Is there chart for PIC intRC osc temp stability?

[Mr RB]

I've just done a timer with a PIC using the internal RC osc 4MHz. It was easy enough to calibrate the PIC to better than 1 second per hour accuracy. (Yes I know it's best to use a xtal, but in this app thats not possible.)

I just did a google search but still couldn't find any good info on the temperature stability of the PIC internal RC osc. Does any one have a chart showing the % change etc of the PIC internal 4MHz oscillator (12F675) over room temperature variation (ie 10'C to 40'C ??)

 

[Pommie]

Isn't it in the data sheet. Page 110, figure 13-15.

Mike.

 

[Mr RB]

Darn now I feel a bit silly. Mine is datasheet DS41190A and doesn't have many charts at all. It definitely doesn't have any charts on page 110.

Thanks Pommie i'll download a newer version of the DS.

(edit) Ok I downloaded the NEW datasheet DS41190F (Oct 2009) and as far as I can tell from the rather crappy chart on page 112;

0'C = 3.99 MHz
25'C = 4.00 MHz
85'C = 4.01 MHz

That equates to;
15'C = 3.996 = 1hour - 3.6 seconds
25'C = 4.000 = 1hour
35'C = 4.002 = 1hour + 1.8 seconds

So for an indoor timer it should give less than 1 part in 1000 error. That seems workable, it's for a dryer timer so the timing isn't that critical.

This is the first time i've calibrated an internal RC osc to much finer resolution than the OSCCAL is capable of, and it's keeping surprisingly good time so far. Much better than the figures above, but the temp variation has only been about 4 degrees so far.