0
\$\begingroup\$

I've a problem to choose correctly which Crystal to use with an ARM Cortex M4. In particular, I can't understand relationship between datasheet specifics and a Crystal used in an ARM Cortex M4 EVB.

Here's schematics of ARM Cortex M4 EVB crystal (Crystal used is FQ5032B-12-C-C-C-200-1 Datasheet: http://www.mouser.com/ds/2/160/fq5032-9986.pdf ):

enter image description here

And here's datasheet regarding crystal specifics with this type of MCU:

enter image description here

I can't understand why is calculated Cext as 20pF but used 18pF. Also, Max. Allowed Crystal Capacitance Load is 17.5pF and in this EVB is used 20pF, why? And which problem it can generate(I found it can generate Freq. Drift)?

\$\endgroup\$
1
\$\begingroup\$

Probably they just split the difference between bending the limit on the maximum and the optimal load. The 18pF value is also very common, so maybe they had it in stock.

The likely consequences of exceeding the datasheet maximum for the MCU by a small amount is that some small percentage of units may fail to start oscillating at temperature extremes (where the gm of the amplifier is low). Since this is just an evaluation board, not a commercial product, nobody really cares that much. Using a different value from the optimal load for the crystal will cause a slight systemic error in the oscillation frequency. Again, probably not all that important.

Of course the crystal oscillation frequency will change with temperature, but the amount of change should be in line with the specifications of the crystal. A possible non-temperature-related drift can occur from exceeding the maximum drive power of the crystal, which can be rather small for some types of crystal, but in this case is specified to be very reasonable (less than 50uW).

I must say that the specifications for the oscillator section of that chip are quite excellent in terms of how complete they are. Usually the specs are vague and seem more aimed at deflecting blame. Who is the manufacturer of that particular ARM chip? Looks a bit like Freescale's datasheet format.

\$\endgroup\$
1
\$\begingroup\$

The more the used value differs from the calculated value, the higher the expected drift in the crystal will be.

They calculated 20pF, but this is not a very common value for capacitors, so probably for cost, they rounded off. Now they had to choose between rounding to 22pF and 18pF. Going to 18pF has two reasons going for it:

  1. Rounding down with the same absolute value will have a marginally lower effect than rounding up with the same value. So taking away 2 will have a smaller effect than adding 2. It's very tiny though.
  2. Since the MCU doesn't like driving above 17.5pF rounding up to 22pF would make the problem even worse, where 18pF is only fractionally more than the maximum, so it'll probably be fine if the traces on the PCB are nice and short and don't add much.
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.