S35390A datasheet(20/51 Pages) SII

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FOR AUTOMOTIVE 105°C OPERATION 2-WIRE REAL-TIME CLOCK

S-35390A H Series

Rev.2.2_00

Seiko Instruments Inc.



Low Power Supply Voltage Detection Circuit

The S-35390A H Series has a low power supply voltage detection circuit, so that users can monitor drops in the power

supply voltage by reading the BLD flag (B1 in the status register 1). There is a hysteresis width of approx. 0.15 V typ.

between detection voltage and release voltage (refer to " Characteristics (Typical Data)"). The low power supply

voltage detection circuit does the sampling operation only once in one sec for 15.6 ms.

If the power supply voltage decreases to the level of detection voltage (VDET) or less, "1" is set to the BLD flag so that

sampling operation stops. Once "1" is detected in the BLD flag, no sampling operation is performed even if the power

supply voltage increases to the level of release voltage or more, and "1" is held in the BLD flag.

Furthermore, the S-35390A H Series does not initialize the internal circuit even if "1" is set to the BLD flag. If the BLD flag

is "1" even after the power supply voltage is recovered, the internal circuit may be in the indefinite status. In this case, be

sure to initialize the circuit. Without initializing, if the next BLD flag reading is done after sampling, the BLD flag gets reset

to "0". In this case, be sure to initialize although the BLD flag is in "0" because the internal circuit may be in the indefinite

status.

VDD

BLD flag

Stop

Sampling pulse

Hysteresis width

0.15 V approximately

BLD flag reading

Detection voltage

Release

voltage

15.6 ms

1 s

Time keeping power

supply voltage (min.)

Figure 22 Timing of Low Power Supply Voltage Detection Circuit



Circuits Power-on and Low Power Supply Voltage Detection

Figure 23

shows the changes of the POC flag and BLD flag due to VDD fluctuation.

VDD

BLD flag

Status register 1

reading

POC flag

VSS

Low power supply voltage

detection voltage

Low power supply voltage

detection voltage

Figure 23 POC Flag and BLD Flag

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