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Fosc in PICs

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mora

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I have connected an oscillator of 20 Mhz to the PIC18F452 circuit.

Data sheet says that the operating frequency of PIC18F452 is 40MHz.

In this case, does Fosc = 20 Mhz OR Fosc = 40 MHz ??
 
That is the max operating feq. it can be as low as 30 khz to save power(The faster you run a PIC the more power it needs)
 
Yes the maximum is 40Mhz. The PIC can run from DC (Static!) to 40Mhz. But you can only use 20Mhz crystal (maximum), for higher frequencies you need to use 10Mhz crystal and 4x on-chip PLL to get 40Mhz or use External clock source.
 
To mora,

The Fosc is definitely 20MHz. You can't use HS+PLL mode because frequency is too high. If you connect a 10MHz crystal or use a 10MHz oscillator and enable the HS+PLL mode, then Fosc=40MHz.

Jay.slovak said:
Yes the maximum is 40Mhz. The PIC can run from DC (Static!) to 40Mhz. But you can only use 20Mhz crystal (maximum), for higher frequencies you need to use 10Mhz crystal and 4x on-chip PLL to get 40Mhz or use External clock source.
And it is interesting to know(from the datasheet) that in case of 4xPLL mode using the PIC's on chip oscillator, one can't use a crystal of lower than 4MHz. So one can use only crystals with frequency between 4 MHz and 10MHz. At +85C or higher, the max freq. limit even drops to 6.25MHz.
 
I don't think that 4-10Mhz PLL range is a problem.

Would you ever design a project that would run at 85C ? (Like car computer installed directly on the engine). So that isn't an issue as well.
 
Heard of higher temperatures

Hola Jay,

Few days ago I've read in the Microchip forum, some posts about designs running above 100ºC :!: One of them was an automotive application.

BTW, the micros controlling a car engine, what temperature do they have to sustain Always intrigued by that :?:
 
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