Power reduction is essential for battery-powered devices. While software developers play a key role, it's also the responsibility of hardware engineers to design efficient circuits, select appropriate components, and collaborate to ensure power consumption stays within acceptable limits.
MCU power optimization can be approached from six main areas:
1. **Device Selection**
Choosing the right components is crucial for low power consumption. Sometimes, cost constraints lead to using parts that aren't ideal in terms of energy efficiency. For example, some sensors consume a lot of power, and reducing their usage through software can be complicated. It’s better to pick sensors with low-power modes. Similarly, when selecting an MCU, it’s wise to go for one that has low power consumption while still meeting functional needs. Some 8-bit MCUs use just tens of microamps in active mode and around 1 µA in sleep mode, making them a top choice. However, these MCUs often have limited resources, such as small ROM and RAM sizes, which can make software development more challenging.
2. **Reduce Clock Frequency**
The higher the clock frequency, the more power the chip consumes. To save energy, many designs avoid using external crystal oscillators and instead rely on internal ones. While a 32.768 kHz oscillator provides good accuracy at the second level, it’s not sufficient for millisecond or microsecond-level timing. In such cases, a higher frequency (over 1 MHz) is necessary, even though it may increase power consumption slightly.
3. **Sleep Mode**
Sleep mode is one of the most effective ways to reduce power. The MCU can enter a low-power state, and other modules can also be put into sleep. When an external event wakes up the MCU, it activates the necessary modules, performs the required tasks, and then returns to sleep. Many modules offer multiple sleep levels, allowing for flexible power management without compromising functionality.
4. **Turn Off Unused Resources**
When using a more complex MCU, it's important to disable peripherals that are not in use. Even with simpler MCUs, functions like I²C, SPI, or UART may be implemented via pins, but they should still be turned off before entering deep sleep. Additionally, sensors or other devices should be powered down or placed in a power-saving mode before sleep, and reinitialized upon waking.
5. **Configure GPIO Pins Properly**
Before putting the MCU into sleep, it’s easy to forget to configure the GPIOs correctly. If a pin is connected to an external pull-up resistor and the MCU goes to sleep, there could be a voltage difference across the resistor, causing unnecessary current draw. This subtle issue can significantly impact power consumption, so each pin must be carefully configured based on the external circuit.
6. **Intermittent Operation**
Intermittent operation involves alternating between active and inactive periods, effectively cutting power supply on and off. This method can cut power consumption by half, provided the device doesn’t require a warm-up period after power-on. Some components can operate with pulsed signals, and adjusting the duty cycle can help balance the load and improve efficiency. However, this approach requires careful planning and is more complex to implement.
These strategies provide a solid foundation for reducing power consumption in MCU-based systems. Power optimization is a continuous challenge, and the more stringent the requirements, the more creative solutions you’ll need to find.
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