DS14285 Datasheet, PDF (7/25 Pages) Dallas Semiconductor – Real Time Clock with NV RAM Control

DS14285/DS14287
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 V_CC input. When V_CC 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
Register A). This time period allows the system to stabilize after power is applied. When V_CC falls below
4.25 volts (typical), the chip select input is internally forced to an inactive level regardless of the value of
CS 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
V_CC falls below a level of approximately 3 volts, the external V_CC 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 V_CCO 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
V_CC slews down during a power fail, CEO is driven to an inactive level regardless CEI . This write
protection occurs when V_CC is less than 4.25 volts (typical).
During power up, when V_CC 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 V_CC is above a level of approximately 3V, the external SRAM will be powered by V_CC through the
V_CCO pin. When V_CC is below a level of approximately 3V, the external SRAM will be powered by the
internal lithium cell through the V_CCO pin. An internal comparator and switch determine whether V_CCO is
powered by V_CC or the internal lithium cell.
When the device is in battery backup mode, the energy source connected to the V_BAT 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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