At 20.25 2012.01.17, you wrote: > > Nathan, > >Regarding your question about capacitance. There are series cut >crystals and parallel cut crystals. You can use either one in a given >circuit and it will work. However, it won't be ideal, and if you use a >parallel crystal in a series circuit, the frequency of the crystal won't >be what it is stated to be. and vice versa for a series cut crystalin a >parallel circuit. What this boils down to is use the correct crystal >for the circuit you have. > >Regarding your question about using the PLL is a PLL generates some >amount of noise. The bulk of this noise results from jitter of the >incoming frequency being processed (up or down converted by the PLL). If >the circuit you are working with can accept this noise, then by all >means use it. The original intent of using a PLL in a microcontroller >has to do with RF radiation. With a crystal of let's say 20Mhz >connected directly to the microcontroller, if there is any nonlinearity >in the oscillator circuit, there will be some spurious frequencies >generated, and radiated. These spurs will generally be harmonics of the >fundamental. ie 40Mhz, 60Mhz...etc. By using a PLL, internal to the >microcontroller, you can use a lower crystal frequency connected to the >microcontroller, but it can run faster internally, and the generated >and/or radiated RFI will be less. So, you can get the best of both >worlds, assuming you application and circuit can work with the extra >noise. Chances are it can. Only in cases where low noise is a critical >factor would you probably not get by using the internal PLL, and in >those cases you'd almost have to use a crystal cut for the frequency you >want to operate at. What do You mean with noise exactly? What is the concern, additional EMI susceptibility? Thanks. > >Hope this helps you out. > >Regards, > >Jim=20 > >Regards, > >Jim > >> -------- Original Message -------- >> Subject: [PIC] A few questions about crystals >> From: Nathan House >> Date: Tue, January 17, 2012 12:11 pm >> To: "Microcontroller discussion list - Public." >>=20 >>=20 >> I'm making a board for a PIC18 microcontroller and have a few >> questions about crystal selection. Firstly, on Digi-Key, under the >> heading "Load Capacitance," there is the option "Series" and then a >> bunch of different capacitor options below that. Will a series crystal >> work, or should I get one that uses capacitors? One other question I >> have is regarding the crystal frequency. Most PIC18 microcontrollers >> (all of the ones I've used, at least) have a PLL built in, so a >> variety of different crystals can be used with the same resulting >> system clock speed. If the PIC I'm using has a PLL that would allow me >> to use, say, a 4MHz, 8MHz, 12MHz, 20MHz, or 24MHz crystal, is there >> any reason to use a higher speed crystal as opposed to a lower speed >> one? >>=20 >> Appreciate your advice! >>=20 >> Nathan >>=20 >> --=20 >> Student Hobbyist >> www.roboticsguy.com >> --=20 >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > > >--=20 >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .