DS87C530/DS83C530 EPROM/ROM Microcontrollers with Real-Time Clock
11 of 47
automatically to occur once per second, once per minute, once per hour, or once per day. Enabling
interrupts with no match will generate an interrupt 256 times per second.
Software enables the timekeeper oscillator using the RTC enable bit in the RTC Control register (F9h).
This starts the clock. It can disable the oscillator to preserve the life of the backup energy-source if
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.
Figure 2. Real-Time Clock
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 32.768kHz 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 32.768kHz crystal as the RTC time base. There are two
versions of standard crystals available, with 6pF and 12.5pF load capacitance. The tradeoff is that the 6pF
uses less power, giving longer life while V
CC
is off, but is more sensitive to noise and board layout. The
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