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.
Key Features
Direct replacement for IBM AT computer clock/calendar
Functionally compatible with the DS1285/DS1287
Available as chip (DS14285, DS14285S, or DS14285Q) or stand-alone module with embedded lithium battery and crystal (DS14287)
Automatic backup supply and write protection to make external SRAM nonvolatile
Counts seconds, minutes, hours, days, day of the week, date, month, and year with leap year compensation valid up to 2100
Binary or BCD representation of time, calendar, and alarm 12- or 24-hour clock with AM and PM in 12-hour mode
Daylight savings time option
Multiplex bus for pin efficiency
Interfaced with software as 128 RAM locations
14 bytes of clock and control registers
114 bytes of general purpose RAM
Programmable square wave output signal
Bus-compatible interrupt signals (active-low IRQ)
Three interrupts are separately software maskable and testable
Time-of-day alarm once/second to once/day
Periodic rates from 122µs to 500ms
End of clock update cycle
Optional industrial temperature version available DS14285 DIP, SO, and PLCC
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