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D的定义D = decimal equivalent of the 2's complement code that is loaded to the DAC register
代表的是data 的2进制补码 。上电初始，还没有开始写入数据，所以应该为零。
另外，datasheet中有介绍DAC8550内部有POR reset 电路：
The DAC8550 contains a power-on reset circuit that controls the output voltage during power-up. On power-up,
the output voltages are set to midscale; they remain that way until a valid write sequence is made to the DAC.
