DS1746P-70IND+ datasheet(7/16 Pages) MAXIM

Part #	DS1746P-70IND+

Description	Y2K-Compliant, Nonvolatile Timekeeping RAMs

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Manufacturer	MAXIM [Maxim Integrated Products]

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DS1746/DS1746P Y2K-Compliant, Nonvolatile Timekeeping RAMs

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DATA-RETENTION MODE

The 5V device is fully accessible and data can be written or read only when VCC is greater than VPF.

However, when VCC is below the power-fail point, VPF, (point at which write protection occurs) the

internal clock registers and SRAM are blocked from any access. At this time the power fail reset output

signal (RST) is driven active and will remain active until VCC returns to nominal levels. When VCC falls

below the battery switch point VSO (battery supply level), device power is switched from the VCC pin to

the backup battery. RTC operation and SRAM data are maintained from the battery until VCC is returned

to nominal levels. The 3.3V device is fully accessible and data can be written or read only when VCC is

greater than VPF. When VCC falls below the power fail point, VPF, access to the device is inhibited. At this

time the power fail reset output signal (RST) is driven active and will remain active until VCC returns to

nominal levels. If VPF is less than VSO, the device power is switched from VCC to the backup supply

(VBAT) when VCC drops below VPF. If VPF is greater than VSO, the device power is switched from VCC to

the backup supply (VBAT) when VCC drops below VSO. RTC operation and SRAM data are maintained

from the battery until VCC is returned to nominal levels. The RST signal is an open drain output and

requires a pull up. Except for the RST, all control, data, and address signals must be powered down when

VCC is powered down.

BATTERY LONGEVITY

The DS1746 has a lithium power source that is designed to provide energy for clock activity, and clock

and RAM data retention when the VCC supply is not present. The capability of this internal power supply

is sufficient to power the DS1746 continuously for the life of the equipment in which it is installed. For

specification purposes, the life expectancy is 10 years at 25°C with the internal clock oscillator running in

the absence of VCC power. Each DS1746 is shipped from Maxim with its lithium energy source

disconnected, guaranteeing full energy capacity. When VCC is first applied at a level greater than VPF, the

lithium energy source is enabled for battery backup operation. Actual life expectancy of the DS1746 will

be much longer than 10 years since no lithium battery energy is consumed when VCC is present.

BATTERY MONITOR

The DS1746 constantly monitors the battery voltage of the internal battery. The Battery Flag bit (bit 7) of

the day register is used to indicate the voltage level range of the battery. This bit is not writable and

should always be a one when read. If a zero is ever present, an exhausted lithium energy source is

indicated and both the contents of the RTC and RAM are questionable.

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