Post by thread:Oscillator types: HS or XT?

I don't understand very well the difference between the XT and HS oscillations options. I use a PIC 16C84 with a 4 MHz crystal, I haven't current absorb problem, Which is the best choice between the two oscillator operation mode (I need clock stability)? Thanks for your time. =================================================== Alboni Giorgio Faenza (Ra) ITALY E-Mail: spam_OUTrac1337TakeThisOuTracine.ravenna.it www.geocities.com/SiliconValley/Heights/5444 (Last Update: 19/06/97) ===================================================

Giorgio Alboni wrote: > > I don't understand very well the difference between the XT and HS > oscillations options. I use a PIC 16C84 with a 4 MHz crystal, I haven't > current absorb problem, Which is the best choice between the two oscillator > operation mode (I need clock stability)? > Thanks for your time. > Although the Microchip databooks relate the oscillator modes (LP, XT, HS) to frequency, this is only an indirect relationship. In most cases it's good enough to use LP for 32kHz, XT for 4MHz and below and HS for greater than this. (These figures are from memory so no flames if I'm mistaken, please.) When you burn the fuse, you are actually setting the amount of feedback resistance across the cyrstal, the value of which depends upon the crystal characteristics. Typically (and _very_ generally), more feedback resistance gives better stability but worse startup - vise versa for lower feedback resistance. LP mode gives the highest amount of feedback resistance, HS the least. For ceramic resonators you should use HS mode even if the frequency is less than 4MHz (I've never used a resonator but I assume the concern is startup). I use an external oscillator so I program for LP mode for lower current - BTW, it would be nice to have a way to totally disable the feedback. Hope this helps. Don't design any medical instruments based upon this advise in case I've transposed the odd HS/XT/LP. :-) Matt