A plurality of groups of first flip-flops (group 40 of flip-flops A1 An-1 for each of channels CIA CIC) store input data clocked in response to first clock signals (A C). First enable signals (Stack_en) are generated for each group of first flip-flops. A plurality of groups of second flip-flops (group 60 of flip-flops B1 Bn for each of channels CIA CIC) store the input data from the first flip-flops in response to the first enable signals and first clock signals. A second enable signal (Slide_en) is generated in response to a second clock signal (D) and the first enable signal. A plurality of groups of third flip-flops (group 80 for each of channels CIA CIC) store the data in response to the second enable signal and second clock signal. The data is transmitted in serial form at the rate of the second clock signal.