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ak8963.py
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ak8963.py
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#
# This file is part of MicroPython MPU9250 driver
# Copyright (c) 2018 Mika Tuupola
#
# Licensed under the MIT license:
# http://www.opensource.org/licenses/mit-license.php
#
# See:
# https://github.com/tuupola/micropython-mpu9250
# https://www.akm.com/akm/en/file/datasheet/AK8963C.pdf
#
"""
MicroPython I2C driver for AK8963 magnetometer
"""
__version__ = "0.2.0"
# pylint: disable=import-error
import ustruct
import utime
from machine import I2C, Pin
from micropython import const
# pylint: enable=import-error
_WIA = const(0x00)
_HXL = const(0x03)
_HXH = const(0x04)
_HYL = const(0x05)
_HYH = const(0x06)
_HZL = const(0x07)
_HZH = const(0x08)
_ST2 = const(0x09)
_CNTL1 = const(0x0a)
_ASAX = const(0x10)
_ASAY = const(0x11)
_ASAZ = const(0x12)
_MODE_POWER_DOWN = 0b00000000
MODE_SINGLE_MEASURE = 0b00000001
MODE_CONTINOUS_MEASURE_1 = 0b00000010 # 8Hz
MODE_CONTINOUS_MEASURE_2 = 0b00000110 # 100Hz
MODE_EXTERNAL_TRIGGER_MEASURE = 0b00000100
_MODE_SELF_TEST = 0b00001000
_MODE_FUSE_ROM_ACCESS = 0b00001111
OUTPUT_14_BIT = 0b00000000
OUTPUT_16_BIT = 0b00010000
_SO_14BIT = 0.6 # μT per digit when 14bit mode
_SO_16BIT = 0.15 # μT per digit when 16bit mode
class AK8963:
"""Class which provides interface to AK8963 magnetometer."""
def __init__(
self, i2c, address=0x0c,
mode=MODE_CONTINOUS_MEASURE_1, output=OUTPUT_16_BIT,
offset=(0, 0, 0), scale=(1, 1, 1)
):
self.i2c = i2c
self.address = address
self._offset = offset
self._scale = scale
if 0x48 != self.whoami:
print(self.whoami)
# Sensitivity adjustement values
self._register_char(_CNTL1, _MODE_FUSE_ROM_ACCESS)
asax = self._register_char(_ASAX)
asay = self._register_char(_ASAY)
asaz = self._register_char(_ASAZ)
self._register_char(_CNTL1, _MODE_POWER_DOWN)
# Should wait atleast 100us before next mode
self._adjustement = (
(0.5 * (asax - 128)) / 128 + 1,
(0.5 * (asay - 128)) / 128 + 1,
(0.5 * (asaz - 128)) / 128 + 1
)
# Power on
self._register_char(_CNTL1, (mode | output))
if output is OUTPUT_16_BIT:
self._so = _SO_16BIT
else:
self._so = _SO_14BIT
@property
def magnetic(self):
"""
X, Y, Z axis micro-Tesla (uT) as floats.
"""
xyz = list(self._register_three_shorts(_HXL))
self._register_char(_ST2) # Enable updating readings again
# Apply factory axial sensitivy adjustements
xyz[0] *= self._adjustement[0]
xyz[1] *= self._adjustement[1]
xyz[2] *= self._adjustement[2]
# Apply output scale determined in constructor
so = self._so
xyz[0] *= so
xyz[1] *= so
xyz[2] *= so
# Apply hard iron ie. offset bias from calibration
xyz[0] -= self._offset[0]
xyz[1] -= self._offset[1]
xyz[2] -= self._offset[2]
# Apply soft iron ie. scale bias from calibration
xyz[0] *= self._scale[0]
xyz[1] *= self._scale[1]
xyz[2] *= self._scale[2]
return tuple(xyz)
@property
def adjustement(self):
return self._adjustement
@property
def whoami(self):
""" Value of the whoami register. """
return self._register_char(_WIA)
def calibrate(self, count=256, delay=200):
self._offset = (0, 0, 0)
self._scale = (1, 1, 1)
reading = self.magnetic
minx = maxx = reading[0]
miny = maxy = reading[1]
minz = maxz = reading[2]
while count:
utime.sleep_ms(delay)
reading = self.magnetic
minx = min(minx, reading[0])
maxx = max(maxx, reading[0])
miny = min(miny, reading[1])
maxy = max(maxy, reading[1])
minz = min(minz, reading[2])
maxz = max(maxz, reading[2])
count -= 1
# Hard iron correction
offset_x = (maxx + minx) / 2
offset_y = (maxy + miny) / 2
offset_z = (maxz + minz) / 2
self._offset = (offset_x, offset_y, offset_z)
# Soft iron correction
avg_delta_x = (maxx - minx) / 2
avg_delta_y = (maxy - miny) / 2
avg_delta_z = (maxz - minz) / 2
avg_delta = (avg_delta_x + avg_delta_y + avg_delta_z) / 3
scale_x = avg_delta / avg_delta_x
scale_y = avg_delta / avg_delta_y
scale_z = avg_delta / avg_delta_z
self._scale = (scale_x, scale_y, scale_z)
return self._offset, self._scale
def _register_short(self, register, value=None, buf=bytearray(2)):
if value is None:
self.i2c.readfrom_mem_into(self.address, register, buf)
return ustruct.unpack("<h", buf)[0]
ustruct.pack_into("<h", buf, 0, value)
return self.i2c.writeto_mem(self.address, register, buf)
def _register_three_shorts(self, register, buf=bytearray(6)):
self.i2c.readfrom_mem_into(self.address, register, buf)
return ustruct.unpack("<hhh", buf)
def _register_char(self, register, value=None, buf=bytearray(1)):
if value is None:
self.i2c.readfrom_mem_into(self.address, register, buf)
return buf[0]
ustruct.pack_into("<b", buf, 0, value)
return self.i2c.writeto_mem(self.address, register, buf)
def __enter__(self):
return self
def __exit__(self, exception_type, exception_value, traceback):
pass