Minecraft wiki entry for Clock, associated text: A setblock clock works by replacing a block of redstone or a redstone torch repeatedly with a command block activated by the block of redstone it places. A /setblock command takes 0.5 ticks to place a block, so these clocks are capable of producing 20 0-tick pulse per second. Only redstone dust, note blocks, and other command blocks can activate that rapidly – other mechanism components and repeaters powered by a setblock clock usually pulse only 5 times per second (like a 1-clock), while comparators may activate once and then stay on or not activate at all.