DS1677 datasheet(7/17 Pages) DALLAS

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DS1677

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CU (Conversion Update In Progress) – When this bit is a one, an update to the ADC Register

(register 0Eh) will occur within 488 µs. When this bit is a zero, an update to the ADC Register will not

occur for at least 244 µs.

LOBAT (Low Battery Flag) – This bit reflects the status of the backup power source connected to the

VPAT pin. When VPAT is greater than 2.5 volts, LOBAT is set to a logic 0. When VPAT is less than

2.3 volts, LOBAT is set to a logic 1.

IRQF (Interrupt Request Flag) – A logic 1 in the Interrupt Request Flag bit indicates that the current

time has matched the time of day Alarm registers. If the AIE bit is also a logic 1, the INT pin will go

high. IRQF is cleared by reading or writing to any of the alarm registers.

NONVOLATILE SRAM CONTROLLER

The DS1677 provides automatic backup and write protection for an external SRAM. This function is

pro-vided by gating the chip enable signal and by providing a constant power supply through the VCCO

pin.

The DS1677 nonvolatizes the external SRAM by write protecting the SRAM and by providing a back–up

power supply in the absence of VCC. When VCC falls below VPF , access to the external SRAM is

prohibited by forcing CE0 high regardless of the level of CEI . Upon power–up, access is prohibited until

the end of tRPU .

POWER–FAIL COMPARATOR

The PFI input is connected to an internal reference. If PFI is less than 1.25V, PFO goes low. The power–

fail comparator can be used as an undervoltage detector to signal an impending power supply failure.

PFO

can be used as a

µP interrupt input to prepare for power–down. For battery conservation, the

comparator is turned off and PFO is held low when in battery–backed mode

ADDING HYSTERESIS TO THE POWER–FAIL COMPARATOR

Hysteresis adds a noise margin to the power–fail comparator and prevents PFO from oscillating when

VIN is near the power–fail comparator trip point. Figure 8 shows how to add hysteresis to the power–fail

comparator. Select the ratio of R1 and R2 such that PFI sees 1.25 volt when VIN falls to the desired trip

point (VTRIP). Resistors R2 and R3 adds hysteresis. R3 will typically be an order of magnitude greater

than R1 or R2. R3 should be chosen in manner to prevent it from loading down the PFO pin. Capacitor

C1 adds noise filtering and has a value of typically 1.0 uF. See Figure 8 for a schematic diagram and

equations.

DS1677 Datasheet(PDF) 7 Page - Dallas Semiconductor