0

I have used your bootloader for ATmega2560-16AU with 16MHz external crystal and it works like a wonder.

In my new project I have a 3V power supply. Therefore, I am planning to use a low frequency external crystal in order to operate the ATmega at a low frequency so that the voltage will be able to go as low as possible.

However, I have a number of issues/questions:

  1. I uploaded the bootloader using an 8MHz external crystal and it went successfully but I am not able to upload a sketch.

  2. Can I run an ATmega2560-16AU with an 8MHz crystal and will it work at 3V, without any risk?

  3. As I do not have an ATmega2560-8AU right now, I am not able to test this. Will your bootloader work with Atmega2560-8AU and an external 8MHz crystal or, before burning bootloader, do I need to make any changes?

Sorry to ask you such a long query, but I have really spent lots of time looking at this and finally today I made a post everywhere.

  • We have 2 optionn. – Som Nov 2 '15 at 12:38
1

Yes, the ATmega328 will run fine at 8MHz, at 3 volts.

The formulas for clock speed vs. volts are:

  • Below 1.8V - cannot operate
  • Below 4 MHz - use at least 1.8V
  • Between 4 MHz and 10 MHz you require: Volts = 1.8 + ((M - 4) * 0.15) (Where M is Megahertz)
  • Between 10 MHz and 20 MHz you require: Volts = 2.7 + ((M - 10) * 0.18) (Where M is Megahertz)
  • You can supply an absolute maximum of 5.5V

Plugging 8MHz into this formula: volts = 1.8+((8-4)*0.15) = 2.4 volts minimum (5.5 volts maximum). Note that, as batteries are used, their voltage goes down gradually, so the 0.6 volts difference will mean that your project will run a little longer.

Since your supply is > 2.7V, we can reverse the second formula: 3V=2.7+(M-10)*0.18); 0.3=(M-10)*0.18; 0.66=M-10, so you can run it at 10.66MHz at 3V (but it is likely to stop as soon as the battery starts to drain).

Your second question, I think, is the most relevant one - the bootloader is written for 16MHz, and some things will not work at other speeds - serial communications used for programming for example. The same, for that matter, applies to the program - if you put in a delay() instruction in your code, it will delay() for twice as long as it should.

You can get around the programming side by ditching the bootloader altogether - once your code is ready, select file->upload using programmer. This will still affect the speed your sketch runs at (anything time-related - serial communications, delay(), etc). To change this, you will need to add a custom entry to the boards.txt file. See [HOWTO:] Anatomy of the boards.txt file for custom boards.

  • Thank for your input. But here I asked regarding ATMega2560 8au with 8MHZ crystal. For whom who is looking for this: forum.arduino.cc/index.php?topic=51955.new#new I have not tried below on but it should work: sparkfun.com/products/10744 Thank. – Som Oct 28 '15 at 7:47
  • Today I just change CPU cycle to 8MHZ and baudrate to 56700 and bootloader works for ATMega2560 8AU with 8Mhz crystal. Just we need to change cpu and baudrate. Thanks. – Som Nov 2 '15 at 12:37
  • 1
    One may be able to overcome the serial baud rate issue just by talking at half the baud rate configured in the bootloader/sketch - that's not fixing it at the source, but can be a pragmatic workaround if the setup to flash the bootloader isn't handy. Of course, the timing functions will remain off by a factor of approximatley two as well. – Chris Stratton Nov 6 '15 at 2:27

protected by Community Dec 28 '17 at 7:36

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.