Nuts and Volts - January 2016

AVR Fuse Bytes: www.crash-bang.com/programme-avr-fuse Atmel Video — Power-Saving Techniques: www.crash-bang.com/Atmel-Low-Power-Video

Resources Understanding

Nuts & Volts Forum: http://forum.nutsvolts.com

Alternatively, you can use the macro that’s included in the “sleep.h” library to handle this complexity for you:

sleep_bod_disable();

Power Reduction Register

Figure 7: Measuring current draw in Project 2.

have a second project!

Squeezing the Last Drops

The PRR allows you to disable the TWI (I2C) module, Timers 0, 1, and 2, the SPI module, the USART0, and finally the ADC. Each of these are disabled by writing a 1 to the corresponding bit in the register, and enabled by writing a 0. The serial modules We’ve covered the MCU modules that are disabled during sleep mode, but what about modules you don’t need while the microcontroller is running? Additionally, what if the sleep mode you’re using still has modules in an The PRR doesn’t only affect sleep modes. It’s a good way to keep consumption down when the microcontroller is awake, as well. Now that we’ve touched on these in theory, let’s put them into practice!

“awake” state when you don’t actually need them? All sleep modes can be woken by an I2C (TWI) address match, for example, which means a part of the TWI module is running even when asleep. If you don’t use TWI at all, then surely it makes sense to disable it at the outset.

The Second Project: Low Power

Going to this level is really squeezing the last drops of efficiency from your project, but over a period of time it all adds up. There are two registers that enable us to disable unused modules: the MCU Control Register (MCUCR) and the Power Reduction Register (PRR); refer back to Figure 4.

This simple project uses a pin change interrupt to toggle an LED. When you press the momentary pushbutton connected to PB1, the LED on PB0 lights up. You’re probably thinking that sounds pretty boring; after all, we’re now eight articles into the series. Well, this is more of an experiment than a useful project. The idea is that you’ll measure the impact of the various power-saving techniques we’ve discussed so far. Hopefully, things are starting to sound a little more interesting!

MCU Control Register

The MCU Control Register — for our power-saving purposes — gives us the ability to enable or disable the BOD while the microcontroller is asleep. The BOD continuously monitors the voltage supply to the MCU, but if that’s not needed during sleep, then it makes complete sense to turn it off. The BODS and BODSE bits are used to control this, in a somewhat convoluted manner. To

1. Set both BOD and BODSE to “1.”

2. Set BOD to “1” and BODSE to “0” (the latter within four clock cycles).

3. Enter sleep mode within three clock cycles; in other words, almost immediately!

First, connect the LED to PB0 (through a resistor as usual) and a pushbutton to PB1 (connect the button’s terminal to GND, as we’re using the ATmega’s internal pull-up resistor). Then, connect a digital multimeter to the project, so that it is in line with the VCC power supply (i.e., the power needs to “flow through” the DMM). Switch the DMM to measure amps and adjust the scale as you need. We’re ready to start our experiments now.

After each of the below, record the readings on the DMM (Figure 7). The experiments are clearly commented in the code, so you should find them easily enough.

Experiment 0: Obtain a baseline reading. We haven’t implemented any power savings yet, so take a baseline reading to see just how effective the savings are.