Specification :

The ' HC595 devices contain an 8-bit serial-in,
parallel-out shift register that feeds an 8-bit D-type
storage register. The storage register has parallel
3-state outputs. Separate clocks are provided for both
the shift and storage register. The shift register has a
direct overriding clear ( SRCLR) input, serial ( SER)
input, and serial outputs for cascading. When the
output-enable ( OE) input is high, the outputs are in
the high-impedance state.
Both the shift register clock ( SRCLK) and storage
register clock ( RCLK) are positive-edge triggered. If
both clocks are connected together, the shift register
always is one clock pulse ahead of the storage
register.