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Differences among crystal, oscillator and resonator

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powersys

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xt crystal/resonator

I'm attending a training on PIC microcontroller today... We need to select the proper setting (either HS, XT, etc) for a particular microchip controller. I'm confusing with crystal, oscillator, and resonator... Could someone pls help me to differentiate them? Thanks.
 

rkodaira

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In order to work PICmicros need a clock obtained from an oscillator. The oscillator circuit can use several components as base of timing and may be external or internal. The options for PICmicros are:

Internal RC: you don´t need any additional component to set this choice. Its accuracy and stability is not good, but it is the cheapest option and you save one or two pins that can be used as I/O ports. In modern PIC models, it varies from 32kHz to 8MHz. Some PICmicro have a calibration register where you can improve the accuracy.

External RC: you have to connect a capacitor and a resistor to make the RC network for the oscillator circuit (a kind of relaxation oscillator). Again the accuracy and stability is not good. These two types of oscillator are used in insensitive timing applications. Of course you lose some pin/ports.

LP (Low Power): the oscillator needs a low frequency crystal (32kHz to 200kHz) and two load capacitors (see table in the PIC datasheet). Used to save power and for low speed applications together with accuracy needed. Good for battery powered circuits. You lose two pins/ports.

XT (Crystal): used for 200kHz to 4MHz frequencies. You can use either a crystal or a ressonator (ceramic type). The crystal is very accurate (30 to 50ppm deviation) and temperature stable (you need two more ceramic load capacitors). Ceramic resonator is less accurate but doesn´t need the load capacitors (embedded in the resonator package) so it is cheaper than crystal choice. You lose tow pins/ports (unless they are exclusive for the crystal)

HS (High Speed): for oscillator frequencies above 4MHz. This is used with crystal or ceramic resonator. This enables a more powerful driver to the oscillator and is the mode where the power consumption is greater. For high speed processing.
Usually the maximum frequency for this mode is 20MHz.

HSPLL (High Speed Phase Locked Loop): this feature found in some 18F PIC models, is used to clock up to 40MHz, using a 10MHz crystal (together with the 2 load capacitors), which frequency is multiplied by 4 to obtain the full speed of 40MHz. You cannot use 40MHz crystal directly (because you would have EMI and it is hard or impossible to find 40MHz fundamental tone crystals).

There is an option to use external clock signals (coming from an external oscillator circuit, so using only one pin), and options with clock output from the PIC, etc.

The choice depends on the timing of the application, power consumption, temperature stability, use of pins (ports), cost of components, etc.
 

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Usually how many pins or legs crystal, oscillator, and resonator has?
 

rkodaira

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Resonators usually have 3 pins (or 2 but not common)

Crystals have 2 pins

Oscillators modules have 4 pins (dil metal can) to 6 pins (smds) used for the GND, VDD, OUTPUT and Enable/Disable pins

Check the datasheets from the manufacturers
 

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