Change Acceleration Values to milli-g units rather than Raw Counts

src/displayapp/apps/CMakeLists.txt

@@ -1,7 +1,7 @@
 if(DEFINED ENABLE_USERAPPS)
     set(USERAPP_TYPES ${ENABLE_USERAPPS} CACHE STRING "List of user apps to build into the firmware")
 else ()
-    set(USERAPP_TYPES "Apps::Navigation, Apps::StopWatch, Apps::Alarm, Apps::Timer, Apps::Steps, Apps::HeartRate, Apps::Music, Apps::Paint, Apps::Paddle, Apps::Twos, Apps::Metronome" CACHE STRING "List of user apps to build into the firmware")
+    set(USERAPP_TYPES "Apps::Navigation, Apps::StopWatch, Apps::Alarm, Apps::Timer, Apps::Steps, Apps::Motion, Apps::HeartRate, Apps::Music, Apps::Paint, Apps::Paddle, Apps::Twos, Apps::Metronome" CACHE STRING "List of user apps to build into the firmware")
 endif ()
 
 add_library(infinitime_apps INTERFACE)

src/displayapp/screens/Motion.cpp

@@ -53,9 +53,9 @@ void Motion::Refresh() {
   lv_label_set_text_fmt(labelStep, "Steps %lu", motionController.NbSteps());
 
   lv_label_set_text_fmt(label,
-                        "X #FF0000 %d# Y #00B000 %d# Z #FFFF00 %d#",
-                        motionController.X() / 0x10,
-                        motionController.Y() / 0x10,
-                        motionController.Z() / 0x10);
+                        "X #FF0000 %d# Y #00B000 %d# Z #FFFF00 %d# mg",
+                        motionController.X(),
+                        motionController.Y(),
+                        motionController.Z());
   lv_obj_align(label, nullptr, LV_ALIGN_IN_TOP_MID, 0, 10);
 }

src/drivers/Bma421.cpp

@@ -24,6 +24,17 @@ namespace {
   }
 }
 
+// Scale factors to convert accelerometer counts to milli-g
+// from datasheet: https://files.pine64.org/doc/datasheet/pinetime/BST-BMA421-FL000.pdf
+// The array index to use is stored in accel_conf.range
+const short Bma421::accelScaleFactors[] = {
+  1024,  // LSB/g +/- 2g range
+  512,   // LSB/g +/- 4g range 
+  256,   // LSB/g +/- 8g range
+  128    // LSB/g +/- 16g range
+};
+
+
 Bma421::Bma421(TwiMaster& twiMaster, uint8_t twiAddress) : twiMaster {twiMaster}, deviceAddress {twiAddress} {
   bma.intf = BMA4_I2C_INTF;
   bma.bus_read = user_i2c_read;
@@ -74,7 +85,6 @@ void Bma421::Init() {
   if (ret != BMA4_OK)
     return;
 
-  struct bma4_accel_config accel_conf;
   accel_conf.odr = BMA4_OUTPUT_DATA_RATE_100HZ;
   accel_conf.range = BMA4_ACCEL_RANGE_2G;
   accel_conf.bandwidth = BMA4_ACCEL_NORMAL_AVG4;
@@ -102,8 +112,17 @@ void Bma421::Write(uint8_t registerAddress, const uint8_t* data, size_t size) {
 Bma421::Values Bma421::Process() {
   if (not isOk)
     return {};
+  struct bma4_accel rawData;
   struct bma4_accel data;
-  bma4_read_accel_xyz(&data, &bma);
+  bma4_read_accel_xyz(&rawData, &bma);
+
+  // Scale the measured ADC counts to units of 'binary milli-g'
+  // where 1g = 1024 'binary milli-g' units.
+  // See https://github.com/InfiniTimeOrg/InfiniTime/pull/1950 for
+  // discussion of why we opted for scaling to 1024 rather than 1000.
+  data.x = 1024 * rawData.x / accelScaleFactors[accel_conf.range];
+  data.y = 1024 * rawData.y / accelScaleFactors[accel_conf.range];
+  data.z = 1024 * rawData.z / accelScaleFactors[accel_conf.range];
 
   uint32_t steps = 0;
   bma423_step_counter_output(&steps, &bma);

src/drivers/Bma421.h

@@ -9,6 +9,10 @@ namespace Pinetime {
     public:
       enum class DeviceTypes : uint8_t { Unknown, BMA421, BMA425 };
 
+      // Scale factors to convert accelerometer counts to milli-g
+      // The array values are initialised in Bma421.cpp
+      static const short accelScaleFactors[];
+
       struct Values {
         uint32_t steps;
         int16_t x;
@@ -41,6 +45,7 @@ namespace Pinetime {
       TwiMaster& twiMaster;
       uint8_t deviceAddress = 0x18;
       struct bma4_dev bma;
+      struct bma4_accel_config accel_conf;   // Store the device configuration for later reference.
       bool isOk = false;
       bool isResetOk = false;
       DeviceTypes deviceType = DeviceTypes::Unknown;