The 74HC164D653 is an 8-bit serial-in parallel-out shift register in SOIC-14 from NXP. Data is shifted into the register through two serial data inputs (DSA and DSB) that are ANDed together: both must be HIGH to shift in a logic 1. On each rising edge of the clock (CP), the data at the DSA/DSB gate output is loaded into Q0, and the previous contents shift one position (Q0 to Q1 to Q2 through Q7). The MR (master reset) pin, when driven LOW, clears all outputs to LOW regardless of clock or data inputs. The device is commonly used for I/O expansion: a microcontroller uses 2-3 pins (data, clock, optional reset) to control up to 8 output lines. Multiple 74HC164 devices can be cascaded by connecting Q7 of one stage to DSA/DSB of the next. The 74HC family operates from 2V to 6V with typical propagation delay of 14ns at 4.5V.