Rainbow Electronics DS1511 - Manual (Page 12)

DS1501/DS1511 Y2KC Watchdog Real-Time Clocks
12 of 20
DATA RETENTION MODE
The DS1501/DS1511 are fully accessible, and data can be written and read only when V
CC
is greater than V
PF
.
However, when V
CC
falls below the power-fail point V
PF
(point at which write protection occurs) the internal clock
registers and SRAM are blocked from any access. While in the data retention mode, all inputs are don’t cares and
outputs go to a high-Z state, with the possible exception of KS, PWR, SQW, and RST. If V
PF
is less than V
BAT
and
V
BAUX
, the device power is switched from V
CC
to 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 device power is switched from V
CC
to the larger of V
BAT
and V
BAUX
when V
CC
drops
below the larger of V
BAT
and V
BAUX
. RTC operation and SRAM data are maintained from the battery until V
CC
is
returned to nominal levels (Table 1). If the square-wave and battery-backup 32kHz functions are enabled, V
BAUX
always provides power for the square-wave output, when the device is in battery-backup mode.
AUXILIARY BATTERY
The V
BAUX
input is provided to supply power from an auxiliary battery for the DS1501/DS1511 kickstart and square-
wave output features in the absence of V
CC
. This power source must be available to use these auxiliary features
when V
CC
is not applied to the device.
This auxiliary battery can be used as the primary backup power source for maintaining the clock/calendar and
external user RAM. This occurs if the V
BAT
pin is at a lower voltage than V
BAUX
. If the DS1501/DS1511 are to be
backed up using a single battery with the auxiliary features enabled, then V
BAUX
should be used and connected to
V
BAT
should be grounded. If V
BAUX
is not to be used, it should be grounded.
OSCILLATOR CONTROL BIT
When the DS1511 is shipped from the factory, the internal oscillator is turned off. This feature prevents the lithium
energy cell from being used until it is installed in a system. The oscillator is automatically enabled when power is
first applied.
POWER-ON RESET
A temperature-compensated comparator circuit monitors the level of V
CC
. When V
CC
falls to the power-fail trip point,
the RST signal (open drain) is pulled low. When V
CC
returns to nominal levels, the RST signal continues to be pulled
low for a period of t
REC
. The power-on reset function is independent of the RTC oscillator and therefore operational
whether or not the oscillator is enabled.
TIME AND DATE OPERATION
The time and date information is obtained by reading the appropriate register bytes. Table 2 shows the RTC
registers. The time and date are set or initialized by writing the appropriate register bytes. The contents of the time
and date registers are in BCD format. Hours are in 24-hour mode. The day-of-week register increments at
midnight. Values that correspond to the day of week are user defined but must be sequential (i.e., if 1 equals
Sunday, then 2 equals Monday, and so on). Illogical time and date entries result in undefined operation.
READING THE CLOCK
When reading the clock and calendar data, it is possible to access the registers while an update (once per second)
occurs. There are three ways to avoid using invalid time and date data.
The first method uses the transfer enable (TE) bit in the control B register. Transfers are halted when a 0 is written
to the TE bit. Setting TE to 0 halts updates to the user-accessible registers, while allowing the internal registers to
advance. After the registers are read, the TE bit should be written to 1. TE must be kept at 1 for at least 366µs to
ensure a user register update.
The time and date registers can be read and stored in temporary variables. The time and date registers are then
read again, and compared to the first values. If the values do not match, the time and date registers should be read
a third time and compared to the previous values. This should be done until two consecutive reads of the time and
date registers match. The TE bit should always be enabled when using this method for reading the time and date,.