Real Time Crystal

[Experimentalist]

Hi

Paul suggests some crystals to use for the real time clock here https://www.pjrc.com/teensy/td_libs_Time.html

The Citizen one he mentions is 6.5pF but in the text he specifies that the crystal should be 12.5pF

I have this crystal http://www.hobbytronics.co.uk/watch-crystal?keyword=crystal

Spec:
C-Type Watch Crystal - 32.768KHz Frequency. Ideal for clock circuits
• 32.768KHz Frequency
• 20ppm Tolerance
• 1.1mm Lead Pitch
• 6pF Load Capacitance
• 3mm diameter
• 8mm case length


Once again this has 6pF load capacitance and Paul repeatedly specifies 12pF in the posts I have seen.

What will be the effect of the different load capacitance? Will it matter?

Anyone?

Thanks
Ex

 

[PaulStoffregen]

It'll probably run a bit too fast. 12 pF is recommended.

 

[Experimentalist]

My Mistake

Hi

It appears I made a mistake by searching for "CFS-206" I got to THIS page, which is a 6.5pF crystal. However, all the part numbers Paul specifies are indeed for 12.5 pF crystals. It turns out that CFS-206 is the number of the series of crystals rather than a specific part number.

Having just offered the one I have up to the board it seems the size is the main issue anyway with the can being 8mm long. All the ones Paul specified are 6.2mm long. Doesn't sound like much but we are talking Teensy here :0) and at 8mm it will foul the pin headers I am using.

I am using the breakout board from Tindie https://www.tindie.com/products/loglow/teensy-31-breakout/ and so will only get one shot at it so I will have to wait until I can get one of those specified in Pauls post above.

FYI from reviewing the specs of all those Paul suggests the best option appears to be the 300-8763-ND as this has the best frequency tolerance at +-5 ppm.

The main lesson, as ever, is that it is foolish to doubt Paul :0)

Ex

 

[Experimentalist]

It'll probably run a bit too fast. 12 pF is recommended.

Paul, thanks for that, you got back in whilst I was writing my second post. I presume I am correct that the lowest ppm with regard to frequency tolerance relates to the least clock drift?

 

[pxgator]

You might consider using this T3.x function to compensate your 6pf xtal.

Teensy3Clock.compensate(-x);

With Teensy3Clock.compensate(int), a positive number speeds the clock up
and a negative number slows the clock down

 

[Experimentalist]

You might consider using this T3.x function to compensate your 6pf xtal.

Teensy3Clock.compensate(-x);

With Teensy3Clock.compensate(int), a positive number speeds the clock up
and a negative number slows the clock down

Thanks for the tip

 

[manitou]

FYI
Down in the following file:

https://github.com/manitou48/crystals/blob/master/crystals.txt

you'll see some experiments with different Teensy internal load capacitance settings and their effect on ppm for 32khz crystal