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. 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 .