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40Mhz on 18F458

 
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Thomas R
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40Mhz on 18F458
PostPosted: Wed Mar 26, 2003 6:32 am     Reply with quote

I need some help on connecting a 40Mhz xtal to a pic18F458

Is there someone that has a tip on the cap. size to use
my xtal data say that it has to haw a 30pF load

the book from Microchip only gives eks. Up to 25Mhz..

try to give my design 2*15pF…But it seems to be unstable
any ideas ?

And NO I do not want to use the PLL whit a 10Mhz

Best Regards
Thomas
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Original Post ID: 13080
john cutler



Joined: 06 Sep 2003
Posts: 82
Location: Hot Tub, California

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Re: 40Mhz on 18F458
PostPosted: Wed Mar 26, 2003 9:50 am     Reply with quote

I just looked at the pdf, and it says a 0 - 40mhz external clock is ok - are you sure a crysta; will work that fast without the pll?


=I need some help on connecting a 40Mhz xtal to a pic18F458
:=
:=Is there someone that has a tip on the cap. size to use
:=my xtal data say that it has to haw a 30pF load
:=
:=the book from Microchip only gives eks. Up to 25Mhz..
:=
:=try to give my design 2*15pF?But it seems to be unstable
:=any ideas ?
:=
:=And NO I do not want to use the PLL whit a 10Mhz
:=
:=Best Regards
:=Thomas
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This message was ported from CCS's old forum
Original Post ID: 13090
Hans Wedemeyer
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Re: 40Mhz on 18F458
PostPosted: Wed Mar 26, 2003 6:31 pm     Reply with quote

:=And NO I do not want to use the PLL whit a 10Mhz

I wonder why you object to using the PLL ?

I have many PIC18 designs using the PLL at 40 and 48MHz they are very stable.
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Original Post ID: 13113
Thomas R
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Re: 40Mhz on 18F458
PostPosted: Thu Mar 27, 2003 4:04 am     Reply with quote

I want to use timer1 whit 40Mhz clock direct so that the resolusion is 25 -09 seconds
and it looks as if i use the pll it is divede by 4 so that it is back to 10Mhz

and i am trying to avoide to use 2 xtals


Thomas



:=:=And NO I do not want to use the PLL whit a 10Mhz
:=
:=I wonder why you object to using the PLL ?
:=
:=I have many PIC18 designs using the PLL at 40 and 48MHz they are very stable.
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This message was ported from CCS's old forum
Original Post ID: 13127
R.J.Hamlett
Guest







Re: 40Mhz on 18F458
PostPosted: Thu Mar 27, 2003 5:10 am     Reply with quote

:=I want to use timer1 whit 40Mhz clock direct so that the resolusion is 25 -09 seconds
:=and it looks as if i use the pll it is divede by 4 so that it is back to 10Mhz
:=
:=and i am trying to avoide to use 2 xtals
:=
:=
:=Thomas
So you have the OSC2 pin, also connected to RC0, and are setting the timer to use the external clock, asynchronoulsy, without a prescaler?.
Your first problem is that the internal crystal oscillator, is _not_ specified to run this fast. To run with a 40MHz source, requires the use of an external oscillator module, and selecting 'EC' as the clock source. Table 27-6, in the data sheet shows this. The maximum oscillator frequency shown with the HS oscillator, is 25MHz.
Also you may well have problems with the input to the counter module. The shortest legal clock period for this input (table 27-10), is 60nsec, in asynchronous mode (16.6MHz), or the processor clock frequency+40nsec, in synchronous mode (15.5MHz).
You are trying to go to far beyond the processors spec...

Best Wishes

:=:=:=And NO I do not want to use the PLL whit a 10Mhz
:=:=
:=:=I wonder why you object to using the PLL ?
:=:=
:=:=I have many PIC18 designs using the PLL at 40 and 48MHz they are very stable.
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This message was ported from CCS's old forum
Original Post ID: 13130
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