How to use SPI with the stack ??

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

moelski

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

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

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 ...

No further explanation needed ...

Dominik

Kabron

Thanks, it's useful.

Rop

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

moelski

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