AVR-Chat

Maybe it's as you say, I don't know otherwise, but I think Asynchronous 
communications CAN recover clock (if not too far off) using any of the 
techniques described in these postings... so if a UART could just do it 
somehow in HW rather than SW...

In synchronous mode, these controllers use a separate clock line, so I don't 
think clock recovery would be needed... so it could be for asynchronous...

----- Original Message ----- 
From: "Jim Wagner" <jim_d_wagner@applelinks.net>
To: <AVR-Chat@yahoogroups.com>
Sent: Thursday, May 31, 2007 5:50 PM
Subject: Re: [AVR-Chat] Re: What serial speed (RS232) can I achieve using 
the built-in osc...


> Cat -
>
> I'm not very clear on this but async data really does not
> use "clock recovery". However, some of the synchronous
> modes do (manchester comes to mind).
>
> The "standard algorithm" is to detect the leading edge of
> the start bit, then sample some number of times during the
> first excpected bit interval (typically 8 or 16). During
> start-bit time, three samples are evaluated near the middle
> of the bit and if they are consistent with it being a start
> bit, then it proceeds. The UART then blindly samples at the
> 8X or 16x rate, taking the majority value of the three
> samples near each expected bit center and calls that the
> bit value. It proceeds blindly through the entire byte
> until the stop-bit is reached, and does the same, there. If
> the stop-bit value is not consistent with what a stop-bit
> ought to be, then typically an error flag is set.
>
> Because the UART samples blindly (that is why it is called
> asynchronous, the "A" im UART), there us a maximum
> allowable difference in the clock rate at the transmitting
> end and the receiving end so that the stop bit samples
> actually fall within the transmitted stop bit. If the
> difference is too great, the stop-bit samples will either
> fall before or after the actual stop bit.
>
> Jim
>
>
> On Thu, 31 May 2007 17:16:17 -0600
> "Cat" <catalin_cluj@hotmail.com> wrote:
>> Page 136 in the ATmega8 doc2486.pdf mentions some "clock
>> recovery" but I'm
>> not sure what it does... could it do some of this
>> adaptation by itself, in
>> the USART?
>>
>> Cat
>> ----- Original Message ----- 
>> From: "np np" <harrabylad@yahoo.co.uk>
>> To: <AVR-Chat@yahoogroups.com>
>> Sent: Thursday, May 31, 2007 4:01 PM
>> Subject: Re: [AVR-Chat] Re: What serial speed (RS232) can
>> I achieve using
>> the built-in osc...
>>
>>
>> > You could try some adaptive software to work out the
>> baud rate ?
>> >
>> > We wanted to use a cheap internal osc so had the PC
>> send out some 55H
>> > characters before transmission.
>> > The PIC measured the bits and used the value as the
>> length of a bit.
>> > It then could match itself to the rx and tx of the PC.
>> >
>> > www.ckp-railways.talktalk.net/pcbcad21.htm
>> >
>> >
>> >
>> > ----- Original Message ----
>> > From: Mike Harrison <mike@whitewing.co.uk>
>> > To: AVR-Chat@yahoogroups.com
>> > Sent: Thursday, 31 May, 2007 10:45:55 PM
>> > Subject: Re: [AVR-Chat] Re: What serial speed (RS232)
>> can I achieve using
>> > the built-in osc...
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >            On Thu, 31 May 2007 15:27:21 -0600 (MDT),
>> you wrote:
>> >
>> >
>> >
>> >>  That is a brilliant tuning suggestion!  I never
>> thought of that one
>> >
>> >><chuckle>.  Another "gotcha" when using the internal RC
>> oscillators is
>> >
>> >>that they are not as stable over temperature
>> >
>> >
>> >
>> > ..or voltage
>> >
>> >
>> >
>> >>as a resonator or crystal.
>> >
>> >>As long as you stay pretty close to the 20-25C
>> temperature range they
>> >
>> >>are fine, otherwise they can drift (see data sheet for
>> the drift rate).
>> >
>> >
>> >
>> > There can also be significant jitter - seems to be
>> worse on some parts
>> > than others - not sure if
>> >
>> > Microchip's tolerance specs take this into account or
>> not.
>> >
>> >
>> >
>> >>DLC
>> >
>> >>
>> >
>> >>> Hi guys,
>> >
>> >>>
>> >
>> >>> Only bit of advice that I can give from my own
>> experience is that the
>> >
>> >>> on chip RC oscillator is FINE for UART when your
>> running on a accurate
>> >
>> >>> 5V supply AND you program the OSCCAL register with
>> the correct value.
>> >
>> >>>
>> >
>> >>> If your VCC is low (3V or 3.3V) etc, you can still
>> use the UART by
>> >
>> >>> setting the value of OSCCAL correctly (Not the value
>> that the
>> >
>> >>> programmer gives you), I have done this by entering
>> into a loop where
>> >
>> >>> I increment the OSCCAL register, then output it's
>> value out the serial
>> >
>> >>> port, as soon as you can read the value, you know you
>> have a value for
>> >
>> >>> OSCCAL that will allow the UART to work correctly.
>> This is NOT a good
>> >
>> >>> way to do things for mass production, but is fine for
>> homers and
>> >
>> >>> on-offs. The loop looks something like:
>> >
>> >>>
>> >
>> >>> OSCCAL = 0;
>> >
>> >>> while(OSCCAL < 255){
>> >
>> >>>
>> >
>> >>>   OSCCAL++;
>> >
>> >>>   UART_Write_Number( OSCCAL);
>> >
>> >>> }
>> >
>> >>>
>> >
>> >>>
>> >
>> >>> Cheers
>> >
>> >>> Hein B
>> >
>> >>> Auckland, New Zealand
>> >
>> >>>
>> >
>> >>> --- In AVR-Chat@yahoogroup s.com, dlc@... wrote:
>> >
>> >>>>
>> >
>> >>>> > On 5/30/07, dlc <dlc@...> wrote:
>> >
>> >>>> >> I've gotten 9600 baud to work well with the
>> internal 1MHz
>> >
>> >>> oscillators of
>> >
>> >>>> >> the Tiny11.  You can handle quite a bit of slop
>> reading serial
>> >
>> >>> data if
>> >
>> >>>> >> your baud rate is low enough.
>> >
>> >>>> >
>> >
>> >>>> > Unfortunately, clock speed error is proportional,
>> so 5% error is 5%
>> >
>> >>>> > error in baud rate, no matter what baud rate you
>> pick.  But, as the
>> >
>> >>>> > baud rates get higher, the granularity of the
>> steps in the UBRR
>> >
>> >>>> > setting get proportionally larger (ie: 20 vs 21 is
>> smaller error than
>> >
>> >>>> > 3 vs 2)
>> >
>> >>>>
>> >
>> >>>>   There is no hardware UART in these small chips, I
>> bit bang my serial
>> >
>> >>>> there and that seems to be more tolerant of bit slip
>> and stretch.
>> >>>> Being
>> >
>> >>>> in software gives me the option of very high
>> granularity.  The hardware
>> >
>> >>>> UARTS are not as tolerant of error.  That is
>> probably the difference.
>> >
>> >>>>
>> >
>> >>>> DLC
>> >
>> >>>> --
>> >
>> >>>> Dennis Clark
>> >
>> >>>>
>> >
>> >>>
>> >
>> >>>
>> >
>> >>>
>> >
>> >>>
>> >
>> >>>
>> >
>> >>> Yahoo! Groups Links
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