The control unit contains the power management functions and the card bus clock divider. Figure 2.5 shows a block diagram of the control unit.

There are three power phases:

The power management logic controls an external power supply unit, and disables the card bus output signals during the power-off or power-up phases. The power-up phase is a transition phase between the power-off and power-on phases, and allows an external power supply to reach the card bus operating voltage. A device driver is used to ensure that the PrimeCell MCI remains in the power-up phase until the external power supply reaches the operating voltage.

The clock management logic generates and controls the MCICLK signal. The MCICLK output can use either a clock divide or clock bypass mode. The clock output is inactive: