How do you use low-power modes and idle mode in PIC microcontrollers?

1 What are low-power modes and idle mode?

Low-power modes and idle mode are special operating modes that reduce the power consumption of the PIC microcontroller by changing the clock source, frequency, and voltage, and by disabling some or all of the CPU, peripherals, and memory. There are four low-power modes: sleep, deep sleep, ultra-low power, and extreme low power. Each mode has different trade-offs between power savings and functionality. Idle mode is a variation of normal mode that stops the CPU clock but keeps the peripherals and memory active.

2 How to choose the right low-power mode?

When choosing a low-power mode for your application, you should consider factors such as the amount of power consumed, the wake-up time, how many peripherals and memory need to remain active, and how often the low-power mode needs to be entered and exited. Generally, lower power consumption means longer wake-up times, fewer active peripherals and memory, and more complex entry and exit procedures. To find out more detailed specifications and limitations of each low-power mode, you should consult the datasheet and the family reference manual of your PIC device.

3 How to configure the low-power modes?

To configure the low-power modes, you need to use the configuration bits, the power control registers, and the oscillator control registers. The configuration bits are set at compile time and determine the clock source, frequency, and voltage for the normal and low-power modes. The power control registers are used at run time and control the entry and exit of the low-power modes. The oscillator control registers are used to switch between different clock sources and frequencies during operation.

For example, to configure the sleep mode for a PIC18F device, you need to set the FOSC configuration bit to select the internal oscillator as the clock source, the IESO configuration bit to enable the internal oscillator switch-over, and the SCS configuration bit to select the system clock source. Then, you need to use the OSCCON register to select the frequency of the internal oscillator, and the WDTCON register to enable or disable the watchdog timer. Finally, you need to set the IDLEN bit in the OSCCON register to 0 and execute the SLEEP instruction to enter the sleep mode.

4 How to use interrupts and wake-up sources?

Interrupts and wake-up sources are events that can trigger the PIC microcontroller to exit the low-power mode and resume the normal operation. Interrupts are signals from peripherals or external sources that can be enabled or disabled by using interrupt control registers, while wake-up sources are specific signals that can wake up the PIC device from some low-power modes even if interrupts are disabled. Examples of common wake-up sources for PIC devices include the reset pin, the watchdog timer, the brown-out reset, and the ultra-low power wake-up module. To use interrupts and wake-up sources effectively, you need to configure their priority, enable, flag, and service routine, as well as clear their flags before entering the low-power mode. After exiting the low-power mode, you should check their flags to determine the cause of wake-up and perform the necessary actions.

5 How to use idle mode?

Idle mode is a special mode that stops the CPU clock but keeps the peripherals and memory active, which can be useful for applications that need to maintain communication or data processing while saving some power. To enter idle mode, you need to set the IDLEN bit in the OSCCON register to 1 and execute the SLEEP instruction. To exit idle mode, you need to generate an interrupt or a reset. This mode has some advantages and disadvantages compared to low-power modes. The advantages include a very fast wake-up time (one instruction cycle), not requiring any configuration bits or power control registers, not affecting the clock source or frequency, and not needing any special handling of interrupts or wake-up sources. However, the disadvantages are consuming more power than low-power modes (about 40% of normal mode), not supporting any wake-up sources other than interrupts or reset, and not preserving the CPU state (registers, stack, program counter) if a reset occurs.

6 Here’s what else to consider

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