When the value of counter 620 is zero, {TAIL ~ the output signal} of {HEAD ~ NOR gate} 621 generates an active signal on terminal R of SR flip flop 631 
When the value of counter 620 is zero , { TAIL ~ the output signal } of { HEAD ~ NOR gate } 621 generates an active signal on terminal R of SR flip flop 631
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH