Experimenting with xtals - Page 1
Forum section: electronics / general

author: PerranOak
author_rank: Frequent Contributor
timestamp: August 24, 2018, 04:28:20 pm
content: I would like to play around with xtals and my PIC16F1827 or PIC16F1829: I want to get a feel for using them. Presently, I am only using the internal osc.Could anyone please recommend a make and a typical freq (and other parameters if necessary) that would be good to try out?Cheers

author: hans
author_rank: Super Contributor
timestamp: August 24, 2018, 06:32:04 pm
content: If you look at figure 5-1 you see the clock architecture.The internal oscillator many frequency options, from 500kHz to 32MHz.The external oscillator is limited to only 20MHz crystals. So if you want to get 32MHz clock speed from an external oscillator, you will need to use a 8MHz crystal combined with 4x PLL.Other than that, you could use any crystal up to 20MHz without the PLL. Some obscure frequencies like 14.7456MHz can be interesting, because it is a of the nice ratios with serial baud rates, for example 115200Baud.In addition, an external oscillator also has a better frequency tolerance than the internal oscillator, which is also nice for serial comms.

author: NorthGuy
author_rank: Super Contributor
timestamp: August 25, 2018, 09:28:26 pm
content: It is often better to use silicon oscillator. They now make them not any worse than crystals and they're easier to work with.

author: hans
author_rank: Super Contributor
timestamp: August 26, 2018, 07:52:22 am
content: A frequency tolerance in the percents (silicon) vs ppm (dedicated) is still pretty significant, though. Not to mention the temperature drift you might see.But MCU vendors do advertise to sell parts that run within USB spec (<0.25%) on internal oscillators, which is good enough for many serial protocol applications.In only a few applications where ppm accuracy really matters, like higher speed communication standards, RTC time tracking and wireless node synchronization.

author: Ian.M
author_rank: Super Contributor
timestamp: August 26, 2018, 08:07:39 am
content: One key parameter to be aware of is load capacitance. Every crystal intended for parallel resonant operation has a manufacturer specified load capacitance, and if you don't provide the correct load capacitance in your circuit, you wont get the specified frequency accuracy, or if its way out, it may fail to oscillate (or worse, be unreliable). Typically parallel resonant crystals used with MCUs etc. have load capacitances in the 10pF to 15pF range, though you may find some outside that.In the classicPierce Oscillatorcircuit, used by PIC MCUs and just about any other digital chip with an integrated crystal oscillator driver with two pins to connect the crystal, the load capacitors either side of the crystal are effectively in series from the crystal's point of view, so their effective value is halved. Allowing a few pF for the stray capacitance of the tracks and IC pins on either end of the crystal, that means you generally need to use the next lower preferred value to *DOUBLE* the crystal's specified load capacitance.

author: NorthGuy
author_rank: Super Contributor
timestamp: August 26, 2018, 12:55:35 pm
content: Quote from: hans on August 26, 2018, 07:52:22 amHere's an inexpensive 50 ppm silicon part:https://www.digikey.com/product-detail/en/abracon-llc/ASDMB-8.000MHZ-LC-T/535-11740-1-NDHere's a 10 ppm part:https://www.digikey.com/product-detail/en/abracon-llc/ASDMB-8.000MHZ-XY-T/535-11170-1-ND