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Providing a clock with an MCU

Started by galapogos October 23, 2006
galapogos wrote:
> Robert Adsett wrote: > > galapogos wrote: > > > Rene Tschaggelar wrote: > > > > The usual approach is to have an inverter 74HC04 or > > > > so at the mcu crystal driver port, sometimes named > > > > XTAL2. > > > > > > Hmm, how exactly does that work? My MCU has a 27MHz crystal. > > > > If you can change one of those a bit you could use a simple divide by > > eight. > The thing is, the MCU clock uses a crystal and not an oscillator, so it > provides a sine wave rather than a square wave.
So? The usual divide technique would use a CMOS logic circuit, that would result in a square wave not a sine wave.
>I'm not sure if smart > cards will play nice with them. I've heard that they require > oscillators.
'All' an oscillator is, is a crystal oscillator with a buffer. I'm just suggesting the buffer include a divide by eight. You do need to make sure you don't overload the oscillator circuit. If that's all your concerned about use an oscillator for the micro and run the divide by eight off of it instead. Robert
Robert Adsett wrote:
> galapogos wrote: > > Robert Adsett wrote: > > > galapogos wrote: > > > > Rene Tschaggelar wrote: > > > > > The usual approach is to have an inverter 74HC04 or > > > > > so at the mcu crystal driver port, sometimes named > > > > > XTAL2. > > > > > > > > Hmm, how exactly does that work? My MCU has a 27MHz crystal. > > > > > > If you can change one of those a bit you could use a simple divide by > > > eight. > > The thing is, the MCU clock uses a crystal and not an oscillator, so it > > provides a sine wave rather than a square wave. > > So? The usual divide technique would use a CMOS logic circuit, that > would result in a square wave not a sine wave. > > >I'm not sure if smart > > cards will play nice with them. I've heard that they require > > oscillators. > > 'All' an oscillator is, is a crystal oscillator with a buffer. I'm > just suggesting the buffer include a divide by eight. You do need to > make sure you don't overload the oscillator circuit. If that's all > your concerned about use an oscillator for the micro and run the divide > by eight off of it instead. > > Robert
So I read up on frequency dividers and apparently I can use 3 DFFs in a row to do a divide by 8. I tried to find some quad/octal/hex DFF chips but they all share a common clock, whereas I'll need 3 separate clocks(DFF1's output into DFF2's clock, and so on). Are there any chips that allow this, or will I have to use 3 seperate single DFF chips?