Rainbow Electronics DS12C887 - Manual (Page 5)

DS12C887
5 of 19
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
DS12C887 and reaches a level of greater than 4.25 volts, 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,
the chip select input is internally forced to an inactive level regardless of the value of CS at the input pin.
The DS12C887 is, therefore, write-protected. When the DS12C887 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.
RTC ADDRESS MAP
The address map for the DS12C885 is shown in Figure 2. The address map consists of 113 bytes of user
RAM, 11 bytes of RAM that contain the RTC time, calendar, and alarm data, and 4 bytes which are used
for control and status. All 128 bytes can be directly written or read except for the following:
1. Registers C and D are read-only.
2. Bit-7 of Register A is read-only.
3. The high order bit of the seconds byte is read-only.
DS12C887 REAL TIME CLOCK ADDRESS MAP Figure 2