Fez Cerb40 and high capacitance on 32.768Khz crystal oscilator


I’m developing my own stm32f4 board and reading cerb40 schematic and reading datasheet.

I think the capacitance on this oscillator is too high. The maximum per datasheet(section 5.3.8, page 93) is 15pF and you recommend installing a 12.5pF crystal on cerb40 page and you have 27pF capacitors already soldered on board plus the board/trace capacitance(around 2pF to 7pF). This gives a very high capacitance on this pins. Is this correct?

Note that I’m not a hardware design I’m just trying to understand your design decision.


Wagner Sartori Junior

Yes correct. That was a design error. the 32Khz will not work with 27pF at all. They need to be removed.

what do you recommend to resolve this problem?

Use a lower capacitance crystal, lets say with 6pF and swap de 27pF capacitors with 8pF?

Yes that would be best. We tried it with no capacitors at all and it worked but this is not recommended.

Will GHI be mailing out capacitor packs to purchasers? If I order a Cerb40 today, will I get a corrected board?


I’m evaluating the Cerb40 as a maple mini replacement for my www.preenfm.net synth.

I’m not sure i get it.
What is the consequences of this problem ?



I do not think you will use RTC so no consequence.

The consequence is that the RTC will not function without modifying the board. Gus, please, it’s a disservice to your customers to sweep this under the rug. We’re big boys (and girls?), we can take it. Products have errata, it’s not the end of the world. Please respect us enough to be up-front and clear about it.

Just to be clear RTC = Real Time Clock? We always use the RTC on the chip. I had thought that the Timer system was dependant on the RTC as well.

The RTC is indeed the Real Time Clock, but it’s not the processor’s “clock” signal. It’s a special piece of hardware (in this case, embedded as part of the chip), that keeps time, analogous to the clock hanging on the wall. It keeps time even when the processor isn’t running, so that when the processor starts back up, it knows what time (of day) it is.

Maybe it would be clearer if whoever named it the “Real Time Clock” would have named it “Real Time Clock/Calendar” instead.

None of the Timer components (neither the NETMF Timer class, nor the timer hardware included in the chip) depends on the RTC.

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We are not sweeping anything this is VERY clearly stated everywhere. His application doesn’t seem to need RTC so he is not affected whatsoever like stated in my last reply.

But for the sake of getting this from “under the rug”. RTC on Cerb40 will NOT work until you replace the 2 capacitors with smaller ones and also add 32.768Khz crystal as shown here http://wiki.tinyclr.com/index.php?title=FEZ_Cerb40_Developer#Real_Time_Clock Do not forget to add a 3V disc battery as well

This will not effect the system or system-time or anything at all, except if you are using RealTimeClock class. You can still set time and get time (DateTime.Now) but you will lose time only when power is turned off.

See this is exactly what happens when I don’t read :slight_smile:

Gus, what is your response to this line in the datasheet (page 93):

Further, AN2867 (Oscillator design guide for STM8S, STM8A and STM32F1 microcontrollers) which is recommeded in the STM32F4 datasheet, on page 19, says:

It seems to me that GHI’s recommended crystal, at 12.5 pF, is still unsuitable, regardless of the load capacitors chosen. Is this not the case?

@ godefroi - I never look at this myself. I will check with the team and update the wiki. Thanks for pointing this out.

AN2867 gives a list (page 19) of recommended parts, from various manufacturers, all with CL of either 6 or 7 pF.

You can get the app note here: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00221665.pdf

Thanks a lot, this was exacly what i was looking for.


Thanks for updating the wiki, Gus, but your Mouser and DigiKey links still point to the wrong 12.5 pF crystal. This problem also affects the Cerberus and Cerbuino boards, so the scope needs to be expanded a bit. May I humbly suggest a new wiki page, linked to from the other pages, explaining in detail the issue and the various possible solutions?

I didn’t update the wiki and thus do not effect other boards as they do not have thd caps I think.

@ godefroi - May you help us with the part number? :slight_smile:

With a part number for a suitable crystal? I don’t have any better suggestions than what’s in the application note. Digi-Key sells one of the suggested Epson crystals, C-002RX, which is a 6 pF crystal: http://www.digikey.com/product-detail/en/C-002RX%2032.7680K-E:PBFREE/SER3205-ND/1022093

I would suggest, however, a call or support request with ST to get the word straight from them on what they recommend. I wish the datasheet went into more detail.