SRAM FPGA instead of a
Philip> CPLD if I did that, simply because most FPGAs have open
Philip> programming specs (it's something of a requirement - if you
Philip> have to reload the fusemap every time you powercycle the
Philip> chip, you're not going to want to drag the manufacturer's
Philip> programmer around with you).
Some of the better nonvolatile FPGAs also have open programming
specs. In particular, there's the protocol called "Jam" (or was that
"Jazz"?) used by Altera. It's not just open, it's also clean,
well-designed, and easy to implement.
>> o Easy / quick connectivity
Philip> Parallel port then. I'd add "Portable across multiple
Philip> platforms" which basically means "parport or nothing". Every
Philip> desktop and laptop machine I've seen has had an
Philip> IEEE-1284-compliant (or compliant to a reasonable degree)
Philip> parallel port.
I guess you haven't looked at Macs then.
The problem with parallel ports is that they typically have up to four
different modes, all of which have drawbacks.
1. Classic output-only mode. Not applicable here, of course.
2. PIO bidirectional mode. Works fine but it's VERY slow. Not just
because of the PIO nature, but also because in this mode the port
uses open collector signaling, so the rising edges are all defined
by RC time constants.
3. ECP mode. This is a block transfer mode useful for printers, not
obviously useable for anything else.
4. EPP mode. Nice, clean handshake protocol, with totem pole tristate
signaling. Not quite so trivial either on the software or the
hardware side, though.
I did a CPLD implementation of #4, which seems to work. In the first
PC where I used it I had to install an add-on card, though; the
built-in port didn't support EPP. But the HDL for the state machine
was fairly complicated, and getting rid of all the metastability
issues (since there is no clock in any of these modes) wasn't trivial
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