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    How to use SPI with the stack ??

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    • M
      moelski
      last edited by moelski

      Hi guys,

      today I spent some time getting my MCP3204 working. It´s a 4 channel ADC with an SPI Bus.
      I connected the MCP like this:

      #define SPI1_CLK  18  // D -> Clk   -> Pin 11  
      #define SPI1_MISO 19  // Q -> D Out -> Pin 10     
      #define SPI1_MOSI 23  //   -> D In  -> Pin  9    
      #define SPI1_SS   16 //    -> CS    -> Pin  8   
      

      It´s working as expected as far as I don´t use any display stuff from the M5 Stack.

      As soon as I ad the following to my code:

       M5.begin(); // true, false, false);
       M5.Lcd.printf("TEST");
      

      things getting wired. The readings of my MCP3204 are wrong.

      I convert the AD values to temperature vales since I use some BBQ sensors. And the results are about 2 degrees wrong when adding the M5 Display.
      Example: Reading temp without Display -> ~21° and with display ~23° or sometime ~17°
      I can do some addition checks with the plain AD values to get a better understanding.

      But anyway ... What´s wrong in using the SPI bus for a second device?
      Are there any drawbacks i don´t know? Any issues?

      Maybe someone has some explanation for this topic.

      Dominik

      1 Reply Last reply Reply Quote 0
      • M
        moelski
        last edited by

        Hi Guys,
        I think I figured out the main problem.

        It has nothing todo with the SPI bus ... It´s the 3,3V line.
        0_1542533354547_IMG_20181118_102318.jpg

        Take a look at the picture. If you set the LCD to 80 (~33%) brightness then the PWM signal is extremly present on the 3,3v line ... And the MCP3204 don´t like that :-)

        And this is a picture with 255 (100% on) brightness ...
        0_1542533457784_IMG_20181118_102352.jpg

        No further explanation needed ...

        Dominik

        RopR 1 Reply Last reply Reply Quote 2
        • KabronK
          Kabron
          last edited by

          Thanks, it's useful.

          1 Reply Last reply Reply Quote 0
          • RopR
            Rop @moelski
            last edited by Rop

            @moelski Have you tried adding a small capacitor to ground?

            1 Reply Last reply Reply Quote 0
            • M
              moelski
              last edited by

              Hi Rob.
              Yes there are 100nF directly at the Power supply of the Mcp3204.
              But that won't help. I Ordered some lm4040 to have a clean voltage reference.
              I think that will help.

              Regards
              Dominik

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