Automotive EEPROMs use two cells per bit for ruggedness, reliability
by Graham Prophet - October 2, 2008
With the aim of ensuring reliable operation and long life in the demanding automotive environment, Rohm has introduced a series of EEPROMs that employ a double-cell structure. The BR25Hxx0 devices, which are SPI (serial-peripheral-interface)-bus memories, withstand the voltage surges, static discharge, heat, and vibration that they may encounter in automotive ECUs (engine-control units). Features include guaranteed operation at 125°C, with 20-year data retention at 85°C, 6-kV ESD (electrostatic-discharge) resistance, gold-pad/gold-wire connections, and a double-reset function for high reliability. The devices’ high-redundancy circuits and a choice of process technology allow the company to specify a life of 1 million rewrites at 85°C and 300,000 rewrites at 125°C.
EEPROMs are increasingly finding use in critical automotive systems to record, for example, status information. As most users know, the devices also have a wear-out mechanism; charge is transferred by charge tunneling across an oxide barrier. Eventually, on a lifetime-related but somewhat-random basis, the oxide in a cell may break down. Rohm addresses this breakdown by fabricating two identical but separate memory cells for each bit and connecting them in an OR configuration, so that the combined cell will continue to function after a failure.
To protect against error conditions during power-up or voltage dips, the EEPROMs integrate a double-protection circuit comprising a POR (power-on-reset) block that resets during start-up of the power source and a low-voltage write-error-correction circuit that resets the internal circuit and prohibits write operations in low-voltage conditions.
Rohm produces variants with Microwire, I2C (inter-integrated-circuit), and SPI interfaces; the 5-MHz SPI is the fastest. The 14 devices in the family come in 6.2×5×1.71-mm SOP-8 and 6×4.9×1.75-mm SOP-J8 packages and in capacities of 1, 2, 4, 8, 16, and 32 kbits. The memories support high-speed- write and page-write modes.
Rohm Electronics, www.rohm.com.