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'[PIC] Error % charts of PIC USART ?'
2005\05\13@112011 by Kenasw

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In the datasheet of 18F2455 we have Table 20-3 Baud Rates For Aynchronous Modes (USART module)
(I guess other pics would have the same thing as well)
The errors seem to range from 0.16% up to -45.75%. Are all these
values valid, or should I be aiming at a certain percentage, I am confused?

2005\05\13@114244 by Mauricio Jancic

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That percentage is the actual error if you compare the actual speed of the
PICmicro using the mentioned (in the datasheet) crystal and the speed
standard.

For example, the standard speed is 19200 baud, but with the 0.16% error you
will have (in the PIC) 19230 baud.

See?

Regards,

Mauricio Jancic
Janso Desarrollos - Microchip Consultants Program Member
spam_OUTinfoTakeThisOuTspamjanso.com.ar
http://www.janso.com.ar
(54) 11 - 4542 - 3519


> {Original Message removed}

2005\05\13@121128 by Jan-Erik Soderholm

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kenasw@btinternet.com wrote :

> In the datasheet of 18F2455 we have
> Table 20-3 Baud Rates For Aynchronous Modes (USART module)
> (I guess other pics would have the same thing as well)
> The errors seem to range from 0.16% up to -45.75%. Are all these
> values valid, or should I be aiming at a certain percentage, I am
> confused?

Note that the table are just examples. A lot of them will
*never* work. Try to stay (in *your* application) within
"a few" % from the nominal speed.

Exactly how many "a few" is varies a little depending on
who you ask, but 1-2 % would probably be "safe"...

Note that there are "special" crystals with "funny" (odd)
speeds that makes it easier to be on the spot.

Jan-Erik.



2005\05\13@121152 by Kenasw

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----- Original Message -----
From: "Mauricio Jancic" <.....infoKILLspamspam@spam@janso.com.ar>
To: "'Microcontroller discussion list - Public.'" <piclistspamKILLspammit.edu>
Sent: Friday, May 13, 2005 4:37 PM
Subject: RE: [PIC] Error % charts of PIC USART ?


> That percentage is the actual error if you compare the actual speed of the
> PICmicro using the mentioned (in the datasheet) crystal and the speed
> standard.
>
> For example, the standard speed is 19200 baud, but with the 0.16% error
you
> will have (in the PIC) 19230 baud.
>
> See?

I see now, but I still do not quite understand what I can choose if I want
to connect to the PC com port. If I am not mistaken every char is 9 to
10 bits including the stop bit(s), so if  the error is -45.75 percent in the
table, I would need to multiply this by 9 for the whole char. This will
give me an error of 450%. Am I doing something wrong here?

>
> Regards,
>
> Mauricio Jancic
> Janso Desarrollos - Microchip Consultants Program Member
> .....infoKILLspamspam.....janso.com.ar
> http://www.janso.com.ar
> (54) 11 - 4542 - 3519
>
>
> > {Original Message removed}

2005\05\13@123123 by Jan-Erik Soderholm

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kenasw@btinternet.com wrote :

> I see now, but I still do not quite understand what I can
> choose if I want
> to connect to the PC com port. If I am not mistaken every char is 9 to
> 10 bits including the stop bit(s), so if  the error is -45.75
> percent in the
> table, I would need to multiply this by 9 for the whole char.
> This will
> give me an error of 450%. Am I doing something wrong here?

Forget about those %-figures that are larger then "a few" % !

No matter what speed the "other end" is using, your PIC
shouldn't be off more then "a few" % from that. Try to stay
within 1-2 %, and you'd probably be safe...

Jan-Erik.



2005\05\13@125905 by olin_piclist

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kenasw@btinternet.com wrote:
> In the datasheet of 18F2455 we have
> Table 20-3 Baud Rates For Aynchronous Modes (USART module)
> (I guess other pics would have the same thing as well)
> The errors seem to range from 0.16% up to -45.75%. Are all these
> values valid, or should I be aiming at a certain percentage, I am
> confused?

Consider anything over about 2.5% bad.

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\05\13@130043 by Kenasw

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----- Original Message -----
From: "Jan-Erik Soderholm" <EraseMEjan-erik.soderholmspam_OUTspamTakeThisOuTtelia.com>
To: <piclistspamspam_OUTmit.edu>
Sent: Friday, May 13, 2005 5:31 PM
Subject: RE: [PIC] Error % charts of PIC USART ?


{Quote hidden}

o.k. 2% times 9 = 18% per char. Assuming we are sampling in the
middle we have 50% margin, right? so 18% will be o.k. , 3 or 4
percent should still work, anything  above this *should* fail.

this would make for an interesting experiment ...

>
> Jan-Erik.
>
>
>
> --

2005\05\13@131430 by Bob Blick

face picon face
> In the datasheet of 18F2455 we have
> Table 20-3 Baud Rates For Aynchronous Modes (USART module)
> (I guess other pics would have the same thing as well)
> The errors seem to range from 0.16% up to -45.75%. Are all these
> values valid, or should I be aiming at a certain percentage, I am
> confused?

Usually if I am designing something using the usart, I choose a crystal
frequency that divides well. For the 16-series PICs and PC baud rates,
19.6608 MHz is readily available.

For the 18 series with PLL, you will need to choose something 10MHz or less.

Cheers,

Bob


2005\05\13@131844 by Dave VanHorn

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At 12:14 PM 5/13/2005, Bob Blick wrote:
> > In the datasheet of 18F2455 we have
> > Table 20-3 Baud Rates For Aynchronous Modes (USART module)
> > (I guess other pics would have the same thing as well)
> > The errors seem to range from 0.16% up to -45.75%. Are all these
> > values valid, or should I be aiming at a certain percentage, I am
> > confused?
>
>Usually if I am designing something using the usart, I choose a crystal
>frequency that divides well. For the 16-series PICs and PC baud rates,
>19.6608 MHz is readily available.
>
>For the 18 series with PLL, you will need to choose something 10MHz or less.

Yup. When using uarts to talk to other standard devices, it's best to
use a baud rate multiple crystal.
Then the error is zero, and you don't have to worry.

2005\05\13@132045 by Jan-Erik Soderholm

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kenasw@btinternet.com wrote :

> > No matter what speed the "other end" is using, your PIC
> > shouldn't be off more then "a few" % from that. Try to stay
> > within 1-2 %, and you'd probably be safe...
>
> o.k. 2% times 9 = 18% per char.

I'm not sure why you are making it harder then it is... :)

I think you are talking about a half-bit time after a full byte, right ?
Remember also that the *other end* can be off in the
other direction.

As have been said before, just stay within 1-2 % and
you will probably be safe.

Jan-Erik.



2005\05\13@133812 by Dave VanHorn

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At 12:20 PM 5/13/2005, Jan-Erik Soderholm wrote:
>KILLspamkenaswKILLspamspambtinternet.com wrote :
>
> > > No matter what speed the "other end" is using, your PIC
> > > shouldn't be off more then "a few" % from that. Try to stay
> > > within 1-2 %, and you'd probably be safe...
> >
> > o.k. 2% times 9 = 18% per char.

2% is 2%.  you'll be 18% of a BIT off at the end of a word.

2005\05\13@140154 by Russell McMahon

face
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>> > No matter what speed the "other end" is using, your PIC
>> > shouldn't be off more then "a few" % from that. Try to stay
>> > within 1-2 %, and you'd probably be safe...
>>
>> o.k. 2% times 9 = 18% per char.
>
> I'm not sure why you are making it harder then it is... :)
>
> I think you are talking about a half-bit time after a full byte,
> right ?
> Remember also that the *other end* can be off in the
> other direction.
>
> As have been said before, just stay within 1-2 % and
> you will probably be safe.

Theoretically you are "allowed" a total error from all sources of
about 50/N% where N is the total bits used per word. For 8+1+1 = 10
its you can tolerate an all up 5% error. As Jan says, if total error
is about 40%of this maximum value things will probably work OK.





       RM

2005\05\13@140614 by Kenasw

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----- Original Message -----
From: "Jan-Erik Soderholm" <RemoveMEjan-erik.soderholmTakeThisOuTspamtelia.com>
To: <spamBeGonepiclistspamBeGonespammit.edu>
Sent: Friday, May 13, 2005 6:20 PM
Subject: RE: [PIC] Error % charts of PIC USART ?


> TakeThisOuTkenaswEraseMEspamspam_OUTbtinternet.com wrote :
>
> > > No matter what speed the "other end" is using, your PIC
> > > shouldn't be off more then "a few" % from that. Try to stay
> > > within 1-2 %, and you'd probably be safe...
> >
> > o.k. 2% times 9 = 18% per char.
>
> I'm not sure why you are making it harder then it is... :)

I am not making it harder, I will choose what you suggested, I am
simply thinking aloud as they say, because I feel it is important to
also understand *why* we do what we do.

>
> I think you are talking about a half-bit time after a full byte, right ?

yes

> Remember also that the *other end* can be off in the
> other direction.

right, good point, my guess is that the pc will have a clock
which is a multiple of the baud rate so that, as others have
said, with such a crystal there will be no error.

>
> As have been said before, just stay within 1-2 % and
> you will probably be safe.
>
> Jan-Erik.
>
>
>
> --

2005\05\13@141401 by Kenasw

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----- Original Message -----
From: "Bob Blick" <RemoveMEbblickspamTakeThisOuTsonic.net>
To: "Microcontroller discussion list - Public." <piclistEraseMEspam.....mit.edu>
Sent: Friday, May 13, 2005 6:14 PM
Subject: Re: [PIC] Error % charts of PIC USART ?


{Quote hidden}

This seems to fit well in the formula of the 18 series, I think

>
> For the 18 series with PLL, you will need to choose something 10MHz or
less.
>
> Cheers,
>
> Bob
>
>
> --

2005\05\13@144330 by Wouter van Ooijen

face picon face
> right, good point, my guess is that the pc will have a clock
> which is a multiple of the baud rate so that, as others have
> said, with such a crystal there will be no error.

Are willing to bet your neck on this? If not you'd better assume that
the PC designer and you make the same assumptions, so each side can
claim half of the error budget (that is, what's left of it after the
transceivers, cable effects etc. have taken their part).

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\05\13@154005 by Kenasw

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----- Original Message -----
From: "Wouter van Ooijen" <EraseMEwouterspamvoti.nl>
To: "'Microcontroller discussion list - Public.'" <RemoveMEpiclistEraseMEspamEraseMEmit.edu>
Sent: Friday, May 13, 2005 7:43 PM
Subject: RE: [PIC] Error % charts of PIC USART ?


> > right, good point, my guess is that the pc will have a clock
> > which is a multiple of the baud rate so that, as others have
> > said, with such a crystal there will be no error.
>
> Are willing to bet your neck on this? If not you'd better assume that
> the PC designer and you make the same assumptions, so each side can
> claim half of the error budget (that is, what's left of it after the
> transceivers, cable effects etc. have taken their part).

Well, obviously I will start with low error values, then see how
far I can push my luck ...

>
> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: http://www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: http://www.voti.nl/hvu
>  
>
> --

2005\05\13@184055 by Jinx

face picon face
> > 19.6608 MHz is readily available.
>
> This seems to fit well in the formula of the 18 series, I think

Yes it does, but you can't PLL (ooops, another noun verbalised)
that. If you want to run the 18F at high speed with 0% comms
error, use 9.8304MHz. Times 4 = 39.3216MHz. For 20MHz
devices, no PLL, 19.6608 or 18.432. IOW divisible by 256,
not 10 (as in the case of 20MHz, 10MHz, 4MHz etc)

PS I really dislike that table because it emphasises comms error
rates when it's so easy to attain 0% with better (sensible ?) xtal
selection

2005\05\13@224507 by Mike Hord

picon face
> > In the datasheet of 18F2455 we have
> > Table 20-3 Baud Rates For Aynchronous Modes (USART module)
> > (I guess other pics would have the same thing as well)
> > The errors seem to range from 0.16% up to -45.75%. Are all these
> > values valid, or should I be aiming at a certain percentage, I am
> > confused?
>
> Consider anything over about 2.5% bad.

Don't forget to take oscillator accuracy into this; for example, the
onboard osc is usually specced at (IIRC) ~1% (for most PICs),
ceramic resonators are frequently ~.5%, and crystals are ~20ppm.

In the US, Mouser sells in onsie-twosie quantities a ceramic
resonator which gives "good" baud rates, 3.68 MHz.  Unfortunately,
it's surface mount only.

Mike H.

2005\05\16@093118 by alan smith

picon face
So, just curious....

The percentage off related to the baud rate, means it
would be hard pressed to communicate with a device
running the standard baud rate.

What about two PICs talking over serial....at that
point if they are both off by the same percentage,
then it really shouldnt matter?



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2005\05\16@113403 by Mike Hord

picon face
> So, just curious....
>
> The percentage off related to the baud rate, means it
> would be hard pressed to communicate with a device
> running the standard baud rate.
>
> What about two PICs talking over serial....at that
> point if they are both off by the same percentage,
> then it really shouldnt matter?

Correct.  In that case, you're simply using a non-standard
baud rate, and unless you go through a bridge of some
sort that expects a standard baud rate, you'll be fine.

You still need to account for oscillator error, though.  For
example, let's say your particular oscillator technology
guarantees +/-1% accuracy.  In that case, you might be
in trouble, because if one device is at +1% and the other
at -1%, the 2% total error may (or may not) cause problems.

Most of the wise elders of the list suggest keeping error
at or below 1%.  I'm not inclined to argue with them.

Mike H.

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