why not use EEprom or NVRAM?

Hallo Paul,

EEPROM needs some miliseconds for burning a byte (or a group of bytes). Battery backed-up CMOS RAM is much much faster (about 100nsec and less). So, when you want to make datalogging e.g., where much data has to be stored quickly, then an EEPROM is much to slow.
EEPROMs are used e.g. to store calibration data, or other information, which differs from system to system, which can differ after changing certain sensor and which must be quasi permamently stored in the system.
EEPROM can also contain certain program code like it is used sometimes with newer 8052AH BASIC systems. Then code is not stored in EPROM, but EEPROM. In former times only EPROMS were able to store data quasi permantly without using a battery. But today there are FLASHs, EEPROMs, etc.

NVRAM is everything, where the content of RAM is retained, even when turning-off power supply. Battery backed-up CMOS RAM is some sort of NVRAM. Either built-up by discretes, as Yogesh did, or by using a ready module from DALLAS or equivalent, where a very high reliability battery and some digital logic circuitry is built-in.

Kai