Post by thread:QRe: tapping xtal oscillator

Dave VanHorn wrote: > > > The obvious thing to do is to tap into the xtal output. > > So, can somebody tell me what sort of electronic wizardry I have > > to do to make the 4MHtz crystal drive some sort of gate so I can > > divide by 4 and drive the ISD chip??? > > Careful. > > Check the spec, there should be an output drive spec for that pin. If not, > you're in shaky territory, no matter how many hobby projects or protos "just > worked". The most interesting part of a data sheet is often what is NOT in > it. > Thanks - I'm with you, that it worked in the prototype means nothing. Otherwise I'd stick a part in the breadboard and have an answer in the same time I could e mail. Buuuuut....... this means I have to *understand* this circuit. Well, found something interesting - To quote the uChip data sheet: OSC2 - Oscillator Crystal output. Connects to crystal or resonator in crystal oscillator mode. In RC mode, OSC2 pin outputs CLKOUT which has 1/4 the frequency of OSC1, and denotes the instruction cycle time. OK - if I can find a reasonable price can oscillator, I don't need the divide by 4 chip. That might justify some/all of the can's price. > and the eng mgr told me that all the protos > worked, and "don't worry.." Guess who got to fly to kaohsiung christmas > day to debug an assembly line full of product that didn't run... :( Uhhh.... let me guess. It was the eng mgr? Right? No??? Did he happen to have po inty hair? -- Anniepoo Need loco motors? http://www.idiom.com/~anniepoo/depot/motors.html

On Sat, Aug 28, 1999 at 08:38:13AM -0700, Anne Ogborn wrote: > > OSC2 - Oscillator Crystal output. Connects to crystal or resonator in > crystal oscillator mode. In RC mode, OSC2 pin outputs CLKOUT which has 1/4 the > frequency of OSC1, and denotes the instruction cycle time. I noticed that, too, and it did seem like a way to do it. But uChip never says anything about Fosc/4 for anything *but* the RC mode. Does it do this in XT or HS mode? --Bob -- ============================================================ Bob Drzyzgula It's not a problem spam_OUTbobTakeThisOuTdrzyzgula.org until something bad happens ============================================================

> Thanks - I'm with you, that it worked in the prototype means nothing. Otherwise > I'd stick a part in the breadboard and have an answer in the same time I could email. > > Buuuuut....... > this means I have to *understand* this circuit. That's the fun bit. Hanging a track on it connected to a gate isn't JUST hanging a little extra C on it. It couples in some noise (Remember that "Route no tracks near the xtal tracks" directive?) and it's a potential EMI source. The chip you drive better have a nice low Z return to the pic. Series R would probably help all around, as long as the edges don't get too fuzzy. You MIGHT be better with a discrete oscillator, feeding both circuits. A simple 3 gate HC14 osc can drive a ton, since you aren't needing to tap into the xtal nodes to do it. > OSC2 - Oscillator Crystal output. Connects to crystal or resonator in > crystal oscillator mode. In RC mode, OSC2 pin outputs CLKOUT which has 1/4 the > frequency of OSC1, and denotes the instruction cycle time. But can you clock it that fast in RC mode? > > and the eng mgr told me that all the protos > > worked, and "don't worry.." Guess who got to fly to kaohsiung christmas > > day to debug an assembly line full of product that didn't run... :( > > Uhhh.... let me guess. It was the eng mgr? Right? No??? Did he happen to have pointy hair? Nope, me.

> You MIGHT be better with a discrete oscillator, feeding both circuits. A > simple 3 gate HC14 osc can drive a ton, since you aren't needing to tap into > the xtal nodes to do it. Kill two birds with one stone and use a (74HC)4060 which has the oscillator amp built in (equivalent to PIC HS mode). You get the oscillator and the divider chain to get clock sources for both chips. Steve. ====================================================== Steve Baldwin Electronic Product Design TLA Microsystems Ltd Microcontroller Specialists PO Box 15-680, New Lynn http://www.tla.co.nz Auckland, New Zealand ph +64 9 820-2221 email: .....stevebKILLspam@spam@tla.co.nz fax +64 9 820-1929 ======================================================