
DS87C530/DS83C530
070898 10/41
NONVOLATILE RAM
The 1K x 8 on–chip SRAM can be nonvolatile if an exter-
nal backup energy–source is used. This allows the de-
vice to log data or to store configuration settings. Inter-
nal switching circuits will detect the loss of V
CC
 and
switch SRAM power to the backup source on the V
BAT
pin. The 256 bytes of direct RAM are not affected by this
circuit and are volatile.
CRYSTAL AND BACKUP SOURCES
To 
use 
the 
unique 
DS87C530/DS83C530, a 32.768 KHz timekeeping
crystal and a backup energy–source are needed. The
following describes guidelines for choosing these
devices.
functions 
of 
the
Timekeeping Crystal
The DS87C530/DS83C530 can use a standard 32.768
KHz crystal as the RTC time base. There are two ver-
sions of standard crystals available, with 6 pF and 12.5
pF load capacitance. The tradeoff is that the 6 pF uses
less power, giving longer life while V
CC
 is off, but is more
sensitive to noise and board layout. The 12.5 pF crystal
uses more power, giving a shorter battery backed life,
but produces a more robust oscillator. Bit 6 in the RTC
Trim register (TRIM; 96h) must be programmed to spec-
ify the crystal type for the oscillator. When TRIM.6 = 1,
the circuit expects a 12.5 pF crystal. When TRIM.6 = 0, it
expects a 6 pF crystal.  This bit will be nonvolatile so
these choices will remain while the backup source is
present.  A guard ring (connected to the Real Time
Clock ground) should encircle the RTCX1 and RTCX2
pins.
Backup Energy Source
The DS87C530/DS83C530 uses an external energy
source to maintain timekeeping and SRAM data without
V
CC
. This source can be either a battery or 0.47 F super
cap and should be connected to the V
BAT
 pin. The nomi-
nal battery voltage is 3V. The V
BAT
 pin will not source
current. Therefore, a super cap requires an external
resistor and diode to supply charge.
The backup lifetime is a function of the battery capacity
and the data retention current drain. This drain is speci-
fied in the electrical specifications. The circuit loads the
V
BAT
 only when V
CC
 has fallen below V
BAT
. Thus the
actual lifetime depends not only on the current and bat-
tery capacity, but also on the portion of time without
power. A very small  lithium cell provides  a lifetime of
more than 10 years.