Reading out SPI-Sensors via Arduino

Hi Everyone,

i have an Issue using my Arduino with SPI K-Type Sensors for Temperature logging. Before I start explaining my Problem, i want to make shure, that using the Arduino IDE is no option for me. I want to use the Temperature Data for building a controller using MATLAB.

The code I'm using here actually does work, but the Data i want to read comes back in a form of a 1x2 uint8 Array. Reading my Datasheet, I assume, that a [0 0] means 0°C and [255 255] means 1023.75°C. Testing the Sensor with warm Hands leads to one of the numbers increasing fast to 255, one going 1 up every 256 the other one is increasing. According to this, i made this loop giving out a "guesstimated Temperature".

I also read the Datasheet ( https://datasheets.maximintegrated.com/en/ds/MAX6675.pdf ) and found out, the MAX6675 has integrated cold junction temperature compensation, but change in the ambient temperature changes the readings significantly (by 10-15°C).

May there be a thing about reading and interpreting the output or is there an option to activate/deactivate ambient temp. compensation I didn't see? Or is this just a Problem i've got to get along with?

best regards

clear
clc
a = arduino('COM3','Uno','Libraries','SPI');
dev = device(a,'SPIChipSelectPin','D10');

dataIn = [0 0];

dur=40; %Sampling Time
SRate=8; %Sampling Rate
t=[0:1/SRate:dur];
r2=zeros(1,SRate*(dur)+1);

for i=1:1:SRate*dur+1
rout = writeRead(dev,dataIn,'uint8');    %Read MAX6675
r0=255*double(rout(1))+double(rout(2));  %Make a Temperature-proportional reading out of whatever I get back from MAX6675
r1(i)=r0/24;
pause(1/SRate);
plot(t,r1)
end

[Uwe Weltin]

be sure to 'shift out' the first three bits. They have nothing to do with the temperature. (Error Codes) See data sheet of the MAX6675

Answer 1

[Uwe Weltin]

% Ktype_Max6675_Due.m

% Ktype Temperatur mit Arduino Due messen

clear all, close all

ComPort = getAvailableComPort

a = arduino(ComPort{1},'Due','Libraries','SPI')

% DUE Interface: ...,'Libraries','SPI')

% Supports hardware SPI: 6 extra Pins next to CPU

% MISO 'D74' Default SDIPin: 'SPI-1' SPI-Port

% 5V: 'SPI-2' |5 3 1|

% SCK 'D76' Default SCLPin: 'SPI-3' |6 4 2|

% MOSI 'D75' Default SDOPin: 'SPI-4'

% RESET: 'SPI-5'

% GND: 'SPI-6'

% CS: 'D10'

MAX6675 = device(a,'SPIChipSelectPin','D10')

tmax= 30;

fs = 1;

dt = 1/fs;

t = [dt:dt:tmax];

il = length(t);

T = zeros(il,1);

dummy = [uint8(0),uint8(0)];

for i=1:il

datOut = writeRead(MAX6675,dummy);

pause(0.5)

BINR = [dec2bin(datOut(1),8), dec2bin(datOut(2),8)];

disp(BINR)

DECR = bin2dec(BINR);

DECT = bitshift(DECR,-3);

BINT = dec2bin(DECT);

MP = 255/1023.75; % Wird 0 ausgelesen hats 0°C,

T(i,1) = MP*DECT; % wird 255 ausgelesen hats 1023.75°C

disp(T(i,:))

end

figure(1)

plot(t,T(:,1),'r')

xlabel('Zeit [s]')

ylabel('Temperatur [°C]')

legend('KTYP')

title('Temperaturmessungen mit DUE,KTYP')