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In this section of the lab we will focus on how power can be conserved using hardware and software.

One of the most important quality standards for battery powered devices is battery life. Handheld medical tools, electricity meters, personal digital assistants, and a goal of the designer and programmer is to lower the power use of the embedded system to negligible levels. This portion of the lab will give an overview of how power can be conserved using hardware and software. In designing battery powered devices, savings can be gained from the choice of electronic components, the arrangement of components, and the software on the design. The exercises will integrate the low power modes of the MSP into existing labs, so that examples of software power savings can be shown.

Measuring power on the ez430

Measuring the current consumption of the ez430 is a slightly tricky task. To do so we must insert an ammeter in serial with the ground pin between the Spy-Bi-Wire programming interface and the target board. Since the program is loaded into flash on the tool, we can run our code, cease debugging, and the tool will run our program whenever it is powered. Thus, the following diagram shows how we can insert an ammeter in serial with the ez430 while the program runs even though we cannot debug it at this point (the two middle data pins aren't connected).

Connection diagram

Connecting the Ammeter in serial as shown will allow for you to measure current consumption.
The following is what it would look like if we actually connected everything together. (Notice that I have added sockets (DigiKey Product ID: **********) to the "fully accessible" pins to make them more fully accessible.

Connection picture

Here, I have created the setup using parts from around the lab.

Shutting off parts in general

Most parts have a shutdown or sleep mode available that will reduce the current consumption of the component considerably. In general, digital parts consume significant current when their transistors switch because of the charging and discharging of the internal capacitances of the transistors. Analog integrated circuits also support shutdown modes to reduce power consumption. Datasheets will specify the current consumption in both on and shutdown modes of the component. It is important to note that when a device is in shutdown mode, power and ground voltages are still powered and connected to the device.

In order to shutdown most integrated circuits, all that is required is a shutdown or sleep pin to be asserted properly. Other devices require a shutdown command to be issued over the bus. The primary disadvantages of shutdown modes, apart from the fact that the device is inoperative is that recovering back into normal operating modes can impose a significant delay. A useful property of the MSP is that its recovery time from some low-power modes is fast enough to meet the response times of interrupts.

Parts without built-in shutdown modes must be shutdown by having its current supply controlled through a transistor or other switching device.

Using the low power modes

The MSP430 was designed with the low power modes in mind from its beginnings. In lower power mode, the processor can achieve current in the microamps while still monitoring its inputs. The principles of utilizing the MSP power modes are described in detail in the second chapter of the MSP User's Guide. The modes vary the degree to which the processor is aware of its surroundings and the clocks that the processor keeps running. The processor lowers power consumption partly by shutting off external and internal oscillators.

There are four low power modes in addition to regular operating mode on the MSP430:

  • Active Mode is the fully powered mode when the processor executes code and all clocks and peripherals are active. The chip consumes about 340 µA with 1 MHz clock at 3.3V in this mode.
  • Low Power Mode 1 (LPM1) disables the CPU and MCLK while leaving the ACLK and SMCLK enabled. This allows timers, peripherals, and analog systems to continue operation while dropping current consumption to about 70 µA with 1MHz clock at 3.3V. Because the timers and other internal interrupt systems still operate, the processor will be able to wake itself.
  • Low Power Mode 2 (LPM2) disables the CPU, MCLK, and the DCO are disabled but the SMCLK and ACLK are active. The DC is disabled if the DCO is not used for MCLK or SMCLK in active mode. Internal interrupts can still operate. Current consumption drops to about 17 µA.
  • Low Power Mode 3 (LPM3) disables the CPU, MCLK, SMCLK, and DCO. The DC and ACLK remain active. This allows some peripherals and internal interrupts to continue. Current consumption drops to about 2 µA.
  • Low Power Mode 4 (LPM4) Current consumption drops to about .1 µA, but all clocks and the CPU are disabled. This prevents any of the on-chip modules from operating, and only off-chip interrupts can wake the device.

To enter a low power mode the status register in the CPU must be set to indicate the desired mode. Specifically the bits SCG1, SCG0, OSCOFF, and CPUOFF. The User's Guide details the specific bits needed. Also provided in the chapter is some example code on changing power modes. To exit low power mode, an interrupt is needed. In the interrupt, the previous status register state can be altered so that exiting the interrupt will leave the processor awake. The User's Guide explains in detail the specifics of entering and leaving low power mode. Example code with the compiler also demonstrates the low power modes.

Principles of low power operation on the msp

The User's Guide for the MSP also explains the principles needed to lower the power consumption of a design. Be sure to minimize wasteful code execution. This is the same idea as improving speed performance because every unnecessary instruction wastes a little bit of battery power. All of the techniques that improve code efficiency will improve power efficiency. Increasing clock speed will not yield similar power savings because faster execution increases power consumption. Similarly, unused peripheral modules on the processor should be de-activated to save power. Use interrupts to handle events to allow the processor to stay in Low Power Mode 3 as much as possible. By reducing the awake time of the processor, the average current consumption of the MSP can be reduced to levels approximately as low as LPM3 while maintaining the same functionality.

Questions & Answers

Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Introduction to the texas instruments ez430. OpenStax CNX. Jun 19, 2006 Download for free at http://cnx.org/content/col10354/1.6
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