Re: Why should I (not) use an internal oscillator for 8-bit micros

From: Neil Bradley (nb_no_spam_at_synthcom.com)
Date: 08/16/04 

Date: Sun, 15 Aug 2004 20:13:38 -0700

"Casey" <cclremovethispart@cox.net> wrote in message
news:JoUTc.21115$pT5.3217@lakeread05...
> Neil Bradley said...
>> "Doug Dotson" <dougdotson@NOSPAMcablespeed.NOSPAMcom> wrote in message
>> Nope, I understand the concept perfectly. When using a UART, it's
>> required
>> that both sides of the serial transmission be synchronized.
> Both sides are NOT synchronized - that's the essence of async
> communications.

They are for the duration of the byte being transmitted. That's what the
start bits are for - to synchronize both ends for the duration of the byte!

>> If you don't
>> believe me, try using a crystal at a low baud rate with a 20% tolerance.
> No, but the two ends can use clocks that are 5% off from each other and
> still communicate successfully.

It depends upon the baud rate. The lower the baud rate the more susceptible
it is to being

>> The question was in reference to the baud rate generating clock, not when
>> the data comes in. For the period of the byte transmission, both sides
>> must
>> be synchronized.
> Incorrect.

Completely correct. Like I said, FOR THE PERIOD OF THE BYTE transmission
they need to be synchronized (or closely enough in time with eachother) in
order to receive the byte successfully.

>> If you have separate
>> clock and data lines, the clock can vary wildly with no adverse effect on
>> communication. No synchronization between devices needed. Is this a hard
>> concept to grasp?
> The clock can't vary - the two ends have to use the same
> "synchronized" clock.

Well, we're mincing words. I'm not saying that a UART is synchronous. I'm
saying that for the duration of the byte being transmitted, they need to be,
for all intents and purposes, synchronized, or damned close to it.

-->Neil

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