Rainbow Electronics DS12887 - Manual (Page 3)

DS12887
3 of 19
Figure 1. BLOCK DIAGRAM
POWER-UP/DOWN CONSIDERATIONS
The RTC function continues 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 DS12887 and
reaches a level of greater than 4.25V, the device becomes accessible after 200ms, provided that the
oscillator is running and the oscillator countdown chain is not in reset (Register A). This time period
allows the system to stabilize after power is applied. When V
CC
falls below 4.25V, the chip-select input is
internally forced to an inactive level regardless of the value of CS at the input pin. The DS12887 is,
therefore, write-protected. When the DS12887 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 3V, the external
V
CC
supply is switched off, and an internal lithium energy source supplies power to the RTC and the
RAM memory.