Hi, The crystal speed defines the speed that the processor executes an instruction (deeper insights into clock gearboxes/instruction pipelines aside) but the 4MHz / 20MHz is the maximum clock speed recommended by the manufacturer. Of course, not eveyone runs the processor at the maximum frequency available but chooses the crystal frequency (through the clock division chain) to provide best definition of baud rates or to satifsfy some internal clock resolution requirement. I imagine that the 4MHz parts are cheaper than the 20MHz parts, too. Regards, Dan To: PICLIST@MITVMA.MIT.EDU cc: From: "MATTHEWS, DEAN (D.)" Subject: Re: [PIC]:a/d converters What is the difference between 4mhz and 20 mhz, is it just the speed that the PIC steps through the program and if it is why use the 4mhz? Dean Matthews Reliability Engineering Ford Engine Plant Bridgend South Wales, U.K Tel No:(01656)672597 Fax No:(01656)672558 Email: dmatth14@ford.com ` Please consider your environmental responsibility before printing this e-mail -----Original Message----- From: Alan B. Pearce [mailto:A.B.Pearce@RL.AC.UK] Sent: 24 September 2001 12:37 To: PICLIST@MITVMA.MIT.EDU Subject: Re: [PIC]:a/d converters >I would like to use an onboard a/d converter (8 bit would do) >to interface a 500mv signal into a PIC. I have used a 16f74 & >84 PIC before but these do not have A/D converters. >Can anyone recommend a PIC? Get a 16F876 from RS components. They have DIL and SM 4Mhz ones at acceptable prices for small quantities. If you need 20MHz parts then check out Crownhill. -- 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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads