Low power techniques

Discussion in 'Embedded Systems and Microcontrollers' started by Anson Nunley 1, Sep 3, 2015.

  1. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Hello Everyone,

    I am working on a project to help me better understand low-power design techniques. The unit will make a temperature measurement, transmit the data via XBee to a computer, where it will be logged in a CSV file, then the unit will go into sleep/low-power mode. The unit I am designing will have the following:
    • Low-power MCU (MSP430, RX111, or some other low-power MCU)
    • Digital temperature sensor (DS18B20)
    • XBee module (Probably Series 2 but still need to look into it more)
    • 3.6V Lithium Ion rechargeable coin cell battery
    • 3.3V LDO regulator
    • Battery management IC (Probably TI or Maxim)
    I was thinking about using FreeRTOS so the system is in idle/low-power most of the time, but I may be able to get by with a super-loop design. Does anyone have any advise about low-power design techniques? From what I have been researched, most of the techniques refer to IC design. I have a few ideas about how to reduce power consumption, such as:
    • Put the MCU, XBee, and battery management in sleep-mode when waiting to take/send a temperature measurement
    • Perhaps run the MCU at a lower frequency
    • Maybe use a transistor on the power pin of the temperature sensor to keep the sensor from drawing power when it's not being used
    I would appreciate any input you have on the design approach, low-power techniques, or any general rule-of-thumbs.

    -Anson
     
  2. BR-549

    Well-Known Member

    Sep 22, 2013
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    Anson, I just ordered a msp432 launch board.

    I have not received it yet. However, I downloaded the code composer software for it. When ever you build a program.............the software automatically suggest how to set chip to save power.

    It shows exactly what you want to do.

    The board comes with many regular examples and also comes with TI rtos examples. These examples have the routines.

    This all comes with the board for $13.
     
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  3. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Thank you BR-549 for your reply. I have a few different MSP430 launch pads. I'll try to set the mode to see if the power is reduced. Thanks for the suggestion!
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Your list shows you already have a pretty good awareness of the kinds of things to look at for power savings. Consider MOSFETs when you need a transistor switch. Although they can be a problem in 3.3V systems, that zero-static-current gate thingy is great for low power work.

    ak
     
  5. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Thank you AK for the suggestion about using a MOSFET for a switch. I know that I will need a VGS that is low since the board's primary source is 3.3V. For the MOSFET, I was considering using the CSD18542KCS from Texas Instruments.

    I just looked at the datasheet for the DS18B20 temperature sensor and it has somewhat of a low current draw during standby mode. I might configure it with some shunts in case I decide to use the MOSFET.

    Anson
     
  6. RichardO

    Well-Known Member

    May 4, 2013
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    I haven't looked at the specifications of either the MSP430 or the DS18B20 -- But, an output port pin may have enough drive to power the sensor directly eliminating the need of an external MOSFET. If you do turn the sensor on and off, don't forget to configure the input pin for the sensor so that it does not float while the sensor is powered down.

    Note that the MOSFET must have a low on resistance, but not really low (say, less than an ohm) and a gate threshold voltage of less than 2.5 volts. It only needs to dissipate a few milliwatts of power so a TO-92 or SOT-23 packaged device is good enough . I have used the FDN338P (a P-MOS FET in a SOT-23 package) in applications similar to this.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Lithium batteries will last longer if see only the average current draw. Use good quality tantalum capacitors to prevent the battery from seeing current spikes. I designed a controller that was lithium battery powered that had a 10 year life spec. When the battery was disconnected the controller would run for one and a half minutes on the energy from 30 uF of capacitor.

    Loose the LDO regulator. This is a instant waste of power. Some of the PIC chips will run as low as 2.0 volts and as high as 5.5volts. (PIC 16F877 for example) No regulator needed. I am not familiar with the MSP430. If it will work without a regulator, then don't use one.
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    I looked up a MSP430. it will run from 1.8 to 3.6 volts. Very Good. No regulator needed here.

    Your temp chip, DS18B20, works from 3.0 to 5.5 Volts. This part is not a good voltage match to the MSP430. I would look for another temp sensor that operated at lower voltages.
     
  9. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Thank you for the advise RichardO! I learned in my senior design project to either use a pull-up or pull-down resistor for a transistor. Always good to know that it's pulled either high or low.

    Did you mean the on resistance for a FET shouldn't be less than a milli-ohm?
     
  10. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Thank you Lestraveled for your response! I do have a tantalum or two in my design to manage the current spikes. That sounds like a very good MCU! I thought about removing the LDO, just needed a second opinion. The MSP430 that I picked (MSP430F5131) has a few low-power modes. It's powered by 3V, but can draw as low as 0.25uA in the lowest mode. I've worked with a few MSP430s before so that's I'm most familiar with but I have been looking into a PIC for some of my other projects.
     
  11. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Could you explain why the DS18B20 is not a good match for the MSP430?
     
  12. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Could it be that I need to find a temperature sensor that operates between 1.8 to 3.6V for example so it's still operational when the battery is discharging?
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    Exactly! Find a temp sensor that matches the operating voltage range of your CPU. In other words, the temp sensor and the CPU should quit running at about the same voltage. Make Sense??
     
  14. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Indeed, it does make sense. Thank you for the advise! I'll do some more research to see what I can find.
     
  15. RichardO

    Well-Known Member

    May 4, 2013
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    Yes, it is a good idea to have a resistor to make sure that the MOSFET gate never floats. This happens during the reset and initialization periods of the microcontoller.

    Also, when the load (your tempeature sensor) is turned off, an pins driving the sensor must be set to the same voltage as the un-powered sensor.
    For instance, if the sensor is switched off using the 3.3-volt side then the sensor's signal pins will go to ground while switched off. Therefore, output pins driving the sensor should be set to ground to match.

    No, the MOSFET can have an on resistance as high as one ohm. The goal is to the switched-on FET drop a small fraction of your power suppl voltage --say less than 50 millivolts. A sensor could draw as much as 50 milliamps though 1 ohm and still only drop 50 millivolts.
     
  16. John P

    AAC Fanatic!

    Oct 14, 2008
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    You may not need a transistor to switch the sensor's power. Just use a port pin of the uC chip. If you want a digital sensor, this one looks good (and it can run down to 1.4V): http://www.ti.com/product/tmp102
    The PIC16F877 is physically large, if you just want to drive just one sensor and a serial interface. If that really is the job, a 14-pin chip would probably be adequate.
     
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  17. Lestraveled

    Well-Known Member

    May 19, 2014
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    John, I agree, perfect chip with all the bells and whistles for low power operation.
     
  18. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Thanks for the temperature sensor suggestion John. I actually did go with a TI temperature sensor (TMP75CID). It has longer leads than the one you suggested to help me with soldering. It also can operate at 1.4V when the battery is running low.
     
  19. Anson Nunley 1

    Thread Starter New Member

    Sep 3, 2015
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    Thank you for the insight about setting the driving pins to match the signal pins from the sensor. I wouldn't have thought to do that. I'm assuming it's to help reduce power consumption and to prevent any damage to the devices?
     
  20. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Not sure but look amongst those from Microchip. IIRC they could be used.
     
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