This has been my observation in practice. As you approach the edges of tolerance you end up with odd failures that can be hard to troubleshoot. I have made it a practice to do my best to design within the constraints provided by the datasheets of the components. My conclusion was that he pain that I earn by not using accurate oscillators is simply not worth the parts savings. On Tue, Sep 25, 2012 at 8:20 AM, Michael Rigby-Jones < Michael.Rigby-Jones@oclaro.com> wrote: > On 9/25/2012 1:48 AM, Lee Mulvogue wrote: > > > > > > I'd always read in the past to not use the internal OSC of a PIC = as > > it was too innacurate, but am now wondering if this is still the case > > for a PIC24F? > > > > I've just started making a board based on a PIC24FJ64GA004 (my > first > > time with a 24F), and added the usual 20MHz crystal and supporting > > caps; but are the modern PICs accurate enough to start considering > > dumping this external setup? The most time sensitive thing I'm doing > > is interrupting @ 16kHz for PCM sound playback (current sample to PWM > > output, set flag for Main loop to process the next sample), and simple > > RS232 comms. The datasheet seems to indicate a max variation of +/- > > 5%, but I don't have a benchmark to compare that against. > > > > I'd possibly be running the 8Mhz internal with the 4x PLL > multiplier, > > resulting in 16MIPS, vs the 10MIPS of the external 20MHz crystal > > without PLL. > > > > Thoughts? > > > > Lee > > -----Original Message----- > From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf > Of Joe Wronski > Sent: 25 September 2012 12:56 > To: Microcontroller discussion list - Public. > Subject: Re: [PIC] PIC24F; dump external OSC? > > >> Check the requirements of the RS232. You might want to consider the > case > >> where one unit is running at +5% and the other at -5% (if both ends ar= e > the > >> same unit). The baud rate accuracy requirement increases with baud > rate, > >> IIRC. Also, your PWM frequency will be affected, so, the pitch of yo= ur > >> sound may vary. The answer is in the requirements, not the opinions o= f > >> people who don't have the numbers. > >> > >> > >>Joe W > > Baud rate accuracy requirements (as a percentage) is independent of the > actual baud rate. An inaccurate baud generator clock will have the same > effect at 300 baud as 115200 baud. > > 5% is already far too much and equates to half a bit slip over a 10bit > word (8,N,1) which will be threshold at which the comms simply can not > work. Reliability problems are likely to occur before you reach 5%, and I > would want no more than about 3% total error including sending and > receiving ends, obviously the lower the better. > > Mike > > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > This e-mail is intended for the person it is addressed to only. The > information contained in it may be confidential and/or protected by > law. If you are not the intended recipient of this message, you must > not make any use of this information, or copy or show it to any > person. Please contact us immediately to tell us that you have > received this e-mail, and return the original to us. Any use, > forwarding, printing or copying of this message is strictly prohibited. > No part of this message can be considered a request for goods or > services. > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 Glen Wiley "A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." - Antoine de Saint-Exupery --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .