1. Field of the Invention
The present invention relates to the field of power management, and more particularly, to power management of embedded subsystems such as embedded modems.
2. Description of Related Art
With the proliferation of portable, battery-powered electronic devices, power management has become a critical issue for device performance. In order to extend the usable life of the battery power source, sophisticated power management techniques have been employed. In personal computers, the system microprocessor (Intel) and operating system (Microsoft) work together to conserve system power by controlling system resources. Various system devices may be monitored, and power use regulated by cutting or reducing power to inactive devices. For example, after a specified period of inactivity, a timer in the operating system may trigger the system display monitor to enter a "sleep" mode to reduce power consumption. After another interval, the monitor may be completely shutdown, further reducing power consumption. Similarly, power to peripheral devices or plug-in cards can be controlled. These power management techniques work due to the wide adoption of the Intel/Microsoft implemented solutions.
In the embedded device market, however, these solutions are inadequate since many different microprocessors and operating systems are used, each with a different power management scheme. With increased circuit integration, many manufacturers are producing a "system-on-a-chip." In other words, many functions that used to be performed by separate circuitry, are now performed on a single chip. For example, modems may now be embedded into a device, without being a separate peripheral. Unless power management techniques are utilized, the various embedded subsystems can severely drain the battery if they are powered on, but are not used. In the case of a modem, power is continually being consumed even though the user may only access the modem 10% of the time. Since the modem is embedded, it cannot simply be removed. Thus, the "standby" power consumption of the embedded systems is a significant source of power drain. In fact, testing has shown that an embedded modem may draw 6-8 mA of current even in a "stop" mode.
In many prior art attempts to manage power usage in an embedded subsystem, additional power management circuitry is used. This additional circuitry, however, is itself a source of power drain. Also, merely cutting the power to the subsystem does not provide satisfactory results. As shown in FIG. 1, a modem subsystem 2 is connected to a power node 24. In order to disable power to the subsystem when the subsystem is not in use, power supplied through the power node 24 is simply cut-off. This may cause several problems, however. First, since the signal applied to the system bus is indeterminate, the system bus may crash, thereby locking-up the system. The modem may also lock-up or otherwise fail, without special power down processing. Thus, there is a need for an improved power management system for embedded subsystems, such as modems.