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