Post by thread:[PIC]: Using PIC oscillator output to drive divide

Hi, I would like to use the PIC oscillator pin (OSC1/2) to drive an external divider as described by the bad ASCII art below eg g=gnd, x=Xtal, inv=internal inverter for HS/XT mode. There is a resistor (or two) missing, potentially. | PIC internal g----| |---------------|----- osc1 | | | X | INV | | | g----| |---------------|----- osc2 | | | |_______________ | |____| 74HC4040N clk-->QE|_________|Seiko 7600 CLK This is similar to the diagram given in the PIC application notes on the page for crysal operation/capactor selection etc Basically, I will be using the crystal to give a nice baud rate multiple but I need to divide this to drive the clock of a Seiko 7600A. What I need to know is will I be able to drive this CMOS divider direct from the PIC oscillator pins or do I need to do some buffering inbetween? I would prefer to not have the xtal frequency affected by this being loaded...... I had thought about making an external clock generator (using an unbuffered inverter and the clock crystal config shown) but why bother when that effectively is the same as what is on the PIC? Any help/suggestions, please? Best regards, Dan -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Dan, That's the same question as, "Can I run two PICs (CMOS devices) off the same xtal" that comes up all the time. Yes. If you are using an xtal, take into account that the second ("slave") device has some capacitive input which must be subtracted from the capacitor normally on the OSC2 pin. (Yes, use OSC2 to drive the slave chip). If your slave device has an OSC2 pin, you can connect it to a third device, etc. You might want to allow for some resistors to prevent overdriving the next stage, too. Andy D Lloyd <.....dan.lloydKILLspam@spam@GB.ABB.COM> on 11/17/2000 08:25:31 AM Please respond to pic microcontroller discussion list <PICLISTKILLspamMITVMA.MIT.EDU> To: .....PICLISTKILLspam.....MITVMA.MIT.EDU cc: (bcc: Andrew Kunz/TDI_NOTES) Subject: [PIC]: Using PIC oscillator output to drive divider? Hi, I would like to use the PIC oscillator pin (OSC1/2) to drive an external divider as described by the bad ASCII art below eg g=gnd, x=Xtal, inv=internal inverter for HS/XT mode. There is a resistor (or two) missing, potentially. | PIC internal g----| |---------------|----- osc1 | | | X | INV | | | g----| |---------------|----- osc2 | | | |_______________ | |____| 74HC4040N clk-->QE|_________|Seiko 7600 CLK This is similar to the diagram given in the PIC application notes on the page for crysal operation/capactor selection etc Basically, I will be using the crystal to give a nice baud rate multiple but I need to divide this to drive the clock of a Seiko 7600A. What I need to know is will I be able to drive this CMOS divider direct from the PIC oscillator pins or do I need to do some buffering inbetween? I would prefer to not have the xtal frequency affected by this being loaded...... I had thought about making an external clock generator (using an unbuffered inverter and the clock crystal config shown) but why bother when that effectively is the same as what is on the PIC? Any help/suggestions, please? Best regards, Dan -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

>You might want to allow for some resistors to prevent overdriving the next >stage, too. And an application for an international broadcasting licence, unless you pay careful attention to ground. A short, low Z path between the chips will be essential. I'd put series Rs in the clock lines, about 100 ohms, but not becuase of any "overdrive" mythology.. (how do you propose to "overdrive" a cmos input, with a signal that runs from ground to VCC?) The series R changes the waveform to trapezoidal, and keeps EMI down. Hard edges will get you hard EMI problems. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

At 06:30 PM 11/17/00 +0000, Bob Blick wrote: >You should have no problems. Andy already mentioned subtracting the ~3 to >10 pF load of the HC4040 from your load capacitor. > >I'd also like to add the maximum frequency limit of the HC4040 is around >20 MHz, they get real flaky real fast when you hit their limit. Can you spell "non-deterministic"? :) We hit this using a 4040 in the bit clock generator for a thermal dot printer. Had to back it down a bit. The CD4040 was hilarous. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

I stand corrected. I also noticed that the OSC2 pin outputs a sine wave. Aren't those better than trapezoids for preventing harmonics? Andy David VanHorn <spamBeGonedvanhornspamBeGoneCEDAR.NET> on 11/17/2000 02:56:23 PM Please respond to pic microcontroller discussion list <TakeThisOuTPICLISTEraseMEspam_OUTMITVMA.MIT.EDU> To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU cc: (bcc: Andrew Kunz/TDI_NOTES) Subject: Re: [PIC]: Using PIC oscillator output to drive divider? >You might want to allow for some resistors to prevent overdriving the next >stage, too. And an application for an international broadcasting licence, unless you pay careful attention to ground. A short, low Z path between the chips will be essential. I'd put series Rs in the clock lines, about 100 ohms, but not becuase of any "overdrive" mythology.. (how do you propose to "overdrive" a cmos input, with a signal that runs from ground to VCC?) The series R changes the waveform to trapezoidal, and keeps EMI down. Hard edges will get you hard EMI problems. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestEraseME.....mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu

At 03:22 PM 11/17/00 -0500, Andrew Kunz wrote: >I stand corrected. > >I also noticed that the OSC2 pin outputs a sine wave. Aren't those better than >trapezoids for preventing harmonics? It's "choose your poison". Sines have lots of even harmonics, squares have lots of odds. People often think that a sine dosen't cause EMI because it dosen't have hard edges. You can't get away from it completely, but you can control it. The R serves two functions, toning down the edges, and also providing a terminating impedance for the track. The track will be somewhere in around 100 ohms or so. (very dependent on layout) But, a 100 ohm resistor will be a lot closer, and support less ringing, than an open circuit (idealized gate) or short circuit (idealized driver) You can trim up the resistor values on the production boards, to get the best match. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEEraseMEmitvma.mit.edu

Hi Dave, I'm sure you know that a pure sine wave would have no harmonics, so it seems to me that you would have to know how it was being distorted in order to know what harmonics are present. I suppose you could make the argument that many types of nonlinearities tend to have the effect of applying an x^2 function to the waveform, thereby introducing mainly 2nd harmonics, but I wonder how generally that applies. Sean On Fri, 17 Nov 2000, David VanHorn wrote: > It's "choose your poison". Sines have lots of even harmonics, squares have > lots of odds. People often think that a sine dosen't cause EMI because it > dosen't have hard edges. You can't get away from it completely, but you can > control it. > -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu

> ... Sines have lots of even harmonics, A sine (or cosine) is a "pure" frequency which contains no harmonics by definition. ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, RemoveMEolinTakeThisOuTspamembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu

At 04:58 PM 11/17/00 -0500, Olin Lathrop wrote: >> ... Sines have lots of even harmonics, > >A sine (or cosine) is a "pure" frequency which contains no harmonics by >definition. True, but if you've got a pic that outputs a pure sine, let me know, I want to buy stock. The osc drive circuits will give a distorted sine, that contains even harmonics. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestKILLspammitvma.mit.edu

part 1 1062 bytes content-type:text/plain; charset=us-ascii Hi, Thanks to everyone who responded to this. Regards, Dan (Embedded David VanHorn <dvanhornSTOPspamspam_OUTCEDAR.NET>STOPspamspam_OUTMITVMA.MIT.EDU>> image moved 17/11/2000 22:51 to file: pic03035.pcx) Please respond to pic microcontroller discussion list <spamBeGonePICLISTSTOPspamEraseMEMITVMA.MIT.EDU> Sent by: pic microcontroller discussion list <KILLspamPICLISTspamBeGoneMITVMA.MIT.EDU> To: EraseMEPICLISTEraseMEMITVMA.MIT.EDU cc: Subject: Re: [PIC]: Using PIC oscillator output to drive divider? Security Level:? Internal At 04:58 PM 11/17/00 -0500, Olin Lathrop wrote: >> ... Sines have lots of even harmonics, > >A sine (or cosine) is a "pure" frequency which contains no harmonics by >definition. True, but if you've got a pic that outputs a pure sine, let me know, I want to buy stock. The osc drive circuits will give a distorted sine, that contains even harmonics. -- Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9 -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-request@spam@spam_OUTmitvma.mit.edu part 2 165 bytes content-type:application/octet-stream; (decode) part 3 131 bytes -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.