2.2 FRAM Cell

A single FRAM cell can be considered a dipole capacitor that consists of a film of ferroelectric material (ferroelectric crystal) between two electrode plates. Storing a 1 or 0 (writing to FRAM) simply requires polarizing the crystal in a specific direction using an electric field. This makes FRAM very fast, easy to write to, and capable of meeting high endurance requirements. Reading from FRAM requires applying an electric field across the capacitor similar to a write. Depending on the state of the crystal, it may be repolarized, thereby emitting a large induced charge. This charge is then compared to a known reference to estimate the state of the crystal. The stored data bit 1 or 0 is inferred from the induced charge. In the process of reading the data, the crystal that is polarized in the direction of the applied field loses its current state. Hence, every read must be accompanied by a write-back to restore the state of the memory location. With TI's MSP430 FRAM MCUs, this is inherent to the FRAM implementation and is completely transparent to the application. The write-back mechanism is also protected from power loss and completes safely under all power-fail events. The FR59xx power management system achieves this by isolating the FRAM power rails from the device supply rails in the event of a power loss. The FRAM power circuitry also uses built-in low dropout regulator (LDO) and a capacitor that store sufficient charge to complete the current write-back in the event of a power failure.