DS87C530_1 Datasheet, PDF (10/44 Pages) Dallas Semiconductor – EPROM/ROM Micro with Real Time Clock

DS87C530/DS83C530
unneeded. Values in the RTC Control register are maintained by the backup source through power failure.
Once enabled, the RTC maintains time for the life of the backup source even when V_CC is removed.
The RTC will maintain an accuracy of ±2 minutes per month at 25°C. Under no circumstances are
negative voltages, of any amplitude, allowed on any pin while the device is in data retention mode (V_CC <
V_BAT). Negative voltages will shorten battery life, possibly corrupting the contents of internal SRAM and
the RTC.
REAL TIME CLOCK Figure 2
NONVOLATILE RAM
The 1k x 8 on-chip SRAM can be nonvolatile if an external backup energy source is used. This allows the
device to log data or to store configuration settings. Internal 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 functions of the DS87C530/DS83C530, a 32.768 kHz timekeeping crystal and a
backup energy source are needed. The following describes guidelines for choosing these devices.
Timekeeping Crystal
The DS87C530/DS83C530 can use a standard 32.768 kHz crystal as the RTC time base. There are two
versions 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 specify 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
10 of 44