Nuts and Volts - January 2016

this command will look familiar. Remember that this just enables global interrupts. You’ll need to have already configured your interrupt source (e.g., the ADC/external interrupt/timer interrupt).

sleep_mode(); This final function (well, macro actually) really does three things:

• Sets the Sleep Enable bit: SMCR |= (1<<SE);

• Puts the microcontroller to sleep: __sleep();

• Clears the Sleep Enable bit; this will happen when the MCU wakes up: SMCR &= ~(1<<SE);

That’s it! We’ve successfully put the microcontroller to sleep. So, what happens when it wakes up?

When the Microcontroller Wakes Up

We’ve configured one (or more) interrupts to wake the MCU from its sleep mode. Now what? When the interrupt triggers, the microcontroller wakes up (taking at least 10 clock cycles to get up to full steam) and enters the Interrupt Service Routine (ISR). When the code in the ISR has completed, execution returns to the statement immediately after the sleep instruction that initially put the microcontroller to sleep. As we usually try to keep code in the ISR as short as possible, it’s likely that we’d want to use the ISR to set a flag that then gets processed back in the main while(1) loop.

Once we’ve finished executing the code that we woke up to run, we simply put the microcontroller back to sleep. I usually put the sleep commands in the main while(1) loop so the MCU goes back to sleep on each iteration of the loop.

Yep. That is all there is to sleep modes. I’m sure you’re chomping at the bit to see this all in action, so let’s dive into our first project.

The First Project: Sleep

I’ve lined up two projects this month. The first is a simple one that visually shows us the workings of sleep mode. The second one I’m going to leave to you to play with — with a bit of guidance, of course. Let’s get cracking!

The first project has two LEDs attached via current-limiting resistors to PB0 and PB1. Listing 1 shows the bulk of the code and Figures 5 and 6 show the layout. The first LED (I call it LED_BLINK in the code) is blinked by an interrupt on Timer2. Last month, we worked with Timer1, but Timer1 isn’t able to wake a microcontroller from sleep. So, we need to use Timer2 for this project. We use it in the same way, although the control registers are slightly different.

Also, it is an eight-bit timer, so the most we can count up to is 255. This means that the LED will flash pretty quickly! To combat that, I’ve used the Timer’s ISR to increment a variable (delayCounter) on each interrupt;

when delayCounter reaches 10, I toggle the LED. You could also use a slower crystal. If you’re currently using a 16 MHz one, the lowest you can go without reconfiguring the fuse bytes is an 8 MHz crystal.

The second LED (LED_SLEEP) is there to prove to you that the microcontroller is actually going into sleep mode. I needed it to prove the same to me! This LED is toggled before the sleep command is executed. If the sleep command succeeds, then execution of the program will halt until the timer interrupt wakes it; this delay causes the LED to blink (very fast). If the microcontroller doesn’t go to sleep, then execution will continue and the LED will be toggled a few lines of code later, causing the LED to glow constantly (although dimly). Test this functionality by commenting out the sleep_mode(); command and watching how the LED behaves.

The rest of the code is pretty straightforward and should consolidate what we’ve discussed earlier in this article. The problem with this project is that it doesn’t really demonstrate how the sleep mode is reducing the power consumption of the project. It’s for that reason we