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

From: David Brown (david_at_no.westcontrol.spam.com)
Date: 08/16/04 

Date: Mon, 16 Aug 2004 09:08:13 +0200

"Neil Bradley" <nb_no_spam@synthcom.com> wrote in message
news:10hvv8scv52i489@corp.supernews.com...
> "Doug Dotson" <dougdotson@NOSPAMcablespeed.NOSPAMcom> wrote in message
> news:ismdnTKuDKd_T4LcRVn-hQ@cablespeedmd.com...
> >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. If you don't
> believe me, try using a crystal at a low baud rate with a 20% tolerance.
> Devices won't be able to talk to it. When the byte comes is the
asynchronous
> part, and that wasn't even the topic being discussed.
>

The phrase "when you are in a hole, stop digging" springs to mind...

In the context of serial communications, "synchronous" means there is a
clock line (or an "embedded clock" in the data signal) going between the
sender and receiver, while "asynchronous" means there is no directly shared
clock, so each side uses its own time reference to clock the transfers. A
"clock" signal does not have to be regular - for synchronous transfers, it
can vary as much as you want. For asynchronous transfers, you normally have
a regular clock - while you could theoreticly have a varying one, it would
be a challange to implement reliably. But if the clocks on the two sides
are not joined directly to synchronize their clocks, they are not
synchronous - it doesn't matter if their speeds match at 0.00001% accuracy.

For standard uart asynchronous communication, the limit for communication
over a good line (noise-free, and nice sharp edges) is a 5% match between
sender and receiver, so that by the 10th bit they are no more than 50% of a
bit in difference. Normally, that means ensuring that each side is within
2.5% of the nominal baud rate. The absolute baud rate does not matter.

> 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. There is no common clock between them. 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?
>
> You do know that the words synchronous and asynchronous can mean different
> things depending on the context, right?
>
> -->Neil
>
>

Relevant Pages

  • Re: "The Observational Collapse of Einsteinian Physics"
    ... > |> You are still not fooling anyone, ... > | If at the point A of space there is a clock, ... Clocks do not synchronize without infinite time. ... Whoever gave you the idea that *I* was the peer in ...
    (sci.physics)
  • Re: Getting NTP to correct only the clock skew
    ... to synchronize B's clock to the source of your CBR data. ... The data stream is provided by a PCI board connected to A. The jitter between the board and A is small enough to be negligible. ... at least the maximum network latency, the packets out of B will have the ... same timing relationship as those out of A, subject only to clock errors. ...
    (comp.protocols.time.ntp)
  • Re: definition of simultaneity and on the relativity of lenghts and times by a. einstein
    ... > Let us take a system of co-ordinates in which the equations of Newtonian ... > If at the point A of space there is a clock, an observer at A can determine ... the clocks at B and C also synchronize with each other. ... > stationary system, and the time now defined being appropriate to the ...
    (sci.physics.relativity)
  • definition of simultaneity and on the relativity of lenghts and times by a. einstein
    ... Let us take a system of co-ordinates in which the equations of Newtonian ... If at the point A of space there is a clock, an observer at A can determine ... the clocks at B and C also synchronize with each other. ... stationary system, and the time now defined being appropriate to the ...
    (sci.physics.relativity)
  • Re: Why should I (not) use an internal oscillator for 8-bit micros
    ... >> that both sides of the serial transmission be synchronized. ... try using a crystal at a low baud rate with a 20% tolerance. ... >clock, so each side uses its own time reference to clock the transfers. ... >are not joined directly to synchronize their clocks, ...
    (comp.arch.embedded)