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FM3164-GTR Fiches technique(PDF) 8 Page - Cypress Semiconductor |
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FM3164-GTR Fiches technique(HTML) 8 Page - Cypress Semiconductor |
8 / 33 page FM3164/FM31256 Document Number: 001-86391 Rev. *C Page 8 of 33 Backup Power The real-time clock/calendar is intended to be permanently powered. When the primary system power fails, the voltage on the VDD pin will drop. When VDD is less than 2.5 V, the RTC (and event counters) will switch to the backup power supply on VBAK. The clock operates at extremely low current in order to maximize battery or capacitor life. However, an advantage of combining a clock function with F-RAM memory is that data is not lost regardless of the backup power source. The IBAK current varies with temperature and voltage (see DC Electrical Characteristics table). The following graph shows IBAK as a function of VBAK. These curves are useful for calculating backup time when a capacitor is used as the VBAK source. The minimum VBAK voltage varies linearly with temperature. The user can expect the minimum VBAK voltage to be 1.23 V at +85 °C and 1.90 V at -40 °C. The tested limit is 1.55 V at +25 °C. Note The minimum VBAK voltage has been characterized at -40 °C and +85 °C but is not 100% tested. Trickle Charger To facilitate capacitor backup the VBAK pin can optionally provide a trickle charge current. When the VBC bit (register 0Bh, bit 2) is set to ‘1’, the VBAK pin will source approximately 15 µA until VBAK reaches VDD or 3.75 V, whichever is less. In 3 V systems, this charges the capacitor to VDD without an external diode and resistor charger. In 5 V systems, it provides the same convenience and also prevents the user from exceeding the VBAK maximum voltage specification. Figure 7. Real-time Clock Core Block Diagram 32.768 kHz crystal Oscillator Clock Divider Update Logic 512 Hz or Square Wave Seconds 7 bits MInutes 7 bits Hours 6 bits Date 6 bits Months 5 bits Years 8 bits Days 3 bits User Interface Registers 1 Hz W R OSCEN CF Figure 8. IBAK vs. VBAK Voltage VBAK (V) Figure 9. VBAK(min.) vs Temperature Temperature (°C) |
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