[Gb-ccb] epp data rates

[Martin Shepherd]

On Mon, 15 Mar 2004, John Ford wrote:
> Obviously, none of the software you've tried uses DMA.  This should be
> a trivial task CPU-wise.

EPP mode hardware, whose register-level interface is dictated by the
IEEE 1284 standard, doesn't have DMA capability for software to take
advantage of. The more complex ECP mode does, but whereas EPP mode is
general purpose, simple to use and widely supported in SBCs, ECP mode
is optimized for printers, has a more complicated protocol, including
mode negotiation, and run-length encoding for compression, and I
haven't seen any SBCs which advertsize ECP-capable parallel ports.

Since the IEEE-1284 EPP standard was originally defined for use with
the ISA bus, and an outb() CPU instruction to the parallel port was
defined to not return until the corresponding byte had been sent, at a
maximum rate originally dictated by the ISA-bus timing, I believe that
the excessive CPU time that I was seeing, reflects the fact that the
CPU must be executing lots of "wait" instructions while it waits for
the EPP hardware to complete its transaction.

> If you believe that USB 1.1 will do the job, you should probably just
> get on with it...

Although I believe that USB 1.1 will do the job, I can't be sure yet
whether the chosen USB chip will. That is today's job.

Martin