first nmos type transistor 841 the source of the first nmos type transistor 841 is connected to the drain of the second nmos type transistor 842 the source of the third nmos type transistor 843 is connected to the drain of the fourth nmos type transistor 844 the source of the second nmos type transistor 842 and the source of the fourth nmos type transistor 844 are connected to ground 0 volts the drain of the third pmos type transistor 833 and the drain of the third nmos type transistor 843 are connected to the hold unit 849 via the node 804 as shown in fig6 people skilled in the art could easily recognize that the first pmos type transistor 831 the second pmos type transistor 832 the first nmos type transistor 841 and a second nmos type transistor 842 constitute a nand gate the node 802 and the data input 803 are two inputs of the nand gate and the node v 8 is the output of the nand gate if the node 802 is at low level the node v 8 would be at high level whether the data input 803 is at high level or low level in such conditions the third pmos type transistor 833.