DS87C530/DS83C530 EPROM/ROM Microcontrollers with Real-Time Clock

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 VCC 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 (VCC < VBAT). 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 VCC

and switch SRAM power to the backup source on the VBAT 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 VCC is off, but is more sensitive to noise and board layout. The

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