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'[PIC]Oscillator RC time constant'
2006\09\21@004209 by genome

picon face
How to compute frequency when using an RC Oscilator in a PIC.. An app note
from microchip says that I should use the recommended RC values from the
datasheet but I've searched the datasheet from top to bottom and cannot find
one..
is the computation similar to that of a 555 timer in astable op...



2006\09\21@005620 by Jinx

face picon face
> is the computation similar to that of a 555 timer in astable op...

It's linear

http://home.clear.net.nz/pages/joecolquitt/0f84rc.html

I've no idea if that graph is applicable to newer PICs, but
10k + 100pF @ 5V is about the frequency for a 16F88, so
maybe the RC circuitry is pretty much the same as always,
as is WDT

Some datasheets do have R and C vs Vdd tables. eg DS30487C
(16F88), Figures 19-11, 19-12, 19-13

2006\09\21@022557 by genome

picon face
Ive tested the timing by turning by turning on and off  1 LED at equal
intervals using timer0 for on and timer1 for off:
I'm using pic18F242.. and I've used for RC oscillator first
1 50k resistor @ 39pf ceramic capacitor
there was a long pause after 2 ON OFF cycle ( ON OFF ON OFF then a long
PAUSE equal to the interval of 1 on or off ). contineously
I then change the resistor to 10K... and this time a long PAUSE happens
after 8 ON OFF cycle...
I have reason to beleive that the code works correctly cause it turned on
and off the LED as expected.. so this pauses must have something to do with
the resistor capasitor pair..

what could be the cause of this pauses...
Is there a resistor capacitor pair that the microcontroller works reliably..
I read somewher that there is an RC table values for different frequency in
the datasheet but I could not find it for this particular PIC...
hope anyone have an IDEA...

"Jinx" <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote in message
news:0c4901c6dd3a$410c61d0$0100a8c0@ivp2000...
{Quote hidden}

> --

2006\09\21@035719 by Jinx

face picon face
> 1 50k resistor @ 39pf ceramic capacitor there was a long
> pause after 2 ON OFF cycle ( ON OFF ON OFF then a
> long PAUSE equal to the interval of 1 on or off ). contineously
> I then change the resistor to 10K... and this time a long PAUSE
> happens after 8 ON OFF cycle...

> I have reason to beleive that the code works correctly cause
> it turned on and off the LED as expected.. so this pauses must
> have something to do with  the resistor capasitor pair..

I'll be honest and say that sounds more like a code problem. I
can't see any reason why an RC oscillator should change like
that, and not so dramatically

> Is there a resistor capacitor pair that the microcontroller
> works reliably..

Microchip recommend (eg Fig 2-3 of 18Fxx2 datasheet) R
between 3k and 100k and C > 20pF

The measurements I made

http://home.clear.net.nz/pages/joecolquitt/0f84rc.html

show that the F84A, at least, can be run reliably at any speed from
almost 6MHz downwards

> I read somewher that there is an RC table values for different
> frequency in the datasheet but I could not find it for this particular
> PIC...

Because of the same R and C recommendations in the 18Fxx2
datasheet as in many, if not all, other datasheets, I think you
could assume that the RC oscillator is the same or similar in
every PIC

2006\09\21@102023 by genome

picon face
Yes your right silly me.. I forgot to turn off WDT.. the PIC just keep
reseting.. I notice it when it seems that no matter what capacitor resistor
I change the time interval between the pauses seems constant and only the
blink speed changes...
thanks...

"Jinx" <.....joecolquittKILLspamspam@spam@clear.net.nz> wrote in message
news:0ca801c6dd53$807170e0$0100a8c0@ivp2000...
{Quote hidden}

> --

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