Advanced Digital
Flip Flop with Chip-Select

Positive-edge Flip-Flop with Chip Select

flip_flop_pos_edge_with_cs

three_star

VHDL Code 

 1) flop_process : PROCESS(clk_sig, reset_sig)
 2) BEGIN
 3)      If(reset_sig = '0') THEN
 4)         ff_out_q_sig <= '0';
 5)      ELSIF(clk_sig 'event AND clk_sig = '0') THEN  
 6)          IF(enable_sig = '1') THEN
 7)             ff_out_q_sig <= data_bit_sig;
 8)          END IF;
 9)      END If;
10) END PROCESS;

The figure above shows a +ve edge flip-flop with enable pin. enable, chip enable or chip-select all are same. The chip-enable pin gets third priority (reset -> clock edge -> chip-enable). This means that the output will change only when enable pin = '1'. This is archived by putting one more condition inside clock edge check i.e if conditional statement (line no 6,7 and 8).
Negative Edge Flip-Flop
Coding Clock And Reset