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Using Electronic Modules with Jim Rowe MicroMag3 3-axis Magnetic Sensor The MicroMag3 can measure the strength of a magnetic field in three orthogonal axes (eg, North-South, East-West and Up-Down). In effect, it combines the functions of a magnetic compass and an inclinometer. T he MicroMag3 can measure magnetic fields over a wide range of strengths with high resolution and operates from 3V DC, drawing less than 0.5µA of current. It has SPI (serial peripheral interface), so it can communicate with just about any microcontroller. As you can see from the photos, this module is quite small, measuring only 25.4 × 25.4 × 19mm, with the last dimension including both the Z-axis sensor mounted vertically on the top of the PCB and the two 7-pin headers under the sides of the PCB. Manufactured by US firm PNI Sensor Corporation based in Santa Rosa, California, it uses a patented technology called Magneto-Inductive Sensing. The module is specified as being able to measure magnetic fields over a wide range from -1100µT to +1100µT. 1T = 1 tesla = 10,000 gauss = 10,000G. So 1100µT = 1.1mT or 11.0G. The measurement resolution is specified as 0.015µT or 0.00015mG. The MicroMag3 and later versions using the same technology have found their way into a significant number of navigation devices for automotive, marine, aeronautical and even space vehicles. Before we delve deeper into how the MicroMag3 works and how it can be used, we should mention its availability. We bought a couple of the modules from Altronics, which, at the time of writing, has them available (Cat Z6300) for $5.90 each, plus delivery costs. It looks as if Altronics obtained them from the US firm SparkFun Electronics, but when you go to their website (www.sparkfun.com), they advise that the product has been ‘retired’ from their catalog and is no longer for sale. Then, if you go to the PNI Sensor Corporation’s website (www.pnicorp. com), they have dropped all references to the MicroMag3 and only provide data on later versions. You can still find the data sheet for the MicroMag3 on the SparkFun website if you go to www.sparkfun.com/ products/retired/244 So Altronics is the only current supplier of the MicroMag3 that we could find, suggesting that if you want to get hold of one, you may have to be quick! How it works 64 Silicon Chip Fig.1: the MicroMag3 sensor module uses a PNI 11096 ASIC (application-specific integrated circuit). The upper right-corner of the diagram shows how the sensors are orientated. Looking at the photos, you will see a single IC on the PCB, in a compact 28-pin SMD package. It is labelled PNI 11096 and is described in their data sheet as an ‘ASIC’ or application-­ specific IC. Apart from some SMD resistors and capacitors, the only other components on the PCB are the three tiny magneto-­ inductive sensors. Labelled MS1, MS2 and MS3, these each measure only 6.0 × 2.1 × 2.21mm. They are used to sense and measure the magnetic field in one of the three axes. Australia's electronics magazine siliconchip.com.au Fig.1 shows the circuit for the MicroMag3 module, with the PNI 11096 ASIC in the centre and the three magneto-inductive sensors to its right – each with a pair of biasing resistors. Along the bottom are the pins of the 7-pin header provided to allow control by and communication with an MCU (microcontroller unit). The first three pins (SCLK [serial clock], MISO [master-in, slave-out] and MOSI [master-out, slave-in]) are the SPI interface, while the other pins control the ASIC. Up the top are the pins of the second 7-pin header, with only two used to supply the ASIC with 3V DC power. At upper right in Fig.1 is a small diagram showing the way the three sensors are configured to measure the three magnetic axes. The MS1 sensor measures the field in the X or North-South axis, MS2 measures the field in the Y or East-West axis, while MS3 measures the field in the Z or up-down axis. According to the PNI data, the sensors are arranged in a south-west-down or ‘SWD’ configuration. We’ll explain the significance of that later on. Before we look at how the three magneto-inductive sensors measure surrounding magnetic fields, here’s a rundown of the basic measurement procedure, shown graphically in Fig.2. Bear in mind that the ASIC can only measure via one sensor at a time. First, the controlling device drops the voltage on the ASIC’s SS (slave select) pin to indicate that a measurement is to start, then it sends a short positive pulse (>100ns) to the RESET pin. After that, it sends an 8-bit command via the MOSI pin, specifying the sensor to be used (MS1, MS2 or MS3) and the measurement period. The measurement period specifies how many oscillator cycles should be used for the measurement, with choices ranging from 32 to 4096 cycles. The measurement resolution increases with the number of cycles, but 2048 cycles is usually sufficient. The next step involves the MCU either waiting for the ASIC to pull up the voltage on the DRDY (data ready) pin, or just waiting long enough for the ASIC to have made the measurement and have the data available. In either case, the MCU must then send out 16 clock pulses on the SCLK line, to receive the 16-bit measurement data via the MISO line. Finally, the MCU raises the voltage on the SS pin, to signal the end of that measurement sequence. Making measurements of the field intensity in all three axes requires three of these sequences to be completed, one for each axis. Magnetic sensing Now let’s look at how the magneto-­ inductive sensors are used to make the measurements. Each sensor consists of a solenoid coil wrapped around a very high-permeability magnetic core. As shown in Fig.1, each sensor coil has four connections to the ASIC. So the MS1 coil has direct connections to the APXIN and ANXIN pins, plus connections to the APXDRV and ANXDRV pins via the two 62W series resistors. The other two sensor coils are connected to the corresponding pins for the Y and Z axes. Inside the ASIC, each sensor’s coil is used in a simple L-R relaxation oscillator, with its inductance determining the oscillation frequency. As its inductance varies according to the The MicroMag3 module shown at twice actual size. magnetic field passing through its core, the external field can be measured by alternately driving a direct ‘bias’ current from one end of the coil to the other and then back the other way. When there is no external magnetic field, the sensor coil’s inductance will be identical when the bias current flows in either direction because the inductance will be swinging symmetrically on either side of the core’s ‘zero field’ peak. As a result, the oscillator frequency will be the same in both directions. But when there is an external magnetic field, the inductance and frequency will differ depending on the direction of bias current flow. This allows the PNI 11096 chip to measure the strength of the external field by measuring the time taken to complete a fixed number of oscillations in the ‘forward biased’ and ‘reverse biased’ directions, and taking the difference between the two. That is the principle of PNI’s magneto-­ i nductive sensing technology. Fig.2: the microcontroller sends a command byte on the SPI bus, then waits a certain period before reading back 16 bits of measurement data. siliconchip.com.au Australia's electronics magazine June 2024  65 If that explanation isn’t clear enough, there is a PNI ‘white paper’ called Magneto-Inductive Technology Overview, written by Andrew Leuzinger and Andrew Taylor, which you can download as a PDF file from several sources on the web. I found it at siliconchip.au/link/abs5 Connecting it to an Arduino Fig.3 shows how easily the module can be connected to an Arduino Uno. It should be just as straightforward to connect it to any other versions of the Arduino, including the new Uno R4 Minima, or many other microcontrollers such as the Micromite or Maximite. All you need to do is connect the module’s VDD and GND pins to the +3.3V and GND pins of the MCU, then connect its SCK, MISO and MOSI pins to IO13, IO12 and IO11 of the MCU. Those are the pins that the Arduino’s SPI library expects you to use for SPI communication. The only other connections required are those for the module’s SS, DRDY and RESET lines, which, as shown in Fig.3, connect to pins IO7, IO6 and IO5, respectively. Note, however, that if you use our sketch to control and communicate with the MicroMag3, you don’t need to connect the module’s DRDY pin to the Arduino’s IO6 pin. We found it easier to rely on a time delay before requesting the measurement data, as should become clear shortly. Software We need a sketch to use the MicroMag3 module with an Arduino, so I looked around the web to see if suitable sketches had already been written. I found a couple, but they both used a ‘bit-banging’ approach, rather than using the Arduino SPI library and the microcontroller’s built-in SPI peripheral. That seemed a bit clumsy, so I decided to see if I could come up with a more elegant solution. Producing a working sketch turned out to be more work than I anticipated. The main hurdle I encountered was in trying to use the module’s DRDY pin to sense when the module had measurement data available. That is the approach recommended in PNI’s data sheet, by the way. After many puzzling ‘hung sketch’ results, I tried analysing the module’s operation with a DSO. I discovered that the module’s DRDY did go high after each measurement, but only after about 36ms (milliseconds). That seemed to be too long of a wait. After discussing this with my Silicon Chip colleagues, Nicholas Vinen and Tim Blythman, we concluded that it would be better to forget about using the DRDY line and simply wait a little longer than the expected processing time before requesting the measurement data. Suddenly, the sketch sprang to life! The sketch then printed the field measurements for the three axes via the Arduino IDE’s Serial Monitor. Encouraged by this success, I added a section to work out the module’s ‘compass heading’ from the X-axis and Y-axis readings. It was clear that I would need an arctangent function to work out the compass heading from the X-axis and Y-axis readings, yet there seemed to be no such function listed in the Arduino Language Reference. Thinking I might have to include a special ‘maths’ library to get one, I went onto the Arduino forum to find the answer. I discovered that you didn’t need a special library because the existing library includes two such functions, even though they were not listed or even referred to in the Language Reference. The functions are atan() and atan2(), with the second able to work out angles in all four quadrants. Editor’s note – those are standard C library functions from the “math.h” include file, which might explain why they are available but not listed in the Arduino documentation. Arduino is built on C++, which is built on C, so you can access those underlying functions if necessary. Fig.3: how to connect the MicroMag3 module to an Arduino Uno or similar microcontroller. Note that if you’re using our example sketch, then the DRDY pin does not need to be connected. Fig.4: this is the orientation provided by our demo sketch. It could be changed with some extra calculations if required. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au Once I understood that, it wasn’t too long before I could get the compass heading part of the sketch working. There was just one minor complication: the MicroMag3 module’s X axis is aligned with the white line with its arrowhead at upper right on the module PCB, which suggests that the sketch should read ‘true north’ when the module is facing north when pointed in the direction of the arrow. However, I could only get the heading function to work correctly once I reversed the module orientation so that the end of the PCB nearest the MS1 sensor and the ASIC is used as the ‘compass pointing’ end. I suspect this is because of the way the MicroMag3 is set up with the “SWD” scheme (south-west-down). It would be possible to fix that by adding 180° to the output of the arc­ tangent function, modulus 360. Still, I thought it was simple enough to use the module’s rear as the compass pointing end, as shown in Fig.4. Doing that produces the expected bearings without any additional mathematical steps. The resulting sketch file is named Screen 1: example sketch output 14:20:27.626 -> A sketch to communicate with the MicroMag3 14:20:29.781 -> X reading = 1112 14:20:29.781 -> Y reading = 17 14:20:29.781 -> Z reading = -1375 14:20:29.828 -> Heading = -0.02 radians or 0 in degrees 14:20:29.875 -> 14:20:40.045 -> X reading = 971 14:20:40.091 -> Y reading = -602 14:20:40.091 -> Z reading = -1472 14:20:40.138 -> Heading = 0.55 radians or 31 in degrees 14:20:40.138 -> 14:20:50.355 -> X reading = 826 14:20:50.355 -> Y reading = -764 14:20:50.401 -> Z reading = -1519 14:20:50.401 -> Heading = 0.76 radians or 42 in degrees 14:20:50.448 -> 14:21:05.820 -> X reading = 119 14:21:05.820 -> Y reading = 759 14:21:05.820 -> Z reading = -1576 14:21:05.867 -> Heading = -1.42 radians or -81 in degrees 14:21:05.913 -> “Silicon_Chip_MicroMag3_control_ sketch_V2.ino” and you can download it from siliconchip.au/Shop/6/330 Screen 1 shows a screen grab of the Serial Monitor listing when running my sketch, and first pointing north, then towards north-east (+31°, and +42°), and then towards the west (-81°). The sketch does not provide any readout of the magnetic field’s inclination, just the Z-axis reading. I will leave doing that as an exercise for our SC readers. PIC Programming Adaptor Our kit includes everything required to build the Programming Adaptor, including the Raspberry Pi Pico. The parts for the optional USB power supply are not included. Use the Adaptor with an in-circuit programmer such as the Microchip PICkit or Snap to directly program DIP microcontrollers. Supports most newer 8-bit PICs and most 16-bit & 32-bit PICs with 8-40 pins. Tested PICs include: 16F15213/4, 16F15323, 16F18146, 16F18857, 16F18877, 16(L)F1455, 16F1459, 16F1709, dsPIC33FJ256GP802, PIC24FJ256GA702, PIC32MX170F256B and PIC32MX270F256B Learn how to build it from the article in the September 2023 issue of Silicon Chip (siliconchip.au/Article/15943). And see our article in the October 2023 issue about different TFQP adaptors that can be used with the Programmer (siliconchip.au/Article/15977). Complete kit available from $55 + postage siliconchip.com.au/Shop/20/6774 – Catalog SC6774 siliconchip.com.au Australia's electronics magazine June 2024  67