I could not resist to buy a Raspberry Pi Pico when I visited my local electronic supplier. It costed less than 10 €, but at that time I did not really know what to do with it. I tested the analog input and it seemed too noisy for battery condition measurements. Another project involved a electronic compass together with a radar, and there was problems with the calibration. I thought I could build a simulator for that but then I found the I2C magnetometer- accelometer which I had used for other purposes. Pin connections: LM303D RPi Pico Vin . Pin 36 GND Pin 38 SDA Pin 1 (I2C0) SCL Pin 2 (I2C0) With the help of some I2C tutorials for RPi Pico I found out how to access the registers and the math I had already working in RPi 3 and RPi Zero. The tricky part for me was to handle byte data and Hex formatting. After solving that I was ready to test the system in OpenCPN, which runs on a RPI 4. After adding the necessary instrument (Settings-Plugins-Dashboard) and adding the serial USB port (/dev//ttyACM0) and adjusted Baudrate (115200) it started immediately to show the numeric value of True heading. It seems that OpenCPN corrects the deviation automatically.

The break-out board should be placed text upwards and the X arrow pointing towards the nose.

The dashboard shows now the GPS .position GPS compass, True Magnetic Heading,  Pitch and Heel.

The NMEA0183 sentence uses the keywoard "ROLL", but OpenCPN dispays the title "Heel".

The RPi Pico also has a 3.3 V logic level serial port, which can be configured to desired baudrate and amplified to RS232 levels, if the system is used with plotters that have serial NMEA0183 inputs.

## Erik Nordenswan, Finland, (the country that won Russia in icehockey in the Olympic Games Final 20.2.2022)
## 22.2.2022
## Micropython script for Pololu break-out board with LM303 Accelerometer and Compass
## Compass is tilt-compensated
## Copy this script to Raspberry Pi Pico with the name main.py
## RPi Pico starts sending every second a NMEA0183 sentence with the magnetic heading with 115200 baud
## The receiving USB port on a Raspberry Pi will be ttyACM0
## The on-board LED blinks when the script is running
## You can stop the script with Thonny editor (first start Thonny, then power-on, then Thonny stop icon)
## Thanks to several programmers, who have published their code for LM303 on the Internet


import machine
sdaPIN = machine.Pin(0)
sclPIN = machine.Pin(1)
i2c=machine.I2C(0,sda=sdaPIN, scl=sclPIN, freq = 400000)
from utime import sleep
import math
from machine import Pin

led = Pin(25, Pin.OUT)
devices = i2c.scan()


def twocc(h,l):  #two:s complement combine
    twocc = 256*h + l 
    if twocc >= 32768:
        return twocc - 65536
    else:
        return twocc
    


i2c.writeto_mem(0x1D,0x20,b'\x57')
i2c.writeto_mem(0x1D,0x21,b'\x00')
i2c.writeto_mem(0x1D,0x24,b'\x64')
i2c.writeto_mem(0x1D,0x25,b'\x20')
i2c.writeto_mem(0x1D,0x26,b'\x00')
sleep(1)

while 1:
    mxl = i2c.readfrom_mem(0x1D,0x08,1)
    mxh = i2c.readfrom_mem(0x1D,0x09,1)
    myl = i2c.readfrom_mem(0x1D,0x0A,1)
    myh = i2c.readfrom_mem(0x1D,0x0B,1)
    mzl = i2c.readfrom_mem(0x1D,0x0C,1)
    mzh = i2c.readfrom_mem(0x1D,0x0D,1)
    axl = i2c.readfrom_mem(0x1D,0x28,1)
    axh = i2c.readfrom_mem(0x1D,0x29,1)
    ayl = i2c.readfrom_mem(0x1D,0x2A,1)
    ayh = i2c.readfrom_mem(0x1D,0x2B,1)
    azl = i2c.readfrom_mem(0x1D,0x2C,1)
    azh = i2c.readfrom_mem(0x1D,0x2D,1)

    mxli = int(mxl[0])
    mxhi = int(mxh[0])
    myli = int(myl[0])
    myhi = int(myh[0])
    mzli = int(mzl[0])
    mzhi = int(mzh[0])
    axli = int(axl[0])
    axhi = int(axh[0])
    ayli = int(ayl[0])
    ayhi = int(ayh[0])
    azli = int(azl[0])
    azhi = int(azh[0]) 

    mx = twocc(mxhi, mxli)
    my = twocc(myhi, myli)
    mz = twocc(mzhi, mzli)

    ax = twocc(axhi, axli)
    ay = twocc(ayhi, ayli)
    az = twocc(azhi, azli)
   
    pitch = (math.atan2(ax,math.sqrt(ay*ay+az*az)))
    roll = -(math.atan2(ay,math.sqrt(ax*ax+az*az)))

    if az > 0 and ax > 0:
        pitch = math.pi - pitch
    if az > 0 and ax < 0:
        pitch = - math.pi - pitch
    if az > 0 and ay > 0:
       roll =  math.pi - roll
    if az > 0 and ay < 0: 
       roll = - math.pi - roll

    Xc = mx * math.cos(pitch) + mz * math.sin(pitch)
    Yc = mx * math.sin(roll) * math.sin(pitch) + my * math.cos(roll) - mz * math.sin(roll) * math.cos(pitch)

    Compass = (math.atan2(Yc, -1*Xc))*57.296 +180


    a = "HCHDM," + '{:05.1f}'.format(Compass) + ",M"
    chksum = 0
    le = len(a)
    for j in range (0, le):
        b = ord(a[j:j+1])
        #c = a[j:j+1]
        chksum = chksum^b
  
    c = '{0:02X}'.format(chksum)
    a = "$" + a[0:le] + "*" + c
    led.toggle()
    print (a, end = "\r\n")
    
    a = "IIXDR,A," + '{:05.1f}'.format(pitch*57.296) + ",D,PITCH"
    chksum = 0
    le = len(a)
    for j in range (0, le):
        b = ord(a[j:j+1])
        #c = a[j:j+1]
        chksum = chksum^b
  
    c = '{0:02X}'.format(chksum)
    a = "$" + a[0:le] + "*" + c
    led.toggle()
    print (a, end = "\r\n")
    
    a = "IIXDR,A," + '{:05.1f}'.format(roll*57.296) + ",D,ROLL"
    chksum = 0
    le = len(a)
    for j in range (0, le):
        b = ord(a[j:j+1])
        #c = a[j:j+1]
        chksum = chksum^b
  
    c = '{0:02X}'.format(chksum)
    a = "$" + a[0:le] + "*" + c
    led.toggle()
    print (a, end = "\r\n")    
    
    sleep(1)