Accurate RTC combines FRAM storage and crystal in a 300-mil SO package

Dallas Semiconductor has added a nonvolatile FRAM memory to its real-time clock (RTC) product line with the DS32B35 (2k x 8 bits) and DS32C35 (8k x 8 bits), said to be the industry's most accurate RTCs. The tuning-fork crystal typically used as the time base for RTCs produces an error of one hour per year when exposed to temperature extremes. However, the DS32B35/DS32C35 are capable of keeping time to within one minute per year over the temperature range of 0 to +70 C because they measure temperature and adjust the crystal frequency by varying the load capacitance on the crystal.

The FRAM memory used in the DS32B35/DS32C35 offers true nonvolatility with no energy source required for data retention and none of the write-cycle or block-erase limitations of EEPROM or flash memory. A separate battery or capacitor can be sized for the backup requirements of the real-time clock, giving designers flexibility in the layout and sourcing of components.

Both the RTC and FRAM are accessed through an I2C interface, rather than the byte-wide interface used in battery-backed SRAM modules. For applications that have traditionally stored critical data using battery-backed SRAM modules, which require unlimited read/write cycles, the DS32B35/DS32C35 offer the ability to migrate this functionality to the I2C data bus. This migration greatly reduces the number of interconnects between the memory and CPU, thereby making PCB layout easier.

Dallas Semiconductor, www.maxim-ic.com