ISL1208 precision clock application circuit design

　　ISL1208 is a low-power real-time clock integrated circuit from Intersil, which provides users with 2 bytes of static memory. It requires an external 32.768 kHz crystal oscillator to provide an oscillation source; it provides an I2C bus interface to facilitate communication with a microprocessor, with a maximum transmission rate of 400 kHz. Dedicated registers are used to store hours, minutes, seconds, years, months, days, and weeks. The effective range of the calendar is 2000 to 2099, and it can automatically identify leap years.

　　Compared with other types of real-time clock circuits, the outstanding advantages of ISL1208 are:

　　①Frequency compensation function. The accuracy of the crystal frequency is related to the accuracy of the entire real-time clock. A frequency error of 20 ppm will accumulate to an error of 1 minute within a month. The crystal oscillator will have frequency drift under different ambient temperatures. Therefore, the actual operating frequency must be compensated according to different ambient temperatures to ensure the accuracy of the oscillation signal. ISL1208 uses a digital capacitor adjustment method to correct the crystal frequency, with a correction range of -94 to +140 ppm.

　　② Automatic switching function of backup power supply. It can switch between normal working power supply and backup power supply without adding additional components. The backup power supply can be a rechargeable battery or a large-capacity capacitor.

　　③ Ultra-low power consumption. The typical operating current of ISL1208 under 5 V power supply is only 2 μA, and a 0.47 F capacitor can maintain its normal operation for one month.

　　X1 and X2 are the input and output pins of the internal inverting amplifier. An external 32.768 kHz crystal oscillator is connected to X1 and X2 to generate the basic oscillation signal of the real-time clock. VBAT is the backup power supply terminal. When the VDD power supply fails, the backup power supply at the VRAT terminal takes effect immediately; in the case of external power supply interruption, the internal clock signal generation circuit can still work normally. SDA and SCL are two pins connected to the I2C bus. SDA is an open-drain output structure with bidirectional input and output for transmitting serial data signals; SCL is the clock signal line on the I2C bus. IRQ/FOUT is a multi-function pin. By setting the configuration register, it can be defined as an interrupt output or a fixed frequency output terminal. VDD and GND are the power supply terminal and ground respectively. The operating voltage of ISL1208 is 2.0~5.5 V.