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Author Topic: Thermocouple + MAX31855 + Arduino--SPI wiring Problems  (Read 1994 times)
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Hi, I have a S Thermocouple and a MAX31855SASA+ (two samples from maxim IC) and a K Thermocouple with two samples of MAX31855KASA+. I want to use one of both, but I can´t......I soldered the two MAX31855SASA+ in a SOIC to DIP adapter to use it better in the breadboard. (I´m using an arduino mega 1280).I wired it following Ryan McLaughlin scheme and his library:
https://www.dropbox.com/s/fvfkbnosr07h601/MAX31855.png

And this is my breadboard with the connections:
https://www.dropbox.com/s/az8gr961pzv8or0/2013-03-10%2015.37.03.jpg

If you can´t appreciatte it:

MAX31855---ARDUINO MEGA 1280
SO pin------->MISO (pin 50)
SCK---------->SCK (pin 52)
CS----------->SS (pin 53)
Vcc---------->3V3
GND--------->GND

But I haven´t any result.....This is what I get in the monitor: (if I apply fire from a lighter in the sensor, It didn´t do anything):

https://www.dropbox.com/s/o99ko40io80uplr/Monitor.jpg

And I don´t know if I fried one of the MAX31855SASA+ smiley-roll-blue ...... Anyone can suggest me how to wire it???? Thank you!
« Last Edit: March 10, 2013, 09:54:39 am by peko86 » Logged

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Each SPI device has to have a separate Slave Select signal.  Only one can use the hardware default SS pin (53).  For the other you have to choose another pin.
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Spain
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At first thank you for your fast answer johnwasser...i tried to change cs pin to digital pin 2 but i didn't get response too. the vcc pin is connected to 3,3v..could be the problem that spi outputs need 5v instead 3v3??? Should I amplify outputs of the max31855??
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3.3V logic outputs will generally work with 5V logic inputs.  You should read the 31185 datasheet to see if the inputs are 5V tolerant.  If not you should use voltage dividers to make the 5V signals safe for your 3.3V chip.

Pin 2 should work fine as a Slave Select pin. I suspect bad wiring or a problem in the software.
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Thank you again for your fast answer johnwaser, as you can see in the next capture of the dataseet, it isn´t tolerant with no more than +4V in supply pin and in others a maximum of 4,3V:

https://www.dropbox.com/s/qgqrmj3tfx0wktd/maximum%20ratings.jpg

Your idea is using voltage dividers in the CS and SCK spi outputs of MAX31855 and not in SO? or how? Thank you! smiley
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Hello,

I think that you have to change the voltage from 3.3 V to 5 Volt. If you look at the break-out board of ADAFRUIT http://www.adafruit.com/products/269 than you can you see that they converted the voltage!

Kind regards
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I would make sure that you  have a good solder joint in the SOIC to DIP. use a straight pin contact to the top of the pins of the SOIC  and the regular lead to the DIP. so that you are seeing if the continuity is there. I found an open this way that was pretty elusive.
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Thank you for your replies:

Hello,

I think that you have to change the voltage from 3.3 V to 5 Volt. If you look at the break-out board of ADAFRUIT http://www.adafruit.com/products/269 than you can you see that they converted the voltage!

Kind regards

Instead of using a level shifter chip, I used voltage dividers on CS and SCK pins to put into max31855KASA+ 3.3V and not 5V....But MISO is directly connected to the arduino MEGA Digital pin 50 without any level shifting, because as I read, Arduino recognises 3,3V as a logic 1. Do you think  I will make this output 5V instead 3v3?


I would make sure that you  have a good solder joint in the SOIC to DIP. use a straight pin contact to the top of the pins of the SOIC  and the regular lead to the DIP. so that you are seeing if the continuity is there. I found an open this way that was pretty elusive.

Always I solder any chip I see If the solder joints are made well with my multimeter, for not having problems like this...Thank you for your suggestion, this problem seems similar to this, but it isn´t the case smiley-sad
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Santa Fe
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The reason I said this is because I checked all the connections and they looked good, but the damn thing still didn't work. What happened is that I put just enough pressure on the open pun, to have it make contact with the solder beneath it. When I took off the probe, the pin opened again.

I probably would still be scratching my head except I saw the Arduino readout change if I pressed on the chip.

I had no problem with the 3.3 out to the Arduino. but I fried a chip by allowing 5 volts in, so make sure you use a voltage divider or level shifter.

Even after I got things running I still had problems with seeing large swings in output that got worse as the temperature rose. I never did track it down. I went back to using a AD595 that had no stability problem.

 
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Thank you for your reply, I will try with new  wires soldering it...Maybe my chip could be fried??? How can I check it?
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You should not have done any harm  unless you connected the MAX chip directly to the 5 volt outputs of the Arduino.  What voltage are you reading across the MISO connection? Do you ever get any other reading than the base reading?

« Last Edit: March 28, 2013, 07:54:15 pm by RPCoyle » Logged

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