This invention presents a coding method for binary digits coding and its circuit for digits transmission. With this coding method, the binary digits are corresponding to a sequence of pulse groups. The binary digits "0" and "1" are corresponding to the two pulse groups with same defined number of pulses and with two special defined pulse frequencies respectively. The two pulse groups have the same defined number of pulses. The number is at least 2. The corresponding decoding method divides the sequence of pulse groups, according to the same defined number, into a set of pulse groups. The duration time of each pulse group is measured. The binary digits "0" and "1" are corresponding to the differences of the duration time of the pulse groups.