In this lesson, you will learn how to configure your hardware timers to maintain essential timekeeping while keeping your battery-operated device in a low-power state.
When building battery-powered systems, the goal is to keep the CPU in a deep-sleep mode as much as possible. However, your device still needs a "heartbeat" to trigger tasks like sensor readings or transmission intervals. If you leave a high-frequency timer running at full power, you will drain your battery rapidly.
Reducing Current Draw with Prescalers
The first step in power optimization is using the Hardware Timer Prescaler we discussed in lesson 16. A high-frequency timer clock consumes significant power. By setting the prescaler to the maximum possible value, you reduce the internal toggle frequency of the timer counter. The counter still ticks at the frequency you need to maintain your time-base, but the clock tree internal to the peripheral operates at a much lower power draw.
Leveraging Timer Gating and Low-Power Clocks
Most modern microcontrollers allow you to select the clock source for a timer. Instead of using the main system clock (which powers the CPU and bus matrix), you can often switch your timer to a low-frequency, low-power internal oscillator (like a 32kHz crystal or internal RC oscillator).
Consider this conceptual flow for setting up a power-efficient periodic interrupt:
// Configure the timer to use a low-power 32kHz clock source
Timer_SelectClockSource(TIMER_0, CLK_LOW_POWER_32KHZ);
// Set a large prescaler to extend the period without high-speed switching
Timer_SetPrescaler(TIMER_0, 1024);
// Only wake the CPU when the timer match register is hit
Timer_EnableInterrupt(TIMER_0, INTERRUPT_MATCH);
// Enter low-power sleep; the timer peripheral continues running independently
Enter_DeepSleep();
By decoupling the timer from the main system clock and using a higher prescaler, the timer peripheral requires significantly fewer transitions per second. This allows the core logic to remain in a low-leakage state while the hardware counter increments in the background, only waking the processor when a specific time-base event occurs.
Key Takeaway: Minimize timer power consumption by selecting the lowest frequency clock source required and utilizing large prescalers to keep peripheral switching activity at a minimum.