Your prototype already has a compass or magnetic sensor. You want to try a different module, and the replacement has the same four connections. It is tempting to plug it in, change a library name and get back to the product. Then the program stops, the numbers change scale, or the display keeps showing the same value.
For a small team building an interactive object, an educational robot or a handheld prototype, the useful question is whether the replacement can supply the information the application expects. A matching connector is only the beginning. The software also needs the right device address, a compatible driver, meaningful units and a way to get fresh measurements.
On our bench we ran seven GraftSense magnetometer modules one at a time on a GraftPort-RP2040 controller board under MicroPython: HSCDTD008A, RM3100, MMC5983, LIS3MDL, LIS2MDL, MMC5603 and BMM150. These were separate bring-up tests, not a comparison that picks the best sensor. What they show is where a swap tends to go wrong.
Keep the working setup small while you change the sensor
Before reconnecting the replacement, record the current module, wiring, driver version and one short output log. With power off, check the new carrier's supply and logic requirements, connector order and address configuration. Similar-looking carrier boards can hide different chips and circuitry.
Our tests used GP4 for SDA and GP5 for SCL on a 400 kHz I2C bus, the two-wire link carrying commands and readings between controller and sensor. Start with the replacement connected on its own, running its matching example program. That keeps problems in the new sensor path separate from changes in your display, robot or application code.
The same migration problem shows up in an Arduino Forum discussion from August 2022: a builder replacing an HMC5883L library with a QMC5883L library ran into missing methods after changing the address and library.
A bus scan answers one question, not all of them
An address in a scan means something answered there. It does not identify the chip or tell the driver how to operate it. After scanning, use the chip identity check where the device provides one, and make sure the driver actually receives the detected address.
Our LIS3MDL program illustrates the last point. It scanned for either 0x1c or 0x1e, but then created the driver without passing the address it found. The driver defaults to 0x1c, which matched our board. If the carrier is configured for 0x1e, finding it in the scan will not update that constructor. Pass the address explicitly.
The MMC5983 and MMC5603 programs both use 0x30, but the two chips have different identity values and data handling. Sharing an address does not make them interchangeable, and putting both on one bus would need the address conflict resolved separately.
Our MMC5603 record also has a gap worth admitting. The terminal capture saved with that test is headed MLX90393 and reports 0x0c, while the MMC5603 program expects 0x30. So that capture cannot confirm that the MMC5603 program produced those readings. For your own test, include the detected address and chip identity in the saved log; a photo of the connected module next to that log is far more useful later than three numbers on a screen.
Decide what the application expects the numbers to mean
An application may need a magnetic-field vector, an angle or just a repeatable response to movement. These are different outputs. A magnetometer's three axes describe the field along three directions; they do not automatically form a verified compass heading.
The HSCDTD008A program ran self-test and measurement sequences in forced and continuous operation. Its reading function returns signed register counts, so the printed integers and the calculated vector magnitude are counts unless a valid conversion is applied.
The RM3100 program also reported counts. It scanned the allowed address range, passed the selected address to the driver, ran axis self-tests and showed single and continuous measurements. The result showed address 0x22 with a cycle count of 200 on each axis. That setting belongs with the readings: the manufacturer relates gain to cycle count, so changing it changes how the raw values should be read.
Do not compare the size of those integers with another module's microtesla values. Microtesla (µT) is a physical unit of magnetic flux density; counts are the sensor's digital output before interpretation. Larger numbers are not evidence of better sensitivity or accuracy.
A unit label in a print statement needs support from the driver. In the BMM150 driver package we used, the measurements property extracts and shifts raw register fields but does not apply the manufacturer's trim compensation. Bosch's datasheet describes that compensation, and its BMM150 Sensor API is a reference implementation. Our printed µT label is not enough to accept the output as a calibrated field. The same run requested a 30 Hz rate and read back an 8 Hz label, another inconsistency to resolve before relying on mode settings.
The MMC5983 driver has a scale issue. It divides the centered 18-bit reading by 131072 before converting to µT, while the MMC5983MA datasheet gives 16384 counts per gauss in 18-bit operation. Its low-bit extraction also needs review. Fix the conversion before using those values for a threshold or comparing them with another instrument.
You do not need to audit every driver function before moving on. For the swap, trace the path your application uses: raw bytes, signed values, scale or compensation, axis order and final output. Write the agreement down in plain terms, such as "three axes in µT, in the marked board directions, with invalid readings reported separately."
Check that a new read produces a new measurement
Once the values make sense, check how they arrive. In continuous mode the sensor measures repeatedly. In one-shot mode, software generally has to request a measurement and wait for it to complete before reading the result. Calling a read function repeatedly does not necessarily start that process.
Our MMC5983 run printed changing values in continuous mode, then switched to one-shot and printed the same three values over and over. The driver's read path does not trigger a fresh measurement for each one-shot read. That run is a good illustration of stale data, not a demonstration of repeated one-shot sampling.
The LIS2MDL run shows why console speed and sensor speed are different things. It changed the configured output data rate after groups of readings while pausing half a second between printouts. A displayed setting of 100 Hz describes the sensor configuration, not 100 recorded samples per second.
For a moving interaction, test freshness directly: change the module's orientation while recording a timestamp and the sensor's data-ready or completion indication where available. Then test at the intended application rate, and define what happens if the next sample never arrives, so a missing update does not leave the rest of the prototype waiting. We have not run these checks on these modules.
Bring the replacement back into the product one step at a time
First save a short standalone run with the exact board, address, driver version, units and operating mode. Then connect the readings to one application behavior. For an educational robot, that might mean displaying the interpreted direction before letting it steer a turn. For an interactive object, it might mean logging the gesture response before enabling sound or motion.
Next, check axis orientation after mounting the board and repeat the relevant test with the other electronics running. If the application needs a compass heading, calibration, tilt handling and the magnetic environment each need their own validation. Successful communication and plausible field values do not establish heading accuracy inside a finished enclosure.
The seven tests divide the work into a clear order: establish which device is connected, confirm what its driver returns, verify when values update, then test the behavior your user needs. That is a more useful handover to an engineer than asking whether two modules are simply "compatible."