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I am beginner in Electronics.

While working with MAX31865 and MAX31855 Modules it was working perfectly, but after designing custom PCB based on modules its not working.

RTD shows -242C and 982C randomly and Thermocouple shows -100C, is there any problem with schematic ?

Tried Removing series Voltage level shifter but same result

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Jay Vadoliya
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    Is there any reason why you’re using diodes and pull-ups for SPI? CS and MOSI are outputs from the micro and MISO should tri-state from any device that has CS inactive. The diodes may not allow the signals to pull low enough, try linking across them and maybe remove the pull-ups, which should be unnecessary. – Frog Apr 15 '22 at 06:12
  • SDI and SDO corresponding to MOSI and MISO in your schematic – Frog Apr 15 '22 at 06:16
  • @Frog I tried removing all diodes in SPI lines and sort them, and open Pull up resistor but ended with same result also SDI and SDO is taken as MOSI & MISO respectively – Jay Vadoliya Apr 15 '22 at 06:25
  • Do you have access to an oscilloscope to check the signals? – Frog Apr 15 '22 at 07:23
  • I am purchasing logic analyzer for this but still before that if anyone can find problem in schematic – Jay Vadoliya Apr 15 '22 at 08:11

1 Answers1

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I had a similar issue. As @Frog said in the comments, you would need a oscilloscope to spot the exact issue.

The datasheet does not mention it, but you need a pull-down resistor on MISO (SDO) line. These MAX ICs's SDO line does not pull itself down. It only pulls the line UP, for some reason.

You would need a oscilloscope to detect this issue, not a logic analyzer. You would see the MISO line go like this:

SDO.png

I used a resistor of 560 Ohms to pull the SDO line down and worked like charm.

A second thing to take into account, is the Capacitance of the Diodes you are using. if its too much, it will delay the signal rise up times. A workaround to that (if you don't have other diode types available) is to lower the frequency of the SPI protocol so the voltage will have more time to rise.

Christianidis Vasileios
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