Hello again!

On Thu, 30 May 2002, Lee Jones wrote:

>
> >> If you can arrange to measure the bit widths on the pin then
> >> you will probably arrive at the required baud rate a lot quicker.
>
> Normally, auto-baud is done by measuring the width of a single
> bit.  That bit is normally the start bit of the frame.  To ensure
> that a single bit is measured, the first data bit (low order bit)
> has to be a 1.  This criteriun is met by the sending using an
> ASCII character of carriage return, '1', '3', '5', '7', '9', 'A',
> 'a', 'C', 'c', 'E', 'e', etc.  All that matters is that the low
> order bit of the character is a 1.
>
> Wire the receive data line to a pin.  Watch the pin for voltage
> transition (direction depends on your hardware).  Start a timer.
> When you see the next transtition, stop timer.  Invert bit time
> (via table or If-Else tree) into baud rate.  Program hardware
> UART.  Then check next character for validity & framing.  If it
> doesn't "look right", start over.
> carriage return, ASCII '1', ASCII 'A', ASCII 'a',
>
I was trying this quite sucessfully at first. I was measuring the width of
one bit with every baudrate I need. Everything was working properly and I
was happy, but when I put everything together it wasn't working that good
anymore. One problem I have is that when I turn on the PIC before I send
the character I get one time, and when I send the character and then turn
on the PIC I get another.. So, it looks like I'm measuring an extra bit
when I send the character before turning on my PIC. (the measuring time
was ~2*the first time.) Any ideas what this is? It must be something
before the start bit or am I wrong?

I have been taking some days off with this autobaud, but yesterday I
started to try my first idea, to xorlw the received character with 'A' and
try again with next baudrate if it's wrong. This I got now is improved
from the first one, I've got frame error checking and so one... I also
forgot to flush the receiver buffer in the first version, but now I have
tried with that too. But I'm not really sure how and when to do that so
I've tried many different ways. But since I'm still writing I didn't get
it to work...
I start with 38400 baud and goes down to 19200 and then to 9600 and last
4800 baud. My version works with 38400 without problems, and with 19200
when I press A two times, but when I'm going down to 9600 I don't get it
to work anymore. I have BRGH=1 at 9600 and higher and BRGH=0 at 4800.
I hope someone can give me more ideas what to do.

/Andreas

 > Someone else replied:
>
> > The character for [a US-ASCII] 'A' will have a picture like
> > this on an oscilloscope [...].  A capital S is a start bit, and
> > a lower case S is a stop bit.
> >
> >             _____          _____                         __________
> > waveform         |  S |  0 |  1 |  0 |  0 |  0 |  0 |  0 |  1 |  s |  S |
> >                  -----------    --------------------------         ------
>
> This diagram shows the bits being sent most significant bit first.
> It is reversed.
>
> This is incorrect.  Serial data is transmitted low order bit first.
> It has to be since the number of bits per byte may vary.  Common
> sizes have been 5 bits/byte up to 9 bits per byte.
>

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads