Rainbow Electronics DS1687 - Manual (Page 9)

DS1685/DS1687
9 of 38
POWER-DOWN/POWER-UP 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 DS1685/DS1687
and reaches a level of greater than V
PF
(power-fail trip point), the device becomes accessible after t
REC
,
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.
The DS1685/DS1687 is available in either a 3V or a 5V device.
The 5V device is fully accessible and data can be written and read only when V
CC
is greater than 4.5V.
When V
CC
is below 4.5V, read and writes are inhibited. However, the timekeeping function continues
unaffected by the lower input voltage. As V
CC
falls below the greater of V
BAT
and V
BAUX
, the RAM and
timekeeper are switched over to a lithium battery connected either to the V
BAT
pin or V
BAUX
pin.
The 3V device is fully accessible and data can be written or read only when V
CC
is greater than 2.7V.
When V
CC
falls below V
PF
, access to the device is inhibited. If V
PF
is less than V
BAT
and V
BAUX
, the
power supply is switched from V
CC
to the backup supply (the greater of V
BAT
and V
BAUX
) when V
CC
drops below V
PF
. If V
PF
is greater than V
BAT
and V
BAUX
, the power supply is switched from V
CC
to the
backup supply when V
CC
drops below the larger of V
BAT
and V
BAUX
.
When V
CC
falls below V
PF
, the chip is write-protected. With the possible exception of the KS , PWR , and
SQW pins, all inputs are ignored and all outputs are in a high-impedance state.
RTC ADDRESS MAP
The address map for the RTC registers of the DS1685/DS1687 is shown in Figure 2. The address map
consists of the 14 clock/calendar registers. Ten registers contain the time, calendar, and alarm data, and
four bytes are used for control and status. All registers 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.