clkb=0 at 0
clkb=1 at 900
clkb=0 at 2100
clkb=1 at 2900
clkb=0 at 4100
clkb=1 at 4900
clkb=0 at 6100
clkb=1 at 6900
clkb=0 at 8100
clkb=1 at 8900
clkb=0 at 10100