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The V3021 is a low power CMOS real time clock. Data is transmitted serially as 4 address bits and 8 data bits, over one line of a standard parallel data bus. The device is accessed by chip select (/CS) with read and write control timing provided by either /RD and /WR pulse (Intel CPU) or /DS with advanced R/W (Motorola CPU). Data can also be transmitted over a conventional 3 wire serial interface having CLK, data I/O and strobe. The V3021 has no busy states and there is no danger of a clock update while accessing. Supply current is typically 800nA at VDD = 3.0V. Battery operation is supported by complete functionality down to 2.0V. The oscillator stability is typically 0.3 ppm/V.