Post by thread:crystal oscillator for 74a

Hi I want to clock a 74a with a 20 MHz crystal oscillator from AEL. The oscillator gives me a 17 V peak to peak signal. [-7 ; + 10] I guess I can't feed the pic with such a signal... or can I ? Anyway, is there any chip to convert the signal to the correct range or shall I do it by myself ? Thanks Francois Maletras Kingston University UK

On Tue, 16 Mar 1999 16:38:37 -0000 Francois-Xavier Maletras <spam_OUTK964534TakeThisOuTKINGSTON.AC.UK> writes: >I want to clock a 74a with a 20 MHz crystal oscillator from AEL. >The oscillator gives me a 17 V peak to peak signal. [-7 ; + 10] >I guess I can't feed the pic with such a signal... or can I ? Reduce the signal to 5V p-p (or whatever Vdd you're using) with a resistor divider. Then couple it to the X1 pin of the PIC through a "large" capacitor (1000 pF or more). Set the PIC to HS mode. There's no need to connect anything to X2 or anything else to X1. It's better to err with a little too small an amplitude than too big. Probably anything from 2 V p-p and up will work. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

Plain resistor divder may not work. The negative swing need to be clamped. Hence use a diode to clamp the negative swing. Also coupling through a CAP will remove the DC contents and swing negative. Am I right ? Regards Pailoor Mike Keitz wrote: {Quote hidden}> On Tue, 16 Mar 1999 16:38:37 -0000 Francois-Xavier Maletras > <.....K964534KILLspam@spam@KINGSTON.AC.UK> writes: > > >I want to clock a 74a with a 20 MHz crystal oscillator from AEL. > >The oscillator gives me a 17 V peak to peak signal. [-7 ; + 10] > >I guess I can't feed the pic with such a signal... or can I ? > > Reduce the signal to 5V p-p (or whatever Vdd you're using) with a > resistor divider. Then couple it to the X1 pin of the PIC through a > "large" capacitor (1000 pF or more). Set the PIC to HS mode. There's no > need to connect anything to X2 or anything else to X1. It's better to > err with a little too small an amplitude than too big. Probably anything > from 2 V p-p and up will work. > > ___________________________________________________________________ > You don't need to buy Internet access to use free Internet e-mail. > Get completely free e-mail from Juno at http://www.juno.com/getjuno.html > or call Juno at (800) 654-JUNO [654-5866] ================================================ For Embedded Controls Solutions and Custom Designs ------------------------------------------------------------------------ CHIP TECHNOLOGIES - Member, Microchip Consultant Program Bangalore INDIA Email : chiptechKILLspamvsnl.com Webpage : http://business.vsnl.com/chiptech ( updated 16th Feb 1999 ) ================================================

What about using clamping diodes. Granted, you would have to make sure you ensure the devices switch fast enough and are rated for that amount ofpower. > On Tue, 16 Mar 1999 16:38:37 -0000 Francois-Xavier Maletras {Quote hidden}> <.....K964534KILLspam.....KINGSTON.AC.UK> writes: > > >I want to clock a 74a with a 20 MHz crystal oscillator from AEL. > >The oscillator gives me a 17 V peak to peak signal. [-7 ; + 10] > >I guess I can't feed the pic with such a signal... or can I ? > > Reduce the signal to 5V p-p (or whatever Vdd you're using) with a > resistor divider. Then couple it to the X1 pin of the PIC through a > "large" capacitor (1000 pF or more). Set the PIC to HS mode. There's no > need to connect anything to X2 or anything else to X1. It's better to > err with a little too small an amplitude than too big. Probably anything > from 2 V p-p and up will work. > > > ___________________________________________________________________ > You don't need to buy Internet access to use free Internet e-mail. > Get completely free e-mail from Juno at http://www.juno.com/getjuno.html > or call Juno at (800) 654-JUNO [654-5866] > -------------- Vincent Deno Design Engineer Theta Digital Corp. http://www.thetadigital.com EraseMEdenovjspam_OUTTakeThisOuTemail.uc.edu _____________ | ____ ____ | |/| | | | | |\| | | |/| |\| | | | | | | | | | | | |_/ | | \_| | | | | | | | |_/ /___\ \_|

I would recommend a resistive divider to remove some of the amplitude (probably down to 2V p-p). This would be followed by a coupling capacitor to remove the DC component. You could then run it through a CMOS inverter with the input biased to 1/2 the supply voltage. The output of this should be a good clean CMOS level signal. This would work for whatever supply voltage you are running. The Wenzel and Associates web site at http://www.wenzel.com/documents/library.html has some very good waveform conversion circuits. > {Original Message removed}

On Wed, 17 Mar 1999 12:33:19 +0530 Ravi Pailoor <chiptechspam_OUTVSNL.COM> writes: >Plain resistor divder may not work. The negative swing need to be >clamped. >Hence use a diode to clamp the negative swing. It doesn't matter what DC level exists at the output of the divider since it's coupled through a capacitor to the PIC. But, if the oscillator's amplitude tends to vary, it would be good to clamp with diodes in order to regulate the amplitude. >Also coupling through a CAP will remove the DC contents and swing >negative. The PIC oscillator circuit has internal feedback that sets the DC bias at the X1 input to about Vdd / 2. So it will charge the capacitor enough to prevent the input from swinging negative as long as the input amplitude is small enough (less than Vdd/2 volts peak). ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]