----- Original Message ----- From: "Gustaf J. Barkstrom" To: Sent: Saturday, February 21, 2004 1:07 PM Subject: [OT]: Greetings from a neophite PIC user. > I purchased a MAX232 RS-232 controller/driver chip, and I've noticed > in the archives that people have had varying results with resonators > and xtals when controlling serial communications in software. I'm wondering > if I have to worry about clock drift between the MAX232 and a PIC, using > the same clock source for both (if possible), and if the MAX232 has the > same problems a software-based RS-232 controller has. I'm still a neophite, > so I will try to make some sensible questions and post to the PIC: threads. > A MAX232 is just a driver. It converts a signal from the 0-Vdd range that the PIC produces to the balanced +/- voltages that the RS232 spec requires. It doesn't have anything to do with the logic or timing of the communication (other than its finite rise time and fall time which limit its maximum usable bitrate) I haven't had any problems with serial communications using the PIC USART through a MAX232 or similar driver. In the datasheet for your PIC in the USART section (p. 114 of the 16F87xA datasheet) you will find a table that shows the % timing error as a function of crystal frequency and serial bitrate. Due to the fact that the baud rate is generated with integer divides (actually just a counter), rounding errors in the timing vary with bitrate. As long as you pick a oscillator/bitrate combination that has a low error you should be fine. Note that this is one reason that you can get off the shelf crystals at non round number frequencies, such as 3.6864 MHz, which divides evenly to many common bitrates for zero error (more precisely, zero division error - there will still be error from crystal drift, etc. but for RS232 bitrates and any decent crystal oscillator circuit this shouldn't cause you any problems) > Anyway, this looks like a very active group, and I'm glad to be with others > interested (and moreover knowledgable) in the electronics world. > It is a fantastic resource. I'm sure you'll find a lot of useful information here and your mechanical knowledge will surely come in handy to the many of the rest of us on the list. Nick > Gustaf > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads