Maxim DS87C530 Clock User Manual

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

12 of 47

12.5pF 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 12.5pF crystal. When TRIM.6 = 0, it expects a

6pF crystal. This bit will be nonvolatile so these choices will remain while the backup source is present.

A guard ring (connected to the RTC ground) should encircle the RTCX1 and RTCX2 pins.

Backup Energy Source

The DS87C530/DS83C530 use an external energy source to maintain timekeeping and SRAM data

without V

. This source can be either a battery or 0.47F super cap and should be connected to the V

BAT

pin. The nominal 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

specified in the electrical specifications. The circuit loads the V

BAT

only when V

has fallen below V

BAT

Thus the actual lifetime depends not only on the current and battery capacity, but also on the portion of

time without power. A very small lithium cell provides a lifetime of more than 10 years.

Figure 3. Internal Backup Circuit

IMPORTANT APPLICATION NOTE

The pins on the DS87C530/DS83C530 are generally as resilient as other CMOS circuits. They have no

unusual susceptibility to electrostatic discharge (ESD) or other electrical transients. However, no pin on

the DS87C530/DS83C530 should ever be taken to a voltage below ground. Negative voltages on any

pin can turn on internal parasitic diodes that draw current directly from the battery. If a device pin is

connected to the “outside world” where it may be handled or come in contact with electrical noise,

protection should be added to prevent the device pin from going below -0.3V. Some power supplies can

give a small undershoot on power-up, which should be prevented. Application Note 93: Design

Guidelines for Microcontrollers Incorporating NV RAM discusses how to protect the

DS87C530/DS83C530 against these conditions.

MEMORY RESOURCES

Like the 8051, the DS87C530/DS83C530 use three memory areas. The total memory configuration of the

device is 16kB of ROM, 1kB of data SRAM and 256 bytes of scratchpad or direct RAM. The 1kB of data

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Maxim DS87C530 Clock User Manual