DS14285SSLASHTR datasheet(7/26 Pages) DALLAS

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DS14285/DS14287

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POWER-DOWN/POWER-UP CONSIDERATIONS

The real time clock function will continue to operate and all of the RAM, time, calendar, and alarm

memory locations remain nonvolatile regardless of the level of the VCC input. When VCC is applied to the

DS14285/DS14287 and reaches a level of greater than 4.25 volts (typical), the device becomes accessible

after 200 ms, provided that the oscillator is running and the oscillator countdown chain is not in reset (see

4.25 volts (typical), the chip select input is internally forced to an inactive level regardless of the value of

at the input pin. The DS14285/DS14287 is, therefore, write-protected. When the DS14285/DS14287

is in a write-protected state, all inputs are ignored and all outputs are in a high impedance state. When

VCC falls below a level of approximately 3 volts, the external VCC supply is switched off and an internal

lithium energy source supplies power to the Real-time Clock and the RAM memory.

An external SRAM can be made nonvolatile by using the VCCO and SRAM chip enable pins (see Figure

1). Nonvolatile control of the external SRAM is analogous to that of the real time clock registers. When

VCC slews down during a power fail, CEO is driven to an inactive level regardless CEI . This write

protection occurs when VCC is less than 4.25 volts (typical).

During power up, when VCC reaches a level of greater than 4.25 volts (typical), CEO will reflect CEI

after 200 ms. During power-valid operation, the CEI input is passed to the CEO output with a

propagation delay of less than 10 ns.

When VCC is above a level of approximately 3V, the external SRAM will be powered by VCC through the

VCCO pin. When VCC is below a level of approximately 3V, the external SRAM will be powered by the

internal lithium cell through the VCCO pin. An internal comparator and switch determine whether VCCO is

When the device is in battery backup mode, the energy source connected to the VBAT pin in the case of

the DS14285, or the internal lithium cell in the case of the DS14287 can power an external SRAM for an

extended period of time. The amount of time that the lithium cell can supply power to the external SRAM

is a function of the data retention current of the SRAM. The capacity of the lithium cell that is

encapsulated within the DS14287 module is 130 mAh. If an SRAM with a data retention current of less

than 1

µA is used and the oscillator current is 300 nA (typical), the cumulative data retention time is

calculated at more than 11 years.

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