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I'm a student and I'm working on a low power communication project. I am trying to design a PCB using the TI CC2540 sample design. There is a MC-306 (32.768kHz, 12.5pf, and 20/50ppm).

I don't know what the 20/50ppm rating is. For me, the size is very important, so I decided to replace it with the FX135A, but its ppm is -20/+20. Will it cause a problem if I use this one instead?

What is the ppm rating in the crystal oscillator?

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Like Olin said, ppm stands for parts per million, and it indicates how much your crystal's frequency may deviate from the nominal value. The MC-306 exists in a 20 ppm and a 50 ppm version. For the 20 ppm version this means that the frequency will be between 32.7673 kHz (32.768 - 20 ppm, or x 0.999980) and 32.7687 kHz (32.768 + 20 ppm, or x 1.000020). These numbers may give you a comfortable feeling, but remember that a month is 2.6 million seconds, so if you want to use a 20 ppm crystal to build an alarm clock, it may have an error of 1 minute per month.
Crystals are available is different precisions, +/-20 ppm is more or less standard, for 10 ppm you'll pay more.
Also, this is basic precision. This frequency may deviate depending on environment factors, mainly temperature.

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PPM stands for "parts per million". It's like percent which is really parts per hundred, but based on million (\$10^6\$) instead of hundred (\$10^2\$). Therefore, 1% = 10,000 PPM (0.01 x 1000000), and 20 ppm = 20/1000000 = 0.00002 = 0.002%.

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  • \$\begingroup\$ Is safe to say that for a 20 ppm tolerance, the oscillator will be off by 20 cycles every million cycles? For example, during the duration when a perfect oscillator would give one millions cycles, a 20 ppm oscillator would give between 999.980 and 1 000 020 cycles? \$\endgroup\$
    – Fredled
    Jan 23 '19 at 18:25

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