ExpressCard eases power-management woes

An ExpressCard module is an add-in card with a serial interface based on PCI Express, the USB (Universal Serial Bus) technologies, or both. The host system provides power to the ExpressCard slot as the ExpressCard standard specifies. The PCMCIA Committee in the early 1990s developed the PC Card standard (references 1 and 2). The committee maintained the PC Card standard's proven methodology for power management during the development of the ExpressCard standard. However, some differences between the implementations exist in both the host and the module. It is important to note that, because the ExpressCard technology applies to both notebook and desktop computers, the host power management for ExpressCard modules is the same for the two platforms.

ExpressCard modules, like PC Cards, receive power through the connector after a user has inserted the card and the host system has detected it. A host system that accepts PC Cards follows the sequence for applying power as the PC Card standard stipulates. This sequence involves the PC Card controller, which resides on the host, monitoring the card-detection and voltage-sense pins on the connector. As their name suggests, the card-detection pins signal the presence or absence of a PC Card in a socket, and the voltage-sense pins inform the PC Card controller of the card's voltage requirements. Upon detecting that a user has inserted a card into the socket, the PC Card controller reads the logic levels on the voltage-sense pins and then sends a command to the PC Card power switch to turn on the corresponding voltages to the module. This command can be either a serial stream or a parallel interface, depending on the type of power switch the application uses.

The PC Card power switch has two voltage-output connections, V_CC and V_PP, to the module. Depending on the PC Card installed in the socket, the V_CC output can be either 3.3 or 5V, and the V_PP output can be 1.8, 3.3, 5, or 12V. Figure 1 shows a typical implementation of a PC Card application. The power switch serves as a multiplexer, sending only the desired voltages to the PC Card.

The developers of the ExpressCard standard simplified the power management in the host. Instead of having the power switch act as a multiplexer, three voltage rails connect to the ExpressCard module. The availability of these voltages to the module is consistent with a PC Card implementation in which the voltages become available only after a user inserts a card into the slot. The use of card-present inputs from the module is also consistent. However, unlike the PC Card implementation, these inputs go directly to the power switch. This approach thus eliminates the use of the PC Card controller, thereby simplifying the design. The ExpressCard power switch now has to detect whether a user has inserted a card into the slot. Figure 2 shows a typical ExpressCard implementation. Even though the typical implementation looks simpler, the ExpressCard power switch also must decide when to send voltages to the module or which ones to send. The switch makes these decisions based on the state of the host system, which the primary 3.3 and 1.5V_IN and auxiliary 3.3V_{AUX_IN} voltage rails define.

If both the primary 3.3 and 1.5V_IN power and auxiliary 3.3_{VAUX_IN} power at the input of the ExpressCard power switch are off, then all output voltages going to the ExpressCard connector are also off, regardless of whether a card is present. Also, if auxiliary power is available and no card is present, all the voltage outputs to the ExpressCard slot must remain off. If both the primary and the auxiliary power are present at the input of the ExpressCard power switch, then power goes only to the ExpressCard slot after the ExpressCard power switch detects that a card is present.

If either 3.3 or 1.5V primary power at the input of the ExpressCard power switch is off and auxiliary power at the input is available, then the ExpressCard power-switch outputs depend on the state of the host system and on the state of the card-present inputs. If no card is present, then no power goes to the ExpressCard slot. If a user inserts the card after the system has entered this power state, then no power goes to the ExpressCard slot. If a user inserts the card before removing the 3.3V primary power, the 1.5V primary power, or both at the input of the ExpressCard power switch, then only the 3.3 and 1.5 primary power shuts off, and the auxiliary power still goes to the ExpressCard slot. In this scenario, the ExpressCard module can wake the system. This approach differs for a PC Card implementation in that, to enable a PC Card to wake the system from a low-power state, a user must supply main power to the PC Card, the PC Card controller, and the associated power switch. To enable an ExpressCard module to wake the system, the user needs to supply only 3.3V_{AUX_IN} to the module. This approach reduces the total power consumption of a system and eases the implementation of wake-up support from an ExpressCard slot versus a PC Card slot.

For ExpressCard implementations, the ExpressCard power switch should use the 3.3V_{AUX_IN} voltage rail for biasing. The ExpressCard power switch cannot operate if 3.3V_{AUX_IN} is unavailable, and, as a result, the ExpressCard power switch still does not provide power on its output, even if the primary-power voltage rails are present.

The card-present signals  and   are inputs to the host and to the ExpressCard power switch from the ExpressCard module. They signal the host when a user has inserted a card. The ExpressCard standard requires the host to pull up both of these inputs. The signal is an output from the host; PCI Express-based modules use as a reset signal. It is a power-good indicator such that during power-up and -down and whenever power to the ExpressCard module is unstable or outside voltage-tolerance limits, asserts as the ExpressCard standard requires.

The host can use the input to the ExpressCard power switch to place the ExpressCard module in a reset state. Asserting automatically generates a . Generating a  by asserting does not disrupt the voltage rails such that it causes the ExpressCard module to perform a "warm" reset. In a "cold"-start situation,  can also extend the length of time that asserts.

During the development of the ExpressCard standard, the PCMCIA committee wrote the power-sequencing requirements to allow the designer of the host to implement the approach using discrete components. However, in an effort to save space, major power-switch manufacturers, such as Ricoh, Rohm, and Texas Instruments (www.ricoh.com, www.rohm.com, www.ti.com), provide integrated approaches. Figures 3, 4, and 5, respectively, show high-level implementations of each of these approaches. Each of the ExpressCard power-switch implementations differs only in the additional features that each company provides (references 3, 4, and 5). For example, Ricoh and Texas Instruments both provide a discrete output to enable a clock driver. Rohm and Texas Instruments both provide an overcurrent-status output. All three companies provide a means of using discrete inputs to place the ExpressCard power-switch outputs in a standby state, in which only the 3.3V_AUX output goes to the ExpressCard. Consult the respective data sheets for a more detailed description of each of these ExpressCard power switches.

Power management of ExpressCard modules differs depending on whether the module uses a USB or a PCI Express interface. For USB-based ExpressCard modules, the power management is the same as that for a standard USB device that plugs into a USB port. PCI Express-based ExpressCard modules base the power management on the link and device states that the PCI Express base specification defines (Reference 6). PCI Express uses the L0, L0s, L1, L2/L3 Ready, L2, and L3 link states to reduce the power consumption of the PCI Express link and, therefore, the PCI Express device on the ExpressCard module. These link states correspond to device states, which derive from PCI power management; CardBus power management also used these link states. PCI Express-based Express Card modules use the same link states for power management. The only difference is that ExpressCard modules require the L1 active state, which is optional in the PCI Express base specification.

One way for PCI Express-based ExpressCard modules to maximize power management is to monitor how quickly the module enters the L0s and L1 active states. The faster a module enters these low-power states, the more power savings designers can realize. However, they must consider a performance trade-off: If the modules too quickly enter these low-power states, then the data-transfer rates decrease due to the latency necessary to exit the low-power states. The resolution to this trade-off differs based on the application and desired performance of the ExpressCard module. For example, a PCI bridge allows an ExpressCard module to use legacy PCI-based devices. In such a device, a designer could program the PCI Express-to-PCI bridge to enter the L0s active state almost immediately following a PCI Express transaction. Due to the relatively small amount of time to move from the L0s to the L0 state, the performance of the module would remain the same. Using the same device, a designer could adjust the L1 active-state entry time to be slightly longer than the time between back-to-back PCI transactions. This approach allows for maximum performance from the ExpressCard module when data is transferring but allows for quick entry into the L1 active state and maximum power savings when no data is transferring.

Like CardBus cards, PCI Express-based ExpressCard modules also use device states for power savings. These device states, D0, D1, D2, D3hot, and D3cold, allow for software to direct the module to enter a low-power state. The PCI Bus Power Management Interface Specification describes these device states. When in a low-power state, ExpressCard modules can use techniques such as clock gating to turn off inactive portions of the module, thus reducing power consumption.

Another aspect of power management for ExpressCard modules is the presence of a 3.3V auxiliary supply in addition to the 3.3 and 1.5V main supplies. An ExpressCard module can leverage the auxiliary supply to segment the power management and wake-up logic from the rest of the logic in the module. Then, when someone removes main power and the ExpressCard module enters a low-power state, only the power management and wake-up logic remain on. The core logic in the module remains off, greatly reducing power consumption, yet the module retains the ability to wake the system. The PC Card standard includes no auxiliary power supply. As a result, the same supply in a PC Card powers all of the logic, preventing a significant power reduction when the PC Card is in a low-power state.

Though based upon the proven power scheme in the PC Card standard, the ExpressCard standard provides a more robust power-management methodology. On the host side, designers can implement ExpressCard power control without the dedicated controller that PC Cards need. On the module side, link states allow modules to enter low-power states, and a separate 3.3V auxiliary supply eases the implementation of wake-up support. These features enable an ExpressCard system to not only meet, but also improve upon the power savings designers can realize with a PC Card system.