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'[PIC]: Using UART at low Fosc freq.'
2003\05\06@072506 by erholm (QAC)

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Hi.
Looking in the data sheet for the 16F628, I can see
no specified data for 9600 baud below 1Mhz Xtal freq.
Have anyone used the UART at 9600 baud at lower
speeds then 1Mhz ?

I'm looking at a (very) lowpower application, and
one (or "the") way to get low power is low speed, right ?
A couple of 100's kHz would probably be just fine.

It will just be a/some PIC talking on a serial line and
on non-volatile memory, probably FRAM due to the write speed.

The "other side" is fixed to 9600 baud.

Jan-Erik Soderholm

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2003\05\06@073950 by Spehro Pefhany

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At 12:25 PM 5/6/2003 +0200, you wrote:
>Hi.
>Looking in the data sheet for the 16F628, I can see
>no specified data for 9600 baud below 1Mhz Xtal freq.
>Have anyone used the UART at 9600 baud at lower
>speeds then 1Mhz ?
>
>I'm looking at a (very) lowpower application, and
>one (or "the") way to get low power is low speed, right ?
>A couple of 100's kHz would probably be just fine.
>
>It will just be a/some PIC talking on a serial line and
>on non-volatile memory, probably FRAM due to the write speed.
>
>The "other side" is fixed to 9600 baud.

I think the lowest frequency you can operate at would be 153.6 kHz
(use a standard tuning fork style crystal), with BRGH=1 and SPRG=0.
Check it, that's back-of-envelope.

Depending on what you are doing, you might be able to get almost as good
results by sleeping the micro between bursts and running at a more
standard frequency like 4MHz.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
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2003\05\06@074945 by

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Spehro Pefhany wrote:

>I think the lowest frequency you can operate at would be 153.6 kHz
>(use a standard tuning fork style crystal), with BRGH=1 and SPRG=0.
>Check it, that's back-of-envelope.

I'll check that. B.t.w, what is "back-of-envelope" ??

>Depending on what you are doing, you might be able to get almost as good
>results by sleeping the micro between bursts and running at a more
>standard frequency like 4MHz.


Yes, I'd like some of the now power/speed functions of some of
the newer PIC's in a 8 pin package. Besides, I havn't found any
UART in any of the 8-pin PIC, correct ?

And it's "the othere side" that initiates the comm, so I don't
know if sleeping would work ?

If we expand the original question to include bitbang serial
comm, how slow could I go then ?

Jan-Erik.

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2003\05\06@080651 by Alan B. Pearce

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> B.t.w, what is "back-of-envelope" ??

It is a term used to suggest a basic sketch of a circuit on any handy
writing surface. Depending on who is in the conversation a similar term is
"back of cigarette packet" as a variation on the same theme. Generally used
to mean doing something basic to get an idea of how something works.

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2003\05\06@081308 by Alan B. Pearce

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>And it's "the othere side" that initiates the comm, so I don't
>know if sleeping would work ?
>
>If we expand the original question to include bitbang serial
>comm, how slow could I go then ?

Well if you are prepared to bit bang your comms on an 8 pin pic, then
consider using the INT input as the receiver input, and you can then
probably use the interrupt to wake up on the start bit. Then it would not
matter quite so much if you are using the 4MHz internal oscillator.

However if you find that you need to go to an external resonator/crystal
consider using the 16F630 14 pin chip to get the extra pins for the crystal
connection without loosing any IO capability. Still no hardware UART, but a
very cheap chip.

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2003\05\06@081521 by Nigel Orr

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pic microcontroller discussion list <> wrote on Tuesday, May 06, 2003 12:49
PM:

> I'll check that. B.t.w, what is "back-of-envelope" ??

Traditionally, engineers who want to check the feasibility of an idea write
out some initial calculations on the back of a nearby envelope, or some
other piece of scrap paper (napkin in a restaurant, toilet paper in certain
other circumstances...), of course, you need to supply your own pen (I
presume that's where the American pocket protector thing comes in, but I've
never seen one!).

> And it's "the othere side" that initiates the comm, so I don't know
> if sleeping would work ?

Could you either wake up on an interrupt pin or use serial handshaking
lines to hold off the transmitter while the device is in sleep.

> If we expand the original question to include bitbang serial
> comm, how slow could I go then ?

The crystal would probably need to be faster, at a guess you'd need more
than 4 instructions per bit, which is what the 153.6kHz crystal suggested
would need.

Nigel
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2003\05\06@091316 by Bob Ammerman

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> And it's "the othere side" that initiates the comm, so I don't
> know if sleeping would work ?
>
> If we expand the original question to include bitbang serial
> comm, how slow could I go then ?
>
> Jan-Erik.

If you combine sleeping and bitbang serial you should be able to operate at
very low average power. Simply use the start bit from the other end to
interrupt and wake from sleep.

Bob Ammerman
RAm Systems

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2003\05\06@091323 by Olin Lathrop

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> Looking in the data sheet for the 16F628, I can see
> no specified data for 9600 baud below 1Mhz Xtal freq.

So do the math yourself.  The tables are only examples.

I use my UART_BAUD and related macros to compute the UART setup
automatically from the clock frequency and the desired baud rate.  These
macros can be found in STD.INS.ASPIC at http://www.embedinc.com/pic.


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

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2003\05\06@091921 by Mike Harrison

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..but remember that you can't do this with an XT osc due to startup time, and accuracy in RC mode
may not be good enough. One way out of this would be if you can rely on always getting a specific byte first (ideally 00),
either you can time it to calibrate an RC osc, or use it to cover the wake-up time of an XT osc.

On Tue, 6 May 2003 09:01:54 -0400, you wrote:

{Quote hidden}

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2003\05\06@093427 by

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Ah !
A "table napkin sketsh" then, I see...
:-)

Jan-Erik.

Alan B. Pearce wrote:
>> B.t.w, what is "back-of-envelope" ??
>
> It is a term used to suggest a basic sketch of a circuit on any handy
> writing surface.

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2003\05\06@101814 by Spehro Pefhany

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At 03:32 PM 5/6/2003 +0200, you wrote:
>Ah !
>A "table napkin sketsh" then, I see...
>:-)
>
>Jan-Erik.

That's the one! Of course we often call paper napkins "serviettes" here,
but that's another story.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffEraseMEspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2003\05\07@062117 by Peter L. Peres

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>That's the one! Of course we often call paper napkins "serviettes" here,
>but that's another story.

I think that designs scribbled on beer coasters are superior.

Peter

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2003\05\07@085326 by Dale Botkin

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On Wed, 7 May 2003, Peter L. Peres wrote:

> >That's the one! Of course we often call paper napkins "serviettes" here,
> >but that's another story.
>
> I think that designs scribbled on beer coasters are superior.

Yeah, at least until morning.  8-)

Dale
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