Need a recommendation for low power microcontroller

Thread Starter

gpavlov

Joined Mar 1, 2016
5
For a university project I am working on I need a recommendation for a development board. The device will be very simple and it's whole function is to connect via bluetooth to smartphone and switch on/off 2 LEDs based on commands from smartphone. Some alternatives that I looked into are Intel Edison (which is too big and powerful for this task) and cypress CY5676 (which I am not familiar with). I know there are a lot of options out there so my goal is to focus on few recommendations and compare them. Here is the full list of requirements:
  1. Low power (10 hours operation)
  2. Small size (less than credit card)
  3. Can be battery powered
  4. No external devices required for dev (such as jtag or Flash programmer)
  5. Can drive 2 LEDs w/o external power
  6. Free development tools
  7. Easy to connect 2 LEDs with wires (10-20cm long)
  8. Cheap (<$70)
  9. Documentation exists
 

bug13

Joined Feb 13, 2012
1,751
For a university project I am working on I need a recommendation for a development board. The device will be very simple and it's whole function is to connect via bluetooth to smartphone and switch on/off 2 LEDs based on commands from smartphone. Some alternatives that I looked into are Intel Edison (which is too big and powerful for this task) and cypress CY5676 (which I am not familiar with). I know there are a lot of options out there so my goal is to focus on few recommendations and compare them. Here is the full list of requirements:
  1. Low power (10 hours operation)
  2. Small size (less than credit card)
  3. Can be battery powered
  4. No external devices required for dev (such as jtag or Flash programmer)
  5. Can drive 2 LEDs w/o external power
  6. Free development tools
  7. Easy to connect 2 LEDs with wires (10-20cm long)
  8. Cheap (<$70)
  9. Documentation exists

Any MCU can do 10 hours, but I will use TI launcher pad value line, it's about $10.

Or an arduous board can do it.
 

WBahn

Joined Mar 31, 2012
24,964
Whether you can operate for ten hours from a battery depends not only on the power draw of the MCU, but of the loads (the LEDs) and, above all else most likely, the size of the battery.
 

Thread Starter

gpavlov

Joined Mar 1, 2016
5
You are right, a hidden assumption here is that the battery size would be comparable to what bluetooth headsets have, looks like those are between 100mAh and 200mAh. Looks like small LEDs drain between 20mA to 100mA, and assuming that they should be switched on only 20% of the time on average we have 4mA to 20mA. So the power budget for the microcontroller would be between 10mAh to 20mAh, for 10 hours that would give on average 1-2mA consumption
 

WBahn

Joined Mar 31, 2012
24,964
You are right, a hidden assumption here is that the battery size would be comparable to what bluetooth headsets have, looks like those are between 100mAh and 200mAh. Looks like small LEDs drain between 20mA to 100mA, and assuming that they should be switched on only 20% of the time on average we have 4mA to 20mA. So the power budget for the microcontroller would be between 10mAh to 20mAh, for 10 hours that would give on average 1-2mA consumption
I'm not following your math. If you have 20 mA LEDs at 20% duty cycle then that's an average draw of 8 mA for two of them. With a 100 mAh battery at 10 hours that gives you a total current budget of 10 mA, leaving 2 mA for the MCU (and all other circuitry). With a 200 mAh battery that doubles to 4 mA. Are you cutting that in half to provide a cushion?

Note that if you go up in LED current much at all then you exhaust your budget almost immediately.

You can find LEDs that run on only half a milliamp (but they aren't very cheap) and lots of ones that only need a couple milliamps. You might be able to duty cycle them to reduce the average current draw even more.

Will the MCU be able to sleep at all?

What clock speed will it need to run at (this is the big determiner)?
 

Thread Starter

gpavlov

Joined Mar 1, 2016
5
You right again, I forgot the second LED. Please note though that all the numbers are rough estimations so it doesn't matter much.
Good to know that there are even better LEDs.
For the task of getting periodic commands from BT and switching the GPIO I think around 50MHz should be more than enough and I think that the MCU will be able to sleep most of the time, unless BT requires polling.
 

WBahn

Joined Mar 31, 2012
24,964
I don't know what is available, but I would hope that a solution exists in which a BT module could awaken the microcontroller when it had data and then the MCU could pull the data at as slow a rate as was tolerable. If you are just turning on and off two LEDs, running at the speed of a watch crystal (32 kHz) would be more than sufficient and would reduce the power draw of the MCU (if running continuously) by a factor of a thousand.
 

Thread Starter

gpavlov

Joined Mar 1, 2016
5
Thank you for all the useful comments, it helped me to refine the requirements and make a quick choice. Eventually I ordered the TI SensorTag and I am excited in anticipation to play with it :)
 

Joe_M

Joined Feb 19, 2013
8
You might be able to use a Atmel chip, the Arduino IDE, and a bluetooth board from Ebay. They can be made to do all that easily, and be very small. Do you get extra credit for making your own board? The help you would need to do this is all over the internet, it is the most widely supported microcontroller on the planet. Programming would be in C/C++, and it can be powered from a very small battery for 10 hours of use, just use the sleep function, and interrupt to wake.
 
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