serial ports on the '286

[David F. Carlson]

In article <5909 at bigtex.uucp>, james at bigtex.uucp (James Van Artsdalen) writes:
> In article <4690 at b-tech.UUCP>, zeeff at b-tech.UUCP (Jon Zeeff) wrote:
> > I think serious thought should be given to supporting the NS16550AN.  
> > This is a plug in replacement for the existing uart that gives you a 
> > 16 byte fifo.  At least the problem would go from "there is nothing 
> 
> I believe the part has settable high- and low-water marks also.  This
> means that it can not only prevent lost characters at high speed (due
> to the kernel having interrupts disabled for long periods), but is can
> also lessen the system load at high speeds by dramatically cutting the
> number of interrupts to be serviced.
> 
The PIC (interrupt controller) doesn't "lose" interrupts, it allows them
to be masked long enough that the 1 deep fifo on the 8250 family is overrun.
Further interrupt events while a pending interrupt is still flagged does
nothing to the CPU but the events that they represent are possibly
irrevocably lost.

> For example, let's suppose uPort modified the serial driver such that
> when the line was put in raw mode, the 16550AN was set to generate an
> interrupt every tenth character.  The interrupt handler would then get
> ten bytes for the overhead of one interrupt, and would still have six
> character times to respond to the interrupt.
> 
> James R. Van Artsdalen    ...!uunet!utastro!bigtex!james 

But raw mode is defined (often) to return in *less* than 10 characters.
The "win" of the fifo is that overruns are nearly impossible, which allows
the driver to make interrupt latency almost a non-issue.  Getting in and
out of an interrupt routine every 0.5 milliseconds is very tough.  Having
a kernel timer set up to check serial status every 8 milliseconds (when 
interrupts have been received recently) is very much easier on the OS
as a whole and the fifo guarantees that characters would not be lost.



-- 
David F. Carlson, Micropen, Inc.
micropen!dave at ee.rochester.edu

"The faster I go, the behinder I get." --Lewis Carroll