Post by thread:[PIC]: Serial resistor for the oscillator of a PIC

Hi! I have a PIC 16F876 with the oscillator configuration: OSC1 OSC2 ----------CRYSTAL-------------RESISTOR------- | | CAP CAP | | _ _ where CRYSTAL is a 16MHz quartz (-----> UKI brand <----) CAP are 22pF ceramic capacitor The point is that I need to know some suitable value for the serial RESISTOR, since the oscillator doesn't want to start and I need it running with that brand of crystal (UKI). Thank you in advance! -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> I have a PIC 16F876 with the oscillator configuration: > > OSC1 OSC2 > ----------CRYSTAL-------------RESISTOR------- > | | > CAP CAP > | | > _ _ > > where CRYSTAL is a 16MHz quartz (-----> UKI brand <----) > CAP are 22pF ceramic capacitor > > The point is that I need to know some suitable value for the serial > RESISTOR, I always use 0Ohms. Where did you get the idea from to put a resistor there anyway? I don't recall ever seeing a resistor in any Microchip documentation. > since the oscillator doesn't want to start Well, yeah. Can't blame it with a resistor in there. You also have to remember to use HS oscillator mode. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

>OSC1 OSC2 >----------CRYSTAL-------------RESISTOR------- > | | > CAP CAP > | | > _ _ > >where CRYSTAL is a 16MHz quartz (-----> UKI brand <----) >CAP are 22pF ceramic capacitor > >The point is that I need to know some suitable value for the serial >RESISTOR, Why would you put a resistor in there? I've never seen it as you've described above. My datasheet shows an example connection and it doesn't include a resistor anywhere. Leave the resistor out completely and you'll be fine. --Andrew _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.msn.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

part 1 1371 bytes content-type:text/plain; (decoded 7bit) Image was taken from DS30292B-page 123 (PIC16F87X 28/40-pin 8-Bit CMOS FLASH Microcontrollers). Rs may be required in HS mode, as well as XT mode, to avoid overdriving crystals with low drive level specification. Mike. Olin Lathrop wrote: {Quote hidden}> Sent: Tuesday, July 09, 2002 10:24 PM > To: .....PICLISTKILLspam.....MITVMA.MIT.EDU > Subject: Re: [PIC]: Serial resistor for the oscillator of a PIC16F876 > > > > I have a PIC 16F876 with the oscillator configuration: > > > > OSC1 OSC2 > > ----------CRYSTAL-------------RESISTOR------- > > | | > > CAP CAP > > | | > > _ _ > > > > where CRYSTAL is a 16MHz quartz (-----> UKI brand <----) > > CAP are 22pF ceramic capacitor > > > > The point is that I need to know some suitable value for the serial > > RESISTOR, > > I always use 0Ohms. Where did you get the idea from to put a > resistor there > anyway? I don't recall ever seeing a resistor in any Microchip > documentation. > > > since the oscillator doesn't want to start > > Well, yeah. Can't blame it with a resistor in there. You > also have to > remember to use HS oscillator mode. > part 2 15485 bytes content-type:image/gif; (decode) part 3 136 bytes -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> Image was taken from > DS30292B-page 123 (PIC16F87X 28/40-pin > 8-Bit CMOS FLASH Microcontrollers). > Rs may be required in HS mode, as well > as XT mode, to avoid overdriving crystals > with low drive level specification. Thanks Mike, I knew I'd seen it before! If you hadn't posted that, I would have! Sean -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

>Image was taken from > DS30292B-page 123 (PIC16F87X 28/40-pin >8-Bit CMOS FLASH Microcontrollers). > Rs may be required in HS mode, as well >as XT mode, to avoid overdriving crystals >with low drive level specification. ><< resistor.gif >> I noticed that it says "May be required for AT strip cut crystals." Is that what you're using? Series and parallel cuts behave differently, I think. I don't know any more than that. All I know is that I use their recommended crystals and never used a series or parallel resistor. And everything works just fine. --Andrew _________________________________________________________________ Join the world s largest e-mail service with MSN Hotmail. http://www.hotmail.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

{Quote hidden}> -----Original Message----- > From: Olin Lathrop [SMTP:KILLspamolin_piclistKILLspamEMBEDINC.COM] > Sent: Tuesday, July 09, 2002 8:24 PM > To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU > Subject: Re: [PIC]: Serial resistor for the oscillator of a PIC16F876 > > > I have a PIC 16F876 with the oscillator configuration: > > > > OSC1 OSC2 > > ----------CRYSTAL-------------RESISTOR------- > > | | > > CAP CAP > > | | > > _ _ > > > > where CRYSTAL is a 16MHz quartz (-----> UKI brand <----) > > CAP are 22pF ceramic capacitor > > > > The point is that I need to know some suitable value for the serial > > RESISTOR, > > I always use 0Ohms. Where did you get the idea from to put a resistor > there > anyway? I don't recall ever seeing a resistor in any Microchip > documentation. > > > since the oscillator doesn't want to start > > Well, yeah. Can't blame it with a resistor in there. You also have to > remember to use HS oscillator mode. > I thought it was a fairly widely known method to induce crystal oscillators to start reliably. The resistor should be a carbon composition type (i.e. noisy) for best results. Certainly I have seen this technique mentioned in several books, although I have no idea what value would be optimal. Having said that I have never had a problem with crystals or resonators starting reliably on PIC's from 1MHz to 20MHz, even though it seems some people have no end of trouble. Regards Mike -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

> I noticed that it says "May be required for AT strip cut crystals." > > Is that what you're using? Series and parallel cuts behave differently, I > think. I don't know any more than that. AT cut is independent of whether the crystal is intended for series or parallel operation. There is no series or parallel "cut", even though the Microchip docs use that term. > All I know is that I use their > recommended crystals and never used a series or parallel resistor. And > everything works just fine. Same here. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

>I noticed that it says "May be required for AT strip cut crystals." >Is that what you're using? Series and parallel cuts behave differently, I >think. I don't know any more than that. All I know is that I use their >recommended crystals and never used a series or parallel resistor. And >everything works just fine. I suspect that the strip cut crystals may be what are used in low profile cans like the HC49/3U to get the crystal small enough. Normally they are a round disc that would fill the width of an HC49/U can. This is just my surmising, I have not opened any of these to have a look. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

At 07:40 AM 7/10/02 -0400, you wrote: > > I noticed that it says "May be required for AT strip cut crystals." > > > > Is that what you're using? Series and parallel cuts behave differently, I > > think. I don't know any more than that. > >AT cut is independent of whether the crystal is intended for series or >parallel operation. There is no series or parallel "cut", even though the >Microchip docs use that term. There is a difference between "AT strip cut" and "AT cut", however. > > All I know is that I use their > > recommended crystals and never used a series or parallel resistor. And > > everything works just fine. > >Same here. With small SMT crystals this may result in problems (premature aging and even destruction of the crystal) due to overdrive. The maximum drive levels are an order of magnitude or more lower than the larger crystals. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" RemoveMEspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

You are correct Alan. Most any crystal that is packaged in less than a HC49/U can is a strip cut of some kind or another. The designer should always look a the drive power specifications for any crystal they are considering, especially if they are considering SMD or small package crystals. Overdriving the crystal can kill them or degrade them over time. This is especially true of very tiny crystals like 32KHz watch crystals. You often need 180-330K of series resistance for a watch crystal circuit to keep from overdriving it. I've seen *lots* of electronic assemblies where the crystal is seriously overdriven and could have used a series resistor. You can tell if you are overdriving the crystal by looking at the OSC waveform. It should be a nice clean sine wave, slightly less than rail-to-rail. If you are getting a clipped sine wave then you are overdriving the crystal. I put a series resistor in almost all my oscillator circuits. You can always use a 0 Ohm resistor if no resistance is required, but it's very difficult to add the resistor (if necessary) "after the fact". I should add that almost all hardware I work with (prototypes too) are constructed on manufactured printed circuit boards. Matt Pobursky Maximum Performance Systems On Thu, 11 Jul 2002 11:37:58 +0100, Alan B. Pearce wrote: >>I noticed that it says "May be required for AT strip cut crystals." > >>Is that what you're using? Series and parallel cuts behave differently, I >>think. I don't know any more than that. All I know is that I use their >>recommended crystals and never used a series or parallel resistor. And >>everything works just fine. > >I suspect that the strip cut crystals may be what are used in low profile >cans like the HC49/3U to get the crystal small enough. Normally they are a >round disc that would fill the width of an HC49/U can. > >This is just my surmising, I have not opened any of these to have a look. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hi Matt, > You can tell if you are overdriving the crystal by looking at the > OSC waveform. It should be a nice clean sine wave, slightly less > than rail-to-rail. If you are getting a clipped sine wave then > you are overdriving the crystal. Which waveform? The OSC2 or the OSC1? Or both? > > I put a series resistor in almost all my oscillator circuits. You > can always use a 0 Ohm resistor if no resistance is required, but > it's very difficult to add the resistor (if necessary) "after the > fact". I should add that almost all hardware I work with > (prototypes too) are constructed on manufactured printed circuit > boards. For a 20MHz PIC what sort of resistor value do you find you end up using? 1K, 10K, 100K or do you select with a particular batch of crystals? Cheers, John Dammeyer > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On Thu, 11 Jul 2002 07:11:30 -0700, John Dammeyer wrote: >Hi Matt, > >>You can tell if you are overdriving the crystal by looking at the OSC >>waveform. It should be a nice clean sine wave, slightly less than >>rail-to-rail. If you are getting a clipped sine wave then you are >>overdriving the crystal. > >Which waveform? The OSC2 or the OSC1? Or both? I normally look at OSC2 (CLKOUT), since this is the low impedance output drive for the oscillator circuit. Normally it will not significantly effect the oscillator, but the 'scope probe you use has to be considered. A good 10x probe will put about a 10MOhm/10pf load to ground on the PIC output. A cheap 1x probe will many times kill the oscillator. >For a 20MHz PIC what sort of resistor value do you find you end up >using? 1K, 10K, 100K or do you select with a particular batch of >crystals? It really depends on the crystal I'm going to use. If I'm using a typical HC49/U can (1 mW drive level), then no resistor is generally required. If I am using a HC49/US (0.1 mW drive level), then I will go with something around 470 Ohms. For most 32KHz watch crystal circuits, I'll start around 180K Ohms and have gone as high as 330K. 32KHz circuits are extremely difficult to measure as they are very high impedance all around and loading of any kind (even board moisture and dust!) can have a significant effect on the oscillator. >Cheers, > >John Dammeyer P.S. Some interesting (I hope) crystal experiences in an OT post to follow. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads