Teensy 3.2 RTC load capacitance and intermittent failure to tick

Status
Not open for further replies.

Dascyllus

Member
I have been having issues with the Teensy 3.2 RTC not reliably starting using a 12.5 pF crystal. I am using one of the crystals (+/- 10ppm) recommended on this page:
https://www.pjrc.com/teensy/td_libs_Time.html
https://www.digikey.com/product-detail/en/citizen-finedevice-co-ltd/CFS-20632768EZFB/300-8762-ND/2217073

We use a fair number of Teensy's and have seen about 1/10 not correctly start the RTC oscillator. The observed behavior is that the RTC does not tick. We have also seen some work intermittently. There are no shorts and there are good connections to the K20 pins. Changing the crystal does not always solve the issue.

Looking at the core code, it seems the internal capacitors are set to be 20 pF
https://github.com/PaulStoffregen/cores/blob/a587b3dd1e849d1262f49bf17c91c52ecb04c2a2/teensy3/mk20dx128.c
#ifdef KINETISK
// if the RTC oscillator isn't enabled, get it started early
if (!(RTC_CR & RTC_CR_OSCE)) {
RTC_SR = 0;
RTC_CR = RTC_CR_SC16P | RTC_CR_SC4P | RTC_CR_OSCE;
}

Assuming no stray capacitance, then a 10 pF crystal would be more appropriate, and if there is internal stray capacitance something even lower would be called for. Is this correct?

I will try a lower capacitance crystal and also experiment with changing the core code (but I don't like that solution for future Teensy core updates), but I wanted to make sure I understood what could be going wrong.
 
Changing to 26 pF did not solve issue with 12.5 pF crystal
RTC_CR = RTC_CR_SC16P | RTC_CR_SC8P | RTC_CR_SC2P | RTC_CR_OSCE;
 
Normally the total capacitance to ground is supposed to be double the crystal's rated capacitance. From the crystal's point of view, those 2 capacitors are in series.
 
Normally the total capacitance to ground is supposed to be double the crystal's rated capacitance. From the crystal's point of view, those 2 capacitors are in series.

So, with the core code, it is 16 pF and 4 pF in series---so ~3.2 pF capacitance; or is it two 20 pF in series?
Either way, is the recommended crystal 12.5 pF load capacitance?

Bizarre thing is that if I resolder the joint, it starts ticking. The connection was good before this, so wondering if it is a heating effect.
Have had this be repeatable...resolder, works, leave overnight, not running in morning, resolder, works.
 
Last edited:
Status
Not open for further replies.
Back
Top