This program running on an ATMEGA8 exhibits:
1. a fast reliable self-calibration procedure of the internal RC
oscillator when using a 32768Hz crystal with any supply voltage
2. a routine written in assembler for determining the MCU speed
in the above conditions (_Get_MCU_Speed)
2. a way to achieve a high serial baud rate of 115200bps when the MCU
is clocked at very low speeds for preserving battery power
3. a RTC interrupt routine written in assembler which doesn't affect
any register and takes only 67 words from the flash memory.
The RTC keeps the right time and date from 2001 to 2099(_isr_T2OVF).