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

From: Casey (cclremovethispart_at_cox.net)
Date: 08/16/04 

Date: Sun, 15 Aug 2004 20:49:29 -0500

Neil Bradley said...
> "Doug Dotson" <dougdotson@NOSPAMcablespeed.NOSPAMcom> wrote in message
> >
> >I believe that UART stands for "Universal ASYNCRONOUS Receiver
> > Transmitter". You need to go back and study the difference between
> > sync and async.
>
> 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. The clocks at the transmitter and receiver ends are
not exactly the same. They just have to be close enough to sample the
pulses correctly.

> 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.

> 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.

> There is no common clock between them.

Correct.

> 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.
 
> You do know that the words synchronous and asynchronous can mean different
> things depending on the context, right?

You're using the terms exactly backwards. If you insist on using the
terms incorrectly, you'll be unable to communicate with anyone else.

www.inetdaemon.com/tutorials/theory/basics/asynchronous_synchronous.htm

Casey

Relevant Pages