First i apologize about my english, i know it's not the best :D
I'm trying to do a RTC using my arduino board and the AVRStudio, and with an external programmer.
The board have a 16MHz crystal osc and I've set the SUT_CKSEL fuses register in "osc crystal 8MHz - " with 16CK + 4.1ms, and selected de CKDIV8 bit (selected means unprogrammed, am I wrong?). Doing that i suppose i have a System freq equal to 16MHz.
After that I use de CTC mode and obtain the OCR1A value like this:
OCR1A=((Sys Freq/Pre)/Target freq)-1
Well, when I do everything and try to run the program my led doesn't blink each second, it make it about 2 or 3 seconds.
I think that de formula I'm using it's not wrong, then the problem it's that I don't know the system freq.
Sorry about my english one more time