The unique aspect of Dallas Semiconductor’s DS32X35 real-time clock (RTC) module is the use of ferroelectric RAM (FRAM) in place of battery-backed SRAM. The primary motivation for this decision was customer interface requirements, according to Lee Cash, business manager for Real-Time Clocks. Specifically, the use of FRAM enables the use of an I²C communications interface with the module. Customers generally prefer this interface over the 8-bit bus needed to access SRAM.

The other obvious benefits provided by FRAM technology are the elimination of the dedicated in-package battery needed for non-volatile data storage based on SRAM, as well as the elimination of write-cycle limitations or time delays associated with EEPROM or Flash memory.

With these features, the module requires only a single external backup battery to drive the thermally compensated crystal oscillator in the absence of 3.3 V main power. The 3.3 V supply also powers the interface circuitry. The interface circuitry is used to obtain real-time status information, such as the reading of the integrated temperature sensor, as well as the contents of the FRAM.

Aiming to boost the accuracy of the classic 32.768 kHz tuning-fork crystal in real-time-clock (RTC) applications in the same manner as other members of the DS32XX family, the DS32X35 RTC module delivers high accuracy across its full operating temperature range. Crystals can exhibit high stability as discrete components in applications where the thermal environment is relatively constant, such as in wristwatches, states Cash. However, other environments—particularly the insides of electronic enclosures—are prone to wide temperature fluctuations. It is in these applications where the RTC module offers the greatest benefits.

Crystals are high-precision frequency references, and the exact frequency is a highly repeatable function of temperature. This property forms the basis of oven-controlled crystal oscillators. However, the crystal’s exact frequency is also a highly repeatable function of capacitive loading across the nodes of the crystal. It is therefore possible to maintain a constant crystal frequency across a wide temperature range by applying pre-determined amounts of capacitive loading across the crystal according to its temperature.

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