In PT100 + MAX31865 + SKR 1.4 Turbo + Marlin: how to get it all working, I got an RTD sensor working on my printer by sharing its SPI bus connection with the Trinamic motor drivers I’m using. What if you’re using “dumb” drivers like the DRV8825 or A4988, or you’re using some of Trinamic’s other drivers (like the TMC2209) that are configured over a UART connection? This post describes how to use the MicroSD-card SPI connection instead.

One disadvantage is that you can’t use a display that runs on the EXP1/EXP2 headers (such as the ReprapDiscount full-graphic display). (You could, but then you’d need to tap into the SPI bus going to the display’s SD-card slot, which is beyond the scope of this post.) I have one of Bigtreetech’s full-color touchscreens on the way; those plug into a designated connector that uses a UART connection. Until then (and probably after), I have an ESP-01S (running ESP3D) plugged into the 8-pin “WiFi” connector and can control the printer through that. The TFT35 went tango-uniform this past weekend. I pressed the ReprapDiscount display back into service, and it works just fine. I’m still using the MicroSD slot on the SKR instead of the display’s SD slot…not much of a difference.

Hardware Configuration

This is largely similar to my previous post, except that you’ll want to make up your cable with female connectors all around. Connections between the MAX31865 board and the SKR Pro 1.4 Turbo are as follows. The SPI connector is near the MicroSD slot; E1-CLS is the closed-loop motor connector for E1 from which we’re drawing 3.3V. The colored boxes overlaid on the board diagram show visually where everything plugs in.

MAX31865 pin	color	signal	SKR connection type	SKR connection pin
2	white	GND	female	SPI, pin 6
3	black	VDD	female	E1-CLS, pin 2
4	brown	SCK	female	SPI, pin 3
5	red	SDO	female	SPI, pin 1
6	orange	SDI	female	SPI, pin 4
7	yellow	CS	female	SPI, pin 5

Software Configuration

This varies a bit from the software-SPI config. First, the changes to Configuration.h in the thermal-settings section:

#define TEMP_SENSOR_0 -5
#define MAX6675_SS_PIN P0_26
#define MAX31865_SENSOR_OHMS_0 100
#define MAX31865_CALIBRATION_OHMS_0 430
#define MAX31865_USE_60HZ // omit if you're in a 50-Hz part of the world
#define MAX31865_USE_AUTO_MODE
#define MAX31865_USE_READ_ERROR_DETECTION

Make sure SDSUPPORT is enabled:

#define SDSUPPORT

Make sure you’re using the zeleps port of the Adafruit driver, and not the original…the original has lengthy delay loops that will slow your printer considerably. Go into ini/features.ini and look for this line:

TEMP_.+_IS_MAX31865 = Adafruit MAX31865 library@~1.1.0

Change it to this (all on one line):

TEMP_.+_IS_MAX31865 = AdafruitMAX31865Library=https://github.com/zeleps/Adafruit-MAX31865-V1.1.0-Mod-M/archive/master.zip

Next, if you’re using a display plugged into EXP1 and EXP2, you’ll need to disable it. Other features you may have enabled that depend on having a working display may need to be disabled as well. A UART-connected display can be left enabled.

#define REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER // if it's what you're using

In Configuration_adv.h, make sure the onboard MicroSD slot is selected instead of a display-connected SD slot:

#define SDCARD_CONNECTION ONBOARD

Due to a pullup on the SCK line, we need to use SPI mode 3 instead of mode 0. AFAIK, this doesn’t affect performance; it just changes signaling a little bit. Three changes need to be made to Marlin/src/HAL/LPC1768/HAL_SPI.cpp, which will be presented below as a patch…basically, there are three occurrences of SPI_MODE0 that need to be changed to SPI_MODE3:

diff --git a/Marlin/src/HAL/LPC1768/HAL_SPI.cpp b/Marlin/src/HAL/LPC1768/HAL_SPI.cpp
index 99db15f6e9..ee2bc1fa3d 100644
--- a/Marlin/src/HAL/LPC1768/HAL_SPI.cpp
+++ b/Marlin/src/HAL/LPC1768/HAL_SPI.cpp
@@ -123,7 +123,7 @@
       SPI.setModule(2);
     #endif
     SPI.setDataSize(DATA_SIZE_8BIT);
-    SPI.setDataMode(SPI_MODE0);
+    SPI.setDataMode(SPI_MODE3);
 
     SPI.setClock(SPISettings::spiRate2Clock(spiRate));
     SPI.begin();
@@ -187,7 +187,7 @@ SPIClass::SPIClass(uint8_t device) {
 
   #if BOARD_NR_SPI >= 1
     _settings[0].spi_d = LPC_SSP0;
-    _settings[0].dataMode = SPI_MODE0;
+    _settings[0].dataMode = SPI_MODE3;
     _settings[0].dataSize = DATA_SIZE_8BIT;
     _settings[0].clock = SPI_CLOCK_MAX;
     //_settings[0].clockDivider = determine_baud_rate(_settings[0].spi_d, _settings[0].clock);
@@ -195,7 +195,7 @@ SPIClass::SPIClass(uint8_t device) {
 
   #if BOARD_NR_SPI >= 2
     _settings[1].spi_d = LPC_SSP1;
-    _settings[1].dataMode = SPI_MODE0;
+    _settings[1].dataMode = SPI_MODE3;
     _settings[1].dataSize = DATA_SIZE_8BIT;
     _settings[1].clock = SPI_CLOCK_MAX;
     //_settings[1].clockDivider = determine_baud_rate(_settings[1].spi_d, _settings[1].clock);

If you haven’t previously fixed Adafruit_MAX31865.h (look for it under .pio/libdeps/LPC1769/), find this line near the top: Skip this step, as the zeleps driver includes this fix already:

#if (ARDUINO >= 100)

and change ARDUINO to ARDUINOLPC:

#if (ARDUINOLPC >= 100)

Notes

At first, I had noticed a bit of lag in the printer with the temperature sensor using a soft-SPI connection and thought I’d try switching to a hardware SPI port to see if it’ll lighten the load a bit. It didn’t. It was mainly noticeable in the display and when homing the printer. It didn’t seem to affect print speed or quality when printing from local storage, but streaming a print from something like OctoPrint or Pronterface stumbled pretty badly. What finally fixed this issue was the updated MAX31865 driver.

If you want to use the SD-card slot on the ReprapDiscount display, you might try using the SPI bus on the EXP2 connector instead. This might require driving the MAX31865 board with 5V instead of 3.3V to match the display’s 5V interface. The easiest way to tap into the signals would likely be a 2×5 female insulation-displacement connector on the EXP2 ribbon cable, into which pins can be stuck in the appropriate places. Set SDCARD_CONNECTION to LCD instead of ONBOARD if you go this route.

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 pin	color	signal	SKR connection type	SKR connection pin
2	white	GND	male	E1, pin 8
3	black	VDD	male	E1, pin 9
4	brown	SCK	male	E1, pin 3
5	red	SDO	male	E1, pin 5
6	orange	SDI	male	E1, pin 2
7	yellow	CS	female	PWRDET, 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
#define MAX31865_USE_60HZ // omit this if you're in a 50-Hz part of the world
#define MAX31865_USE_AUTO_MODE
#define MAX31865_USE_READ_ERROR_DETECTION

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.

Instead of the above, there’s an updated driver for the MAX31865 that also fixes some serious performance issues with the original. Go into ini/features.ini and look for this line:

TEMP_.+_IS_MAX31865 = Adafruit MAX31865 library@~1.1.0

Change it to this (all on one line):

TEMP_.+_IS_MAX31865 = AdafruitMAX31865Library=https://github.com/zeleps/Adafruit-MAX31865-V1.1.0-Mod-M/archive/master.zip

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).

Display lag and other issues were the result of lengthy delays hardcoded in the Adafruit driver. The zeleps fork of that driver appears so far to have fixed that problem. I currently have the MAX31865 board sharing hardware SPI with the onboard MicroSD slot, but sharing it with the display or with Trinamic drivers should work as well, whether with hardware or software SPI.

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.

Apparently 800 mA (the default setting in Marlin) was too high a drive current setting for the Y-axis motor in my AM8 (and the extruder motor, too, which was getting hot to the touch). Subsequent experimentation on the X and Z axes suggests that 200 mA should be sufficient for reliable operation without burning things up. I’ve also read that the extra-quiet mode (StealthChop) on the stepper drivers I’m using uses more power than the normal, not-as-quiet (but still not too bad) mode.

Fortunately, I have another printer at the ready to produce a replacement motor mount. It’s already sliced…just need to go home, load purple PETG into the Hypercube, and hit “start.”