Rainbow Electronics DS14287 - Manual (Page 7)

DS14285/DS14287
7 of 25
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
µ
A is used and the oscillator current is 300 nA (typical), the cumulative data retention time is
calculated at more than 11 years.