Tips on using CPLDs to reduce system processor power consumption

Designers use several design techniques to significantly reduce overall system power consumption, such as:

* Reducing operating voltage;
* Optimizing system and CPU clock frequency;
* Eliminating spikes of large current consumption during the power up sequence;
* Efficiently managing system battery operation;
* Efficiently managing operating mode of system devices;
* Minimizing bus activity;
* Reducing bus capacitance;
* Reducing switching noise.

Many manufacturers today offer devices with power saving modes that temporarily suspend the device from its normal operation. These devices have the option to power down or transition to a non-functioning state if the device is not active for a specific amount of time.

This feature is available on many of today's microprocessors and MCUs. By taking advantage and managing the operating mode of large power consumers on a PCB, such as the processor, the overall power consumption of the system can be reduced significantly.

Reducing power consumption involves correct management of the operating mode of a device and designing a system to take advantage of the modes a device can operate within.

Offloading operations of the microprocessor allows it to stay in its low-power state for a longer amount of time. One way to reduce system power is to allow a low-power PLD, such as a CPLD, to manage these offloaded operations.

This article describes this possibility, along with types of operations that allow a processor to remain in a low-power state longer, thereby reducing system power consumption.

Figure 1: Shown is the typical power consumption of system components in a Web Pad application.

Microprocessor modes
In some portable applications, the CPU can consume 30 percent of the overall system power. Figure 1 above illustrates the typical power consumption of system components in a Web Pad application.

Microprocessor power consumption can range from 720µW to 1W during normal operation. Microprocessor operating modes vary by part and manufacturer and include modes such as normal; run, sleep, suspend, standby, stop and idle operation.

Operating modes can vary in power consumption as much as 230mW between states. Normal operation of some low-power microprocessors can be as little as 250mW.

Figure 2 below illustrates the power consumption of the Intel StrongARM SA-1110 microprocessor operating modes. The power dissipation numbers shown in Figure 2 are determined by operating at 206MHz with a nominal external voltage supply of 3.3V and internal voltage supply of 1.8V.

Figure 2: Shown is the difference in power consumption of operating modes in a microprocessor.

Operating modes of the StrongARM processor include normal, idle and sleep. In normal operation, the CPU is full-on, with the device fully powered and receiving active clocks.

In idle mode, even though power is applied to the CPU and other components, all clocks to the CPU are stopped, with only clocks to peripheral devices active. In sleep mode, power to the CPU and other peripheral components is disabled. Sleep mode disables all functions except the real-time clock, interrupt controller, power manager and general purpose I/O.

Microprocessors with power saving modes have an on-board power management controller. Operating modes allow the OS or software application to temporarily suspend the CPU. The microprocessor executes a series of instructions to place itself into a power saving state. Once in a power down mode, several components of the microprocessor can still respond to system interrupts.