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The 74HC164 is a serial input 8-bit edge-triggered shift register that has outputs from each of eight stages.
SERIAL DATA INPUT PINS
The serial input data is entered at pin SDA or pin SDB as these are logically ANDED. Either input could be used as an active HIGH enable with data entry on the other pin. If a single input is desired, the pins can be tied together or the unused input can be tied HIGH.
Data is shifted into Q0 from the serial input pins on each LOW to HIGH transition of the CP pin. Also during the CP edge the data is transferred from each Qn to Qn+1. The serial data on pins DSA and DSB must be stable before and after the CP rising edge to meet the set-up and hold timing requirements.
When asserted LOW the Master Reset (MR ) pin sets all Qn to LOW. This action does not depend on the condition of serial input or clock pins. The MR must be asserted HIGH for a recovery time before the next CP positive edge pulse.