pulseLength = 1 -- In ticks
clockSpeed = 20 -- In ticks
while true do
    while not rs.getInput("front") do -- Remove "not" if you want to enable the clock when supplying a signal
        rs.setOutput("back", true)
        sleep(pulseLength / 20)
        rs.setOutput("back", false)
        sleep((clockSpeed - pulseLength) / 20)
    end
	event = os.pullEvent("redstone")
end