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SubjectRe: serial.c (half duplex support)
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tytso@mit.edu said:
> If we have interrupt latency of > 4 character times at 9600 or 19200
> bps, some other driver (or some hardware --- like the cheap sh*t IDE
> contrllers that some boxes 2-3 years ago were inflicted with) has
> serious bugs, and we should fix that driver first.

Agreed. Note that on the transmit side, 1 bit time between chars is
unacceptable - 3 character times (33 bits) is the time we must leave _between_
packets. There must be no gaps inside the packet itself.

> We optimize for the common case, after all, and it's becoming clear
> that the Profibus case is not a friendly case; it looks like you will
> need to disable the FIFO's in order to make it meet the spec anyway.

Yes, possibly - and turn them back on again as soon as you leave the
certification lab :) We can get it to work without this.

Bear in mind that Profibus has a set list of baud rates at which it can work,
and of those, I believe only 9600 and 19200 are actually achievable with the
standard clocks and divisors which we'll find in a PC.

Interrupt latency can be 11 bits @19200 - over half a millisecond. (We have 11
bits because the IRQ is generated when the last octet is _started_).

So I'm not too worried about meeting these requirements - it should be fine.

By all means, push to find out the cause of Rogier's 20ms delays, which are
concerning, but don't worry about the serial side too much on my behalf until
I've actually got time to throw some code together and can give some real
numbers.

Like I said, I only got involved at this stage because someone else was asking
about such things.

---- ---- ----
David Woodhouse David.Woodhouse@mvhi.com Office: (+44) 1223 810302
Project Leader, Process Information Systems Mobile: (+44) 7976 658355
Axiom (Cambridge) Ltd., Swaffham Bulbeck, Cambridge, CB5 0NA, UK.
finger dwmw2@ferret.lmh.ox.ac.uk for PGP key.


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