Introduction to Power Budgeting

The primary goal of the PCI Express power budgeting capability is to allocate power for PCI Express hot plug devices, which can be added to the system during runtime. This capability ensures that the system can allocate the proper amount of power and cooling for these devices.

The specification states that "power budgeting capability is optional for PCI Express devices implemented in a form factor which does not require hot plug, or that are integrated on the system board." None of the form factor specifications released at the time of this writing required support for hot plug and did not require the power budgeting capability. However, form factor specifications under development will require hot plug support and may also require power budgeting capability.

System power budgeting is always required to support all system board devices and add-in cards. The new power budgeting capability provides mechanisms for managing the budgeting process. Each form factor specification defines the minimum and maximum power for a given expansion slot. For example, the Electromechanical specification limits the amount of power an expansion card can consume prior to and during configuration, but after a card is configured and enabled, it can consume the maximum amount of power specified for the slot. Chapter 18, entitled "Add-in Cards and Connectors," on page 685. Consequently, in the absence of the power budgeting capability registers, the system designer is responsible for guaranteeing that power has been budgeted correctly and that sufficient cooling is available to support any compliant card installed into the connector.

The specification defines the configuration registers that are designed to support the power budgeting process, but does not define the power budgeting methods and processes. The next section describes the hardware and software elements that would be involved in power budgeting, including the specified configuration registers.