Subject: Re: High serial port (output) speeds
To: None <>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
List: port-sparc
Date: 10/24/1999 00:05:32
> In fact, the mac versions of this driver copied a feature of the
> com.c driver - there's a 5% tolerance on the baud rate calculation
> code.

I'm not sure I like that, because it means you can ask for one output
rate and get another.  For input, you're right, it doesn't matter so
much, but if you're talking to something that wants exactly 115200,
*it* may not be willing to tolerate 111709+ or 117029- or whatever,
even if *you* are.

>> In each case, there is no reason the chip can't hold the divisor
>> chain cleared until it sees the beginning of the start bit, then
>> [...long outline snipped...]
> What you describe is not the 8530. :-)

Bah.  Cheap hardware.  (Only half a smiley here.)

> While it could be done, that's not what the chips in all of these
> computers do. :-) First off, you can't stop the BRG as that would
> stop transmission.

They drive transmission off the same divisor chain?  How do they do
split speeds?  Or don't they?

> As for the AMD chips being equivalent to the Zilog ones, why would
> you expect them to be different?

I wouldn't *expect* them to, but in general, different things from
different manufacturers are liable to be different.  When it's as
simple as a 7400, perhaps, the differences will be limited to the logo
on the case :-), but with something as complicated as a UART, it
wouldn't surprise me that much if manufacturer B decided to "do it
right".  Especially if manufacturer A's part has been around long
enough to exasperate designers with its more exasperating "features".
(I don't know whether any of this describes the 8530 except to the
extent that what say certainly makes it sound as though it doesn't.)

It sounds as though SPARCs will just have to live with crippled serial
chips.  Feh, and here I was hoping I could actually run the things fast
enough to be useful.

Well, perhaps X1 mode can be useful anyway; there are doubtless
applications where its problems would be tolerable - anything
output-only, for example, won't care.  I wonder if it's possible to run
the transmitter with two stop bits and the receiver with only one; that
would help with the drift problem, though not with the "sampling when
the signal is changing" problem.

					der Mouse

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