PICs and Crystals

Guest
Hello,

I've got a PIC16F627 and two crystals. One is 12MHz, the other is
20MHz. The 12MHz one says it needs a load cap of 32pF, the 20MHz one
says it needs 30pF. What does this mean!??

The circuit in the PIC datasheet has two caps going from either side of
the crystal to ground. Does that mean that these need to be 32pf each
for the 12MHz and 30pf for the 20MHz?

Also, it says a series resistor might be required - how do I know if it
is??

Thanks

Gareth
 
<garethrichardadams@gmail.com> schreef in bericht
news:1164895915.759962.235550@16g2000cwy.googlegroups.com...
Hello,

I've got a PIC16F627 and two crystals. One is 12MHz, the other is
20MHz. The 12MHz one says it needs a load cap of 32pF, the 20MHz one
says it needs 30pF. What does this mean!??

The circuit in the PIC datasheet has two caps going from either side of
the crystal to ground. Does that mean that these need to be 32pf each
for the 12MHz and 30pf for the 20MHz?

Also, it says a series resistor might be required - how do I know if it
is??

Thanks

Gareth
Click to expand...
Knowing the accuracy of the capacitors to be at least 10%, you can use
standard 33pF ones in both cases. If you think for whatrever reason that
they don't have the right value, you can experiment with values fron 22pF to
47pF, but both capacitors must have the same value. I never had a problem
using either 22pF or 33pF but things may differ for different crystals.
Manufacturers advice or datasheet is usually right.

Very small crystals, like the ones found in clocks or watches, cannot handle
the full 5V swing off the micros internal amplifier. I saw values from 1k to
2k2 over time.

petrus bitbyter
 
garethrichardadams@gmail.com wrote:
Hello,

I've got a PIC16F627 and two crystals. One is 12MHz, the other is
20MHz. The 12MHz one says it needs a load cap of 32pF, the 20MHz one
says it needs 30pF. What does this mean!??
Click to expand...
This is the amount of external capacitance across the
crystal that will cause it to act as a parallel resonant
circuit at the specified frequency. If you use it with a
PIC, it will have a capacitor from each side of the crystal
to ground, with the crystal connected between those two
caps. So those two caps are effectively in series across
the crystal. If each capacitor were 68 pf, the series
combination would equal a single 34 pf capacitor across the
crystal. If you are a little off spec on this capacitance,
the parallel resonance frequency will shift, slightly.

The circuit in the PIC datasheet has two caps going from either side of
the crystal to ground. Does that mean that these need to be 32pf each
for the 12MHz and 30pf for the 20MHz?
Click to expand...
I think those caps should each be twice that capacitance, so
that in series they produce that capacitance.

Also, it says a series resistor might be required - how do I know if it
is??
Click to expand...
There is a power limit for the crystal, and connecting it
directly to the oscillator output may over drive it. Adding
the resistor between oscillator output and crystal lowers
the drive level, but also makes it more likely that the
oscillator won't start. At these frequencies, I suspect
that the oscillator inverter in the PIC is losing enough
gain that it will work fine without the resistor. The worst
it will do is damage the crystal. Can you afford a second
one if this happens?

See:
http://www.citizencrystal.com/images/pdf/tech-09.pdf
http://ww1.microchip.com/downloads/en/AppNotes/00826a.pdf
 
petrus bitbyter wrote:
(snip)
Very small crystals, like the ones found in clocks or watches, cannot handle
the full 5V swing off the micros internal amplifier. I saw values from 1k to
2k2 over time.
Click to expand...
At 1.5 volts, the cmos inverter would have that kind of
output impedance. At 3 volts, I have seen 32.768 tuning
fork crystals with a 330k series resistor, and higher values
for higher supply voltages.
 
"petrus bitbyter" <pieterkraltlaatditweg@enditookhccnet.nl> wrote in message
news:456ef0a2$0$10534$847b8a7a@dreader19...
garethrichardadams@gmail.com> schreef in bericht
news:1164895915.759962.235550@16g2000cwy.googlegroups.com...
Hello,

I've got a PIC16F627 and two crystals. One is 12MHz, the other is
20MHz. The 12MHz one says it needs a load cap of 32pF, the 20MHz one
says it needs 30pF. What does this mean!??

The circuit in the PIC datasheet has two caps going from either side of
the crystal to ground. Does that mean that these need to be 32pf each
for the 12MHz and 30pf for the 20MHz?

Also, it says a series resistor might be required - how do I know if it
is??

Thanks

Gareth


Knowing the accuracy of the capacitors to be at least 10%, you can use
standard 33pF ones in both cases. If you think for whatrever reason that
they don't have the right value, you can experiment with values fron 22pF
to
47pF, but both capacitors must have the same value. I never had a problem
using either 22pF or 33pF but things may differ for different crystals.
Manufacturers advice or datasheet is usually right.

Very small crystals, like the ones found in clocks or watches, cannot
handle
the full 5V swing off the micros internal amplifier. I saw values from 1k
to
2k2 over time.

petrus bitbyter

A couple of good bits of information about crystal oscillators are available
Click to expand...
at:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/1017
http://www.ee.washington.edu/circuit_archive/circuits/F_ASCII_Schem.html#ASCIISCHEM_009

Enjoy.

Richard
 
garethrichardadams@gmail.com wrote:
Hello,

I've got a PIC16F627 and two crystals. One is 12MHz, the other is
20MHz. The 12MHz one says it needs a load cap of 32pF, the 20MHz one
says it needs 30pF. What does this mean!??

The circuit in the PIC datasheet has two caps going from either side of
the crystal to ground. Does that mean that these need to be 32pf each
for the 12MHz and 30pf for the 20MHz?

Also, it says a series resistor might be required - how do I know if it
is??

Thanks

Gareth
Click to expand...
Assuming you're using a parallel-mode crystal (most are like this), if
you know the crystal's load capacitance (check its data sheet), you can
use the formula below:

Let: CL = Crystal's load capacitance
CP = Circuit's parasitic capacitance
CI = IC's input capacitance
C1 = Crystal oscillator capacitor 1
C2 = Crystal oscillator capacitor 2

Then: C1 = C2 = 2 * CL - (CP + CI)

Example: For a crystal with 12pF load capacitance (CL)
and assuming CP + CI = 5pF (typical), we have
C1 = C2 = 2 * 12pF - 5pF = 19pF.

Just round to the nearest available value. The two capacitors, C1 and
C2, are connected from the two crystal's pins to ground (as shown in the
PIC data sheet).

--
Regards,
Costas
_________________________________________________
Costas Vlachos Email: c-X-vlachos@hot-X-mail.com
SPAM-TRAPPED: Please remove "-X-" before replying
 
garethrichardadams@gmail.com wrote:

Hello,

I've got a PIC16F627 and two crystals. One is 12MHz, the other is
20MHz. The 12MHz one says it needs a load cap of 32pF, the 20MHz one
says it needs 30pF. What does this mean!??

The circuit in the PIC datasheet has two caps going from either side of
the crystal to ground. Does that mean that these need to be 32pf each
for the 12MHz and 30pf for the 20MHz?

Also, it says a series resistor might be required - how do I know if it
is??
Click to expand...
It sounds like the datasheet is crap to me !

If the oscillator is anything like the one on an 80C51 ( and it probably is )
make both caps 33pF. Almost any 12MHz crystal will happily oscillate with that.
Should be fine with the 20MHz too.

The crystal data is slightly confusing since the pcb layout and PIC will add
some stray capacitance anyway. All crystals for this kind of application will
oscillate with much the same capacitance and the value isn't ultra-critical
anyway - you'd probably get way with +-30%.

Graham
 
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