olin_piclist@embedinc.com (Olin Lathrop) wrote:
> I then modified an EasyProg by shorting pins 7 and 8 on the bottom of the
> DB-9 connector.  This allowed the USB serial port to work with the EasyProg
> directly without any adapter.  It was still slower than a real serial port.
> Programming a full 16F876 and reading it back at two voltages took 64.2
> seconds instead of 55.5 with a real serial port.  That's 16% slower, which
> may be acceptable in many cases.

My guess is that this additional time (8.7 seconds) simply represents
the latency induced by the basic 1 ms USB frame period. Every transaction
will experience a delay that averages 0.5 ms, or half the frame period.
I would estimate that the process you describe above requires about 16k
to 17k individual messages back and forth to the EasyProg. Does that
sound about right?

If you want really high bandwidth on USB, you need to transfer data in
bigger batches (e.g., 1k bytes or more at a time at Full Speed). In this
application, it's a don't-care.

-- Dave Tweed
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