Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!amdahl!pyramid!prls!mips!wyse!vsi1!altnet!uunet!mcvax!ukc!reading!onion!cf-cm!sme
From: sme@computing-maths.cardiff.ac.uk (Simon Elliott)
Newsgroups: comp.os.minix
Subject: Re: AN HISTORIC MOMENT!
Summary: 8250 divisor 1 gives 115200 baud
Message-ID: <477@cf-cm.UUCP>
Date: 19 Jul 88 22:28:58 GMT
References: <841@ast.cs.vu.nl> <4200004@hpihoah.HP.COM>
Organization: Univ. Coll. Cardiff, Cardiff, WALES, UK.
Lines: 33

In article <4200004@hpihoah.HP.COM>, bruce@hpihoah.HP.COM (Bruce LaVigne) writes:
> > It may be that the chip actually works at higher speeds, but this is outside
> > the specification, at the very least.
> 
> As I remember, the bytes you
> actually stuff into the chip are divisors of an externally input clock.  With
> the clock that IBM used, if you go above 9600 you start using non-integer
> divisors.  What this means is that since you can really only use integer
> numbers into the chip as a divisor, you don't get 19200 but something kindof
> close.  If the other side can handle it, fine, but IBM doesn't support it.
> 

I seem to remember that the baud rate divisor on the PC for 9600 is something
like 0x0C;  you can go right down to 1 if you want, giving 115200 baud.
The Brooklyn Bridge and LapLink programs do just that.  There are two
problems here, though:
	1) can your (worst case) 4.77 MHz PC handle data at that rate?
	   At that speed, interrupt latency is VERY significant; to get
	   anything sensible, you have to poll.  Doing handshaken 128
	   byte block transfers is OK, but in the context of Minix's tty
	   driver, not acceptable :-) !
	2) the low divisor has an adverse affect on the UART's bit sample
	   timing, leading to framing errors.  If you want to go at that
	   speed, be prepared to checksum/re-send blocks.

I have used PC to PC speeds of 19200 baud (divisor 0x06 ?) without problems.
Even 38400 baud (divisor 0x03 ? - looks right, gives 115200 = 0x01) works
OK if it's not too far.  It all depends on how much work you're doing with
the byte once you've got it.
-- 
--------------------------------------------------------------------
Simon Elliott, UCC Computer Centre, 40/41 Park Place, Cardiff, Wales
...!mcvax!ukc!cf-cm!sme                 sme@v1.cm.cf.ac.uk