How to use SPI with the stack ??



  • 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



  • 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



  • Thanks, it's useful.



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



  • 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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