Arduino crystal load capacitors

At our last Arduino session, several shrimps were built, and they were all running without the load capacitors across the crystal oscillator. Below is a discussion from the Arduino forum about the capacitors.
From the crystal’s point of view, C1 and C2 are in series, so the total that the crystal sees is 11 pF. Stray capacitance and input capacitance on the pins of the ‘328p may account for 8-10 pF. (Maybe a little more; possibly a little less.)
The ‘328P data sheet says The optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in Table 8-6 on page 31.
The table shows “Recommended Range for C1 and C2” is 12-22 pF.
As an example: If your crystal is cut for 16.0000 MHz +- 50 ppm for a load 20 pF, you may find that the actual frequency is a little high; maybe a little more than 50 ppm. If it’s important, you can make adjustments by padding small values in parallel with C1 and C2 and can probably get the actual frequency (at a given temperature) to within 10 ppm of nominal.
Here’s another thing: It’s usually not practical to measure the frequency directly. You make a test program that uses one of the CPU counter/timers to toggle an output pin at some sub-multiple of the clock frequency and measure that with your high-precision frequency counter.
If the initial frequency is too low, then you might want to replace C1 and C2 by 15 pF caps (or 18 pF) and start again.
But first, you may want to ask yourself how important it is to be dead-nuts on.
Also note that crystals from different vendors may have slightly different characteristics even if they are rated for the same load capacitance, so if precision requirements are really, really important (for mass production), qualifying different vendors might have to be done by you in your lab (not just by reading a data sheet or taking some vendor’s word about compatibility with whatever you have previously tested).
It is strongly recommended not to measure the oscillator frequency directly at the crystal pins. The capacitance at the crystal pins is in the range of 10 pF, and the impedance on this signal line is several megaohms. A typical passive probe has a capacitance in the range of 10 pF and an input impedance of about 10 MΩ. Both values are in the range of the oscillator characteristics and heavily influence the behavior of the crystal oscillators. The MSP430 internal digital ACLK clock signal always carries the clock signal of the 32-kHz crystal oscillator. All MSP430 devices have the capability to output ALCK at one of the I/O pins. Measuring at this digital ACLK output does not influence the crystal oscillator in any way. ACLK still gives all necessary information to determine the stability and performance of the setup.

 2.  a 100 pF, In914 diode, and a multimeter. to test a crystal oscillator.

3. You can make a high impedance probe by putting a 1 pF capacitor on the end of your probe. The amplitude measurement will be wrong, but the frequency will be correct. 

4. If you have a radio receiver or spectrum analyzer, you can make a small loop on the end of a coax and put it near the crystal.
Advertisements
This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s