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Atmega 328p bare minimum with external 8 Mhz Crystal

jeyara

Love  Arduino and its features. Started using it for last 3 months. Looks like it’s one way ticket. Love it.

 

Now, I am working on an arduino project to help my photography hobby. I did some search on google and studied possible datasheets before positing. I feel bit unsure about how to proceed.

 

Hardware I am using,

 

  • Arduino Uno running on 5v.
  • Nokia 5110 LCD display with resistors to match its 3.3v
  • 4 channel optocoupler to connect camera and flash.
  • Few push buttons for LCD navigation.

 

I will be having different sketches to be loaded based on my indented sooting situation using USB to TTL UART RS232. Or may be many clones with different sketches.

 

 

All are wired in breadboard. All working great. Now in a stage of moving to PCB. I intent to create the design to be small to be operated with one hand. This is purely from my photography experience. So I opted for all SMT based solution for PCP.

Also, I wanted whole system to be running on 3.3v.

 

 

This is simply because (i think. correct me if i am wrong) ,

 

  • Easy to work with Nokia 5110 LCD.
  • Less parts than working with 2 voltages in same PCB.
  • Can drive straight from a small 3.7v LiPo Cell
  • LiPo’s can be charged from USB. Especially, when you are on the field, I can use car USB or solar.

 

But i know, arduino will run its half speed at 8Mhz. But its okey for my case.

 

Questions:

 

I have prototyped and tested on 5v and 16Mhz. But Atmega 328p on 3.3v runs on 8Mhz. I am planning to follow http://arduino.cc/en/Main/Standalone schematics for PCB design with replacing 16 MHz crystal with 8 MHz and replacing regulator with LDO 3.3v instead of 5v. I did search on this forum. But there were some example with internal 8 MHz resonator. Not with external. But i have seen few questions saying that internal 8Mhz crystal has issue with usb sync. But it doesn’t mention any about external crystal.

 

 

Atmega chip I am using is here. Crystal I am using is  here.

 

Will burn the boot loader using method described in http://arduino.cc/en/Main/Standalone my PCB will have an ICSP header.

 

  1. Do you guys think of or experienced any issue with this approach?
  2. Can I use 10uF capacitors for 8 Mhz crystal too?
  3. I have chosen a regulator with 100mA supply. Will it be enough for above hardware?

 

 

Hope i am not duplicating same issue.

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  • 0

    First off, I think you mean a 10nF NOT 10uF cap for the crystal, if this is the case then yes you can use a 10nF with the crystal

     

    The Arduino 328 based board does not talk directly to USB, it uses either an FTDI board or a 16U2 chip to off load this responsibility. In your case because your using an ISP 6pin header, this will not be an issue. If the only purpose of having the USB connector is to charge the battery, there will not be an issue

     

    As for internal oscillator vs external Crystal. the biggest thing that will affect things is the stability of the clock. The internal oscillator will not be as precise or as stable. It will drift around a little bit over time. If your not requiring any critical timing better than say 1mS (At a guess) and your not running a real time clock then accuracy then this will not matter

     

    As long as your not drawing a load to drive the camera, this 100mA should be enough

  • 0 in reply to Robert Peter Oakes

    Re. those capacitors:

     

    The two 10uF caps are across the power supply and should be chosen according to the data sheet for the regulator you actually use.

     

    The crystal has two 22pF caps and you may as well keep those the same as well for an 8Mhz crystal (although 15pF would probably get you closer to the ideal).

     

    MK

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