Green Electronics Strategies – Sleep Saves Energy

How Does Sleep Save Energy?

For this post, we will look specifically at Embedded Software techniques to save power and energy. This is a well known Power Saving Strategy which doesn’t always get the recognition it deserves. It is also something you have to design into the Power Management Plan from the beginning.

 

For this example, we will use the MSP430 from TI which has some of the best Power Saving and Power Consumption figures in the industry. We have used them to create devices that run from a pair of AAA batteries for 2 years and which have time based control algorithms so that they can’t be used in a purely event driven mode. Here is how it works:

Low Power Sleep Mode

Low Power Sleep Mode

This shows the power consumption versus time. In Low Power Sleep Mode the consumption is close to zero. Almost no power consumed. Then depending on what is happening it wakes up to varying degrees.

 

Get the best Electronic Sleep

So this is how you take advantage of this:

  • make the time between wake ups as long as possible
  • make the time awake as short as possible
  • only turn on the peripherals needs for a particular wake period

Now if you system only has to wake once every minute then you can get low power operation from a lot of different processors. If it wakes many times a second then you need a processor that gives you lots of ways to reduce power during wake, reduce the time awake, and increase the interval between wakes.

 

MSP430 Sleep

So back to the MSP430. It has Power Conservation features that allow it to do all three better than most. Here is the list:

  • Digitally Controlled Oscillator DCO allows it to wake and run quickly
  • Can run a Timer from a 32KHz crystal making interval timing very low power
  • Can use the DCO to set the run speed and so shorten the wake time
  • Lot’s of Power Down Modes so you can always find one that suits your application
  • Peripherals can be Shut Down when not in use
  • Can run down to 1.8V – more on that later but it can also help here

Low Power System Architecture

To take advantage of all this, you have to develop the System Architecture so that takes advantage of this. An example from a very long life application we did runs like this:

  • 32Hz Oscillator runs a timer that generates a 1 second wake
  • User input keys set up to wake on change of state from high to low
  • Use DCO at 1MHz to quickly wake, execute & sleep again
  • Use State Machines to allow modules to execute predictably with eratic timing
  • Have early exit tests to prevent unnecessary Code Execution

The result is an application that runs a process with User Interaction, LED Indicators, and a 2 week cycle where the average Power Consumption is 20uA at 2.7V or 54uW. Of this, less than half is the processor executing the software and the single biggest energy use is the intermittently flashed LED Indicators.

 

To learn more, check out this more comprehensive article on “Low power MCU selection criteria and sleep mode implementation” from embedded.com which provides more examples.

 

Ray Keefe has been developing high quality and market leading electronics products in Australia for nearly 30 years. For more information go to his LinkedIn profile. This post is Copyright © Successful Endeavours Pty Ltd.

Electronics Manufacture – How to Select a Microcontroller

When it comes to selecting a microcontroller for Electronics Manufacture, the field to choose from is quite large. And while there is no one right answer to this, there are some basic steps to take to ensure you are using the right core component in your latest Electronics Development Project.

 

For this post I will walk you through how we go about this process. As I said earlier, there usually isn’t one right answer but this will give you an idea of the process we use.

 

The first priority to determine is which of the following is critical:

  • power consumption
  • speed or processing power
  • peripherals
  • cost

This is how we go about the selection process:

 

If it is Power Consumption that is most critical, then we go for the TI MSP430 family. These have been designed from the ground up for low power with good processing ability. They also have excellent ADCs and they are fast at 200kSPS.

 

If it is processing power you want, then the ARM Embedded Processors are great value. They are 32bit and most need dual power supplies but they really deliver when its comes to performance. They also come in a wide range of flavours with peripherals to suit a plethora of applications. We particularly like the offerings from Atmel SAM7 and NXP LPC2000.

 

If it is peripherals, then it depends on the level of performance and if there are any specific applications. For automotive you will often want a LIN or CAN peripheral and the processing power will depend on whether it is a simple ECU, Body Computer or Engine Management Unit. If it is networking you want then you can get away with an 8 bit processor for some tasks but it is hard to beat an ARM MCU with on board ethernet controller. For this selection criteria, carefully map out what you need and the performance required then look at the secondary factors such as processing power, power consumption and cost.

 

For cost, we like the Atmel AVR processors. These are also our work horse 8 bit processors. And they also have a good range of peripherals, on board EEPROM as standard, and good power management capabilities.

 

Now let me also make one other point about cost. It’s not just the component cost, but the complete cost equation. So development costs, part costs, manufacturing costs, and product life cycle support costs. Although there are other parts out there that have lower unit component costs, we find the overall cost to be very good with these MCUs.

 

So that is a very cursory look at the process of selecting a microcontroller.

 

Ray Keefe has been developing high quality and market leading electronics products in Australia for nearly 30 years. For more information go to his LinkedIn profile. This post is Copyright © Successful Endeavours Pty Ltd.