DS17285_1 Datasheet, PDF (22/31 Pages) Maxim Integrated Products – Real-Time Clocks

Real-Time Clocks
Century Counter
A register has been added in bank 1, location 48H, to
keep track of centuries. The value is read in either bina-
ry or BCD according to the setting of the DM bit.
RTC Write Counter
An 8-bit counter located in extended register bank 1,
5Eh, counts the number of times the RTC is written to.
This counter is incremented on the rising edge of the
WR signal every time that the CS signal qualifies it. This
counter is a read-only register and rolls over after 256
RTC write pulses. This counter can be used to deter-
mine if and how many RTC writes have occurred since
the last time this register was read.
Auxiliary Battery
The V_BAUX input is provided to supply power from an
auxiliary battery for the DS17x85/DS17x87 kickstart,
wake-up, and SQW output in the absence of V_CC func-
tions. This power source must be available to use these
auxiliary functions when no V_CC is applied to the device.
The auxiliary battery enable (ABE; bank 1, register
04BH) bit in Extended Control Register 4B is used to
turn the auxiliary battery on and off for the above func-
tions in the absence of V_CC. When set to 1, V_BAUX bat-
tery power is enabled; when cleared to 0, V_BAUX
battery power is disabled to these functions.
In the DS17x85/DS17x87, this auxiliary battery can be
used as the primary backup power source for maintain-
ing the clock/calendar, user RAM, and extended exter-
nal RAM functions. This occurs if the V_BAT pin is at a
lower voltage than V_BAUX. If the DS17x85 is to be
backed up using a single battery with any auxiliary
functions enabled, then V_BAUX should be used and
V_BAT should be grounded. If V_BAUX is not to be used, it
should be grounded and ABE should be cleared to 0.
Wake-Up/Kickstart
The DS17x85/DS17x87 incorporates a wake-up feature
that powers on the system at a predetermined date and
time through activation of the PWR output pin. In addi-
tion, the kickstart feature allows the system to be pow-
ered up in response to a low-going transition on the KS
pin, without operating voltage applied to the V_CC pin.
As a result, system power can be applied upon such
events as a key closure or modem ring-detect signal.
To use either the wake-up or the kickstart functions, the
DS17x85/DS17x87 must have an auxiliary battery con-
nected to the V_BAUX pin, the oscillator must be running,
and the countdown chain must not be in reset (Register
A DV2, DV1, DV0 = 01X). If DV2 and DV1 are not in this
required state, the PWR pin is not driven low in
response to a kickstart or wake-up condition while in
battery-backed mode.
The wake-up feature is controlled through the wake-up
interrupt-enable bit in Extended Control Register 4B (WIE,
bank 1, 04BH). Setting WIE to 1 enables the wake-up fea-
ture, clearing WIE to 0 disables it. Similarly, the kickstart
interrupt-enable bit in Extended Control Register 4B
(KSE, bank 1, 04BH) controls the kickstart feature.
A wake-up sequence occurs as follows: When wake-up
is enabled through WIE = 1 while the system is pow-
ered down (no V_CC voltage), the clock/calendar moni-
tors the current date for a match condition with the date
alarm register (bank 1, register 049H). With the date
alarm register, the hours, minutes, and seconds alarm
bytes in the clock/calendar register map (bank 0, regis-
ters 05H, 03H, and 01H) are also monitored. As a
result, a wake-up occurs at the date and time specified
by the date, hours, minutes, and seconds alarm regis-
ter values. This additional alarm occurs regardless of
the programming of the AIE bit (bank 0, register B,
0BH). When the match condition occurs, the PWR pin is
automatically driven low. This output can be used to
turn on the main system power supply that provides
V_CC voltage to the DS17x85/DS17x87 as well as the
other major components in the system. Also at this
time, the wake-up flag (WF, bank 1, register 04AH) is
set, indicating that a wake-up condition has occurred.
A kickstart sequence occurs when kickstarting is
enabled through KSE = 1. While the system is powered
down, the KS input pin is monitored for a low-going
transition of minimum pulse width t_KSPW. When such a
transition is detected, the PWR line is pulled low, as it is
for a wake-up condition. Also at this time, the kickstart
flag (KF, bank 1, register 04AH) is set, indicating that a
kickstart condition has occurred.
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