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2015-12-0001

PT0280-8

12/11/15

16

PT7C43390/43390C

Real-time Clock Module

6. Low Power Supply Voltage Detection Circuit

The RTC 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. 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. After reading the BLD flag, the sampling operation is restarted. 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.

Timing of Low Power Supply Voltage Detection Circuit

7. Circuits Power-on and Low Power Supply Voltage Detection

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

POC Flag and BLD Flag