[SOLVED] Frequency formula for external RC oscillator PIC16C773

[JohnJohn20]

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 R_ext and C_ext?

Thanks.

 

[wp100]

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

 

[JohnJohn20]

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?

 

[keith1200rs]

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

 

[JohnJohn20]

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

 

[wp100]

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

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