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Small, fast and stable
Generating a stable clock is far from easy: while current microcontrollers usually have some kind of embedded clock source, accuracy requires external components. Integrated oscillators provide a very interesting alternative, one of which we will look at here.
The ASEM-Series by ABRACON is a very reliable standard oscillator chip. In principle, the part does not need any kind of external circuitry. Simply look at the pinout, which presents itself as following…
…The datasheet diagram does not include decoupling capacitors.
ABRACON promises an accuracy of 5 PPM for the first year – quite a good value, which should satisfy most applications where extreme timing accuracy is not required.
ABRACON offers a whole family of parts, which can also be programmed to custom frequencies. Information on what you actually get can be obtained by looking at the numbers of the individual parts, which are set up according to the following scheme:
It takes but one look the part numbers to find out what you are dealing with…
The old way…
For comparison’s sake, let us look at traditional crystals such as 9C-8.000MEEJ-T. Its datasheet promises an accuracy of three PPM – this, of course, is quite a bit smaller.
The price is cheaper, too – this can be seen in the OEMsecrets comparison widget embedded. Keep in mind that there is no such thing as a free lunch. Using a crystal requires you to provide additional components – in the end, integrated crystals can be cost-efficient in small, medium also larger designs.
Fool around town…
Oscillators such as the above-mentioned ASEM-Series generate very steep waveforms. This is a problem if your design has to pass EMC certification or needs to work in an EMI-sensitive environment. Playing it safe is easy; place one or two small decoupling capacitors very close to the part. Furthermore, it is also a smart idea to transfer the clock using some kind of transmission line – you are not in the mood to play around with a network analyzer, at least try to stick to a design similar to the one shown in the sketch.
…The ground trace below the signal should minimize the loop area