The DS14285/DS14287 real-time clock with NV RAM control provides the industry standard DS1287 clock function with the additional feature of providing nonvolatile control for an external SRAM. Functions include a nonvolatile time-of-day clock, alarm, 100-year calendar, programmable interrupt, square wave generator, and 114 bytes of nonvolatile static RAM. For the DS14287 a lithium energy source, quartz crystal, and write protection circuitry are contained within a 24-pin dual in-line package. The DS14285 requires an external quartz crystal connected to the X1 and X2 pins as well as an external energy source connected to the VBAT pin. A standard 32.768kHz quartz crystal can be directly connected to the DS14285 via pins 1 and 2 (X1, X2). The crystal selected for use should have a specified load capacitance (CL) of 6pF. For more information on crystal selection and crystal layout considerations, please consult Application Note 58, Crystal Considerations with Dallas Real-time Clocks.
The DS14285/DS14287 uses its backup energy source and battery-backup controller to make a standard CMOS static RAM nonvolatile during power-fail conditions. During power fail, the DS14285/DS14287 automatically write-protects the external SRAM and provides a VCC output sourced from its internal battery.
For the DS14287 the internal lithium cell is electrically isolated from the clock and memory when shipped from the factory. This isolation is removed after the first application of VCC, allowing the lithium cell to provide data retention to the clock, internal RAM, VCCO and active-low CEO on subsequent power-downs. Care must be taken after this isolation has been broken to avoid inadvertently discharging the lithium cell through the VCCO and active-low CEO pins.
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