DS1543 datasheet(4/17 Pages) DALLAS

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DS1543

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active provided that care is taken with the data bus to avoid bus contention. If OE is low prior to WE

transitioning low, the data bus can become active with read data defined by the address inputs. A low

transition on WE will then disable the outputs tWEZ after WE goes active.

DATA RETENTION MODE

The 5-volt device is fully accessible and data can be written and 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. When VCC falls below the battery switch

point VSO (battery supply level), device power is switched from the VCC pin to the internal backup lithium

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

levels.

The 3.3-volt device is fully accessible and data can be written and read only when VCC is greater than

VPF. When VCC falls below VPF, access to the device is inhibited. If VPF is less than VBAT, the device

power is switched from VCC to the internal backup lithium battery when VCC drops below VPF If VPF is

greater than VBAT, the device power is switched from VCC to the internal backup lithium battery when

VCC drops below VBAT. RTC operation and SRAM data are maintained from the battery until VCC is

returned to nominal levels.

All control, data, and address signals must be powered down when VCC is powered down.

BATTERY LONGEVITY

The DS1543 has a lithium power source that is designed to provide energy for the 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 DS1543 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 Each DS1543 is shipped from Dallas Semiconductor 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 DS1543 will be much longer than 10 years since no internal battery energy is

consumed when VCC is present. In fact, in most applications, the life expectancy of the DS1543 will be

approximately equal to the shelf life (expected useful life of the internal lithium battery with no load

attached) of the battery which may prove to be as long as 20 years.

INTERNAL BATTERY MONITOR

The DS15433 constantly monitors the battery voltage of the internal batter. The Battery Low Flag (BLF)

bit of the Flags register (B4 of 1FF0h) is not writable and should always be a 0 when read. If a 1 is ever

present, an exhausted lithium energy source is indicated and both the contents of the RTC and RAM are

questionable.

POWER-ON RESET

A temperature-compensated comparator circuit monitors the level of VCC When VCC falls to the power

fail trip point, the RST signal (open-drain) is pulled low. When VCC returns to nominal levels, the RST

signal continues to be pulled low for a period of 40 ms to 200 ms. The power-on reset function is

independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled.

CLOCK OPERATIONS

Table 2 and the following paragraphs describe the operation of RTC, Alarm, and Watchdog functions.

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