Thank you erik for your replies.

The last procedure worked because I let the RTC time-out twice. The second count of tempcount was the valid value for the period time of the signal. I had to count twice, because the MCU needed some time after the first 'RTCCON = 0x00;' instruction. A time delay or a dummy frequency count (which takes some time) was needed to obtain some delay after that command. Now I have placed 'RTCCON = 0x00;' as the first command in the main program and everything works fine without counting twice or a delay. I don't use a timer at the moment because I use Timer0 as PWM and the second timer is preserved for baudrate generator or to time some others functions. I disabled the interrupts in the beginning of the frequencycount/periodcount-procedure (EA=0), so a simple 'while (!(RTCCON & 0x80)) TempCount++;' (count while not RTC time-out) will do just fine.

Regards,
Mark