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[SOLVED] Frequency formula for external RC oscillator PIC16C773

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JohnJohn20

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Please, an obvious question maybe.

All I can find in the PIC16C77X datasheet for using an external resistor and capacitor to control the oscillator frequency is a description of how to connect them.

While they talk about how the different chips will be slightly different, is there a formula which I can use to make a good guess at suitable values for Rext and Cext?

Thanks.
 

Hi,

Have never used the r/c clock , they say it is not very stable for anything using timers, though good enough for simple projects.

Seems the formula is 1/ ( C1 x R1) R in Meg Ohms and C in uF

Typical values

R C OSC F
3k9 33p 4mhz
4k7 150p 1mhz
3k9 2n2 100khz
120k 150n 50hz

Don't forget that some of the latest 16F chips have very good internal oscillators, and that the C series of chips can only be programmed once
 
wp100. Thanks.

Hard to forget things when I don't yet know them.

I chose the 16C773 chip because they are the only chips with 12 bit ADC resolution that my programmer is capable of talking to. I will go back to the 16F88.

So, is that the difference between Flash and EEPROM Program Memory Type? EEPROM memory can only be programmed once?
 

EEPROM can be reprogrammed as can Flash and EPROM. The early PICs used EPROM which had to be erased using UV light. The one-time programmable ones simply didn't have the erasing window and were in plastic packages (and so were cheaper).

Keith
 
Right. Learn something new everyday.

- - - Updated - - -

Did find a 16F8X datasheet which had some suggested values for Rext and Cext which didn't seem to follow any kind of formula.

See
 

JohnJohn20 said:
Right. Learn something new everyday.

- - - Updated - - -

Did find a 16F8X datasheet which had some suggested values for Rext and Cext which didn't seem to follow any kind of formula.

See
Click to expand...


Hi,

Those figures I quoted were from a book using 16C54 and 16C71 chips

Many chips like the F88 have perfectly good internal oscillators so little point in using a r/c.
Think r/c on modern chips is really a legacy feature.

With crystals and ceramic resonators being so cheap, they seem much more suitablefor chips without internal oscillators like the C773
 

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