PT100 + MAX31865 + SKR 1.4 Turbo + Marlin: how to get it all working

I wanted to upgrade one of my 3D printers to make it capable of printing with high-temperature materials. One of the things needed to pull that off is to replace the usual hotend thermistor (which maxes out around 280°C) with either a thermocouple (which can go into the thousands of degrees) or a platinum RTD (which, as packaged for printers, can usually go to 500° or so). Either of these requires additional electronics to make the device compatible; for a PT100 RTD, the device in question is a little board with a MAX31865 on it, which converts the small change in resistance of the RTD to a digital readout over SPI.

There is a guide to getting all this stuff working, but it seems unnecessarily involved while not providing all of your options. What follows is a simpler explanation that will get you up and running.

(I assume that you’re running the bugfix-2.0.x branch of Marlin, and that your printer has a single hotend with a thermistor that you want to replace with an RTD. For my upgrade, I also replaced the aluminum heater block with a nickel-plated copper block. I also have a 60W heater on order, as the 40W heater I’ve been using is having a hard time maintaining temperature with the blower running. Further, I assume that you’re using Trinamic 2130 or 5160 drivers and that you’re using SPI to configure them…we’re going to share that SPI connection. We’ll tap into most of the SPI signals at the E1 connector. If you have a second extruder, you might want to build something like this adapter.)

Hardware Configuration

When the parts arrive, you’ll most likely need to solder connectors onto the MAX31865 board. They should be in the bag with the board; assembly is self-explanatory. There are also a couple of jumper pads that need to be soldered together to put the chip in 2-pin mode.

You’ll also want to make a cable to connect to the SKR 1.4 Turbo. It should have a six-pin female Dupont connector on the MAX31865 end. The SKR end should have six one-pin Dupont connectors: one female (for the CS signal, along one edge of the cable if you’re using ribbon cable) and the rest male. Let’s say you’re using the same color coding that I used, based on the cables I had laying around:

MAX31865 pincolorsignalSKR connection typeSKR connection pin
2whiteGNDmaleE1, pin 8
3blackVDDmaleE1, pin 9
4brownSCKmaleE1, pin 3
5redSDOmaleE1, pin 5
6orangeSDImaleE1, pin 2
7yellowCSfemalePWRDET, pin 3

Pins 1 (5V) and 8 (RDY) on the MAX31865 aren’t used, as it and the printer motherboard are both 3.3V devices. A color-coded map of connections to the SKR 1.4 Turbo follows:

The RTD should be connected to the middle two terminals on the side opposite the SPI connector. Mine came with a 2-pin Molex connector, so I just knocked together a little adapter with a Dupont connector and some wire. Take note that the 6-pin connector is centered on the board’s 8-pin connector.

Software Configuration

With the hardware sorted, we now turn to configuring Marlin. Most of the changes can go in the thermal-settings section of Configuration.h.

#define TEMP_SENSOR_0 -5 
#define MAX31865_MOSI_PIN TMC_SW_MOSI 
#define MAX31865_MISO_PIN TMC_SW_MISO
#define MAX31865_SCK_PIN TMC_SW_SCK
#define MAX6675_SS_PIN POWER_LOSS_PIN // MAX31865_CS_PIN is copied from this
#define MAX31865_SENSOR_OHMS_0 100
#define MAX31865_CALIBRATION_OHMS_0 430

Trinamic drivers on the SKR 1.4 Turbo use software SPI, so that’s what we end up selecting here. MOSI (SDI), MISO (SDO), and SCK pins are set to the same ones the Trinamic drivers use, while the CS pin is set to a pin that we’re not using so we can still install a second extruder in the future. Any pin that doesn’t have a pullup or pulldown is OK…I’m using the pin on the PWRDET connector.

To use hardware SPI, the MAX31865_*_PIN entries don’t need to be defined. Just define MAX6675_SS_PIN, which is copied internally to MAX31865_CS_PIN. From what I’ve read, if there are multiple hardware SPI buses, the driver will default to using one of them and there’s no way to choose the other. I haven’t had a chance to verify this behavior.

If you’re using a PT1000 instead of a PT100, the MAX31865_*_OHMS values both need to be multiplied by 10 and you need to replace the Rref resistor on the MAX31865 board with a 4.3kΩ part (stock is a 430Ω 0.1% 0805 SMD resistor).

There’s also a change needed in Adafruit_MAX31865.h (in my source tree, it’s in .pio/libdeps/LPC1769/Adafruit MAX31865 library). Line 49 (or thereabouts) looks like this:

#if (ARDUINO >= 100)

ARDUINO isn’t defined by the LPC176x Arduino framework; instead, it needs to look at ARDUINOLPC:

#if (ARDUINOLPC >= 100)

Without this change, you’ll get compile errors saying that WProgram.h can’t be found.

With these changes made, compile, write the firmware.bin file that’s produced to a MicroSD card, pop it into the slot on the SKR, and hit Reset (or power it up).

Other Notes

I’ve noticed that the Marlin UI (on one of the cheap 128×64 graphical LCDs) is a bit laggy, but the printer is otherwise running properly. Switching from the software SPI used for Trinamic drivers to one of the hardware SPI channels might fix that. If I’m not mistaken, there are two: one is brought out on the EXP2 connector for cheap 128×64 graphical LCDs like mine (to go to the SD-card slot on such devices), and another goes to the onboard MicroSD slot and is also available on an SPI header adjacent to that slot. Changing MAX31865_MOSI_PIN, MAX31865_MISO_PIN, and MAX31865_SCK_PIN to use those other channels ought to do the trick (and leave out MAX31865_USES_SW_SPI if you do that).

I’ve also given some thought to trying ReprapFirmware, now that you don’t need to fork over the big bucks for a Duet to run it. How this setup would cooperate with that, I don’t know.

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