ARM system bus starts from CPU core. This is just after MMU and CPU cache unit and it connects to peripheral devices. This is called AMBA or ARM memory bus architecture. Now this CPU bus runs by master clock (fclk). This is the same high speed clock. For mini2440 this is 400Mhz. This clock is very high for memory and low speed peripheral devices. So there is a bridge to divide the clock by 4 (hclk)and this is used for high speed peripherals. This bus is also called AHB or ARM highspeed BUS. Memory and Graphics peripherals should be connected to this bus. Again this AHB goes to a bridge to further divide the clock by 2. This is used for low speed peripheral devices. This is known as APB. ARM peripheral bus (APB). Low speed peripherals like serial port, I2C,SPI.

ARM Memory Bus Arch, AHB, APB