Microchip PIC Forum

Saturday, December 8, 2007

Re: [piclist] Re: 12F675 Startup Problems

And that start bit depends upon the the stop bit of the previous
byte. The longer the string transmitted, the more time drift has to
have an effect. I admit that my experiences have to do with _reading_
async serial using the RC oscillator where my bit-banged serial reader
is likely more affected by drift than a hardware UART reading any serial
that I'm transmitting.

DLC

smxcu wrote:
> --- In piclist@yahoogroups.com, dlc <dlc@...> wrote:
>
>>I have run serial comms at 9600 baud and lower using Microchip internal
>>RC oscillators quite successfully for years. The secret is to keep
>
> your
>
>>dialog length very short, one or two bytes at a time so that clock
>
> drift
>
>>won't affect your data much.
>>
>>DLC
>
>
> Hi DLC,
>
> I must stress here that it is not the length of the packet that
> compensates for the use of an inaccurate oscillator but the start bit
> of each character sent. The leading edge of the start bit is what
> causes the receiver to reset its timer. This happens for every
> character that is sent. Keeping the packets short has no effect on
> this re-sync.
>
> Regards
> Sergio Masci
>
> http://www.xcprod.com/XCSB
>
>
> .
>
>
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
> Yahoo! Groups Links
>
>
>

--
-------------------------------------------------
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www.techtoystoday.com
-------------------------------------------------

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[piclist] Re: 12F675 Startup Problems

--- In piclist@yahoogroups.com, Eirik Karlsen <eikarlse@...> wrote:
>
> Use xtal osc.
> One byte is typically 10bits and that means a max drift of +/-5% if
the last bit is to be detected correctly
> (assuming sampeling in the middle of the bit).
> The practical requirement is more like -/+1% for reliable operation.
Only a xtal or maybe resonator
> can achieve that type of accuracy.
>
> kg4pid wrote:

Hi Eirik,

It's actually more like 2.5% and if you look carefully at what you
have written (turn it around) you can get reliable operation if you
decrease the number of bits sent per byte. Remember that this type of
serial comms sends and receives bytes least significant bit first. You
could use a modified character set of 6 or even 5 bits such as one
shown here:

http://homepages.cwi.nl/~dik/english/codes/stand.html

Regards
Sergio Masci

http://www.xcprod.com/XCSB

.

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[piclist] Re: 12F675 Startup Problems

--- In piclist@yahoogroups.com, dlc <dlc@...> wrote:
>
> I have run serial comms at 9600 baud and lower using Microchip internal
> RC oscillators quite successfully for years. The secret is to keep
your
> dialog length very short, one or two bytes at a time so that clock
drift
> won't affect your data much.
>
> DLC

Hi DLC,

I must stress here that it is not the length of the packet that
compensates for the use of an inaccurate oscillator but the start bit
of each character sent. The leading edge of the start bit is what
causes the receiver to reset its timer. This happens for every
character that is sent. Keeping the packets short has no effect on
this re-sync.

Regards
Sergio Masci

http://www.xcprod.com/XCSB

.

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Friday, December 7, 2007

Re: [piclist] Re: 12F675 Startup Problems

I have run serial comms at 9600 baud and lower using Microchip internal
RC oscillators quite successfully for years. The secret is to keep your
dialog length very short, one or two bytes at a time so that clock drift
won't affect your data much.

DLC

Eirik Karlsen wrote:
> Well...as I've said before serial coms should always be done with x-tal
> or maybe resonator osc.
> Trying to get a RC osc to run within 5 - 10% in accuracy and stability
> just isn't possible
> in practice...you might get it to work but when temperature changes, or
> a wire gets too
> close to chip or tracks, or whatever that can pull the osc happends
> you'll get tagged.
>
>
>
> kg4pid wrote:
>
>> To make it work I had to change the the value that gets moved into
>> OSCCAL. Since the serial routine that I used had been around awhile
>> I can only assume that the problem is in the Pic oscilator. Don't
>> know what to do to fix it though as I don't want to have to
>> calibrate each pic chip.
>>
>> Max
>>
> --
> *******************************************
> VISIT MY HOME PAGE:
> <http://home.online.no/~eikarlse/index.htm>
> LAST UPDATED: 23/08/2003
> *******************************************
> Regards
> Eirik Karlsen
>

--
-------------------------------------------------
Dennis Clark TTT Enterprises
www.techtoystoday.com
-------------------------------------------------

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Re: [piclist] Re: 12F675 Startup Problems

The OSCCAL calibration byte is programmed into top of memory in the
chip, as long as you don't erase it. Good programmers and programmer
software will read that byte in before erasing the chip and put it back
out when the chip is programmed.

DLC

kg4pid wrote:
> To make it work I had to change the the value that gets moved into
> OSCCAL. Since the serial routine that I used had been around awhile
> I can only assume that the problem is in the Pic oscilator. Don't
> know what to do to fix it though as I don't want to have to
> calibrate each pic chip.
>
> Max
>
> --- In piclist@yahoogroups.com, "kg4pid" <kg4pid@...> wrote:
>
>>I'm using a serial routine on a 12F675 and started by sending out
>
> the
>
>>word "HELLO" over and over. Funny thing is while it's powered on I
>
> get
>
>>nothing out, but the instant I remove power it sends out 10 or 11
>>characters. The filter cap is powering it for that fraction of a
>>second. The circuit is being powered by a 9V battery with a 78L05
>
> to
>
>>regulate it at 5V. My meter reads 4.99v at the pic chip. A logic
>
> probe
>
>>connected to the output blinks the whole time it is powered up
>
> which
>
>>should mean that its sending something. I had to include this bit
>
> of
>
>>code at the very beginning before I got anything at all.
>>call 0x3FF ; Retrieve Factory Calibration
>>BANKSEL OSCCAL ; BANK1
>>movwf OSCCAL
>>
>>Anyone have any ideas as to what might cause a problem like this?
>>
>>Thanks, Max
>>
>
>
>
>
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the instructions
> Yahoo! Groups Links
>
>
>

--
-------------------------------------------------
Dennis Clark TTT Enterprises
www.techtoystoday.com
-------------------------------------------------

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Re: [piclist] Re: 12F675 Startup Problems

Use xtal osc.
One byte is typically 10bits and that means a max drift of +/-5% if the last bit is to be detected correctly
(assuming sampeling in the middle of the bit).
The practical requirement is more like -/+1% for reliable operation. Only a xtal or maybe resonator
can achieve that type of accuracy.

kg4pid wrote:

To make it work I had to change the the value that gets moved into
OSCCAL. Since the serial routine that I used had been around awhile
I can only assume that the problem is in the Pic oscilator. Don't
know what to do to fix it though as I don't want to have to
calibrate each pic chip.
Max

--
*******************************************
VISIT MY HOME PAGE:
<http://home.online.no/~eikarlse/index.htm>
LAST UPDATED: 23/08/2003
*******************************************
Regards
Eirik Karlsen

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Re: [piclist] Re: 12F675 Startup Problems

Well...as I've said before serial coms should always be done with x-tal or maybe resonator osc.
Trying to get a RC osc to run within 5 - 10% in accuracy and stability just isn't possible
in practice...you might get it to work but when temperature changes, or a wire gets too
close to chip or tracks, or whatever that can pull the osc happends you'll get tagged.

kg4pid wrote:

To make it work I had to change the the value that gets moved into
OSCCAL. Since the serial routine that I used had been around awhile
I can only assume that the problem is in the Pic oscilator. Don't
know what to do to fix it though as I don't want to have to
calibrate each pic chip.
Max

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Saturday, December 8, 2007

Re: [piclist] Re: 12F675 Startup Problems

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Friday, December 7, 2007

Re: [piclist] Re: 12F675 Startup Problems

Re: [piclist] Re: 12F675 Startup Problems

Re: [piclist] Re: 12F675 Startup Problems

Re: [piclist] Re: 12F675 Startup Problems