It’s proved in practice that CLK signal could be
disconnected on system of ARM. The strobe
single could also synchronous the bus. The time
control logic could produce FCLK single for
CPU、HCLK single for AHB bus and PCLK
single for peripheral device on APB bus[7].
Sometimes there is no peripheral bus clock signal
on system of ARM; the only synchronous signal to
bus clock is crystal signal. If we need the same
frequent signal to bus clock, we can multiple the
crystal signal