/****************************************************************************
 Module
   AccService.c

Description
   This is a service for initializing and getting X, Y and Z readings from the
   accelerometer and calculating heading.

 Notes


 Private Definitions
   CPSDVSR
   SCR
   SSI_ENABLE_D01
   SSI_ENABLE_D23
   SSI_ENABLE
   ALL_SSI_PINS
   BW_RATE
   DATA_FORMAT
   POWER_CTL
   DATAX0
   BYTES_TO_READ
   ACC_TIME
   SIZEOFBUFFER
   n
   MyPriority
   ReadBuffer[7]
   Initialization
   HeadingX
   HeadingY
   HeadingZ
   trash
   bufferX[SIZEOFBUFFER]
   bufferY[SIZEOFBUFFER]
   bufferZ[SIZEOFBUFFER]
   buffer_index
   x[n]
   y[n]
   alpha[n]

 Private Functions
   sendCommand
   startQuery
   Acc_SPI_Init
   readQuery
   AddToBufferX
   GetAvgX
   AddToBufferY
   GetAvgY
   AddToBufferZ
   GetAvgZ

bool InitAccService(uint8_t Priority) ----------------------------------------
  // post the initial transition event
  // start ACC_TIMER

bool PostAccService(ES_Event_t ThisEvent) ------------------------------------
  // post ThisEvent to AccService

ES_Event_t RunAccService(ES_Event_t ThisEvent) -------------------------------
  // process any events
    // if there is an ES_TIMEOUT event with ACC_TIMER as the parameter
      // initiate multi-byte read
      // add to ring buffer for X, Y and Z
      // restart ACC_TIMER
    // if there is an ACC_EOT event
      // check if the Initialization variable is true
        // set Initialization to false
        // make two garbage reads from accelerometer
      // otherwise
        // read X, Y, Z from accelerometer

void Acc_EOT_ISR(void) --------------------------------------------------------
  // clear interrupt
  // Post ACC_EOT to itself

static void Acc_SPI_Init(void) ------------------------------------------------
  // enable the clock to SSI3 module
  // enable clock to Port D
  // wait for the clock at SSI3 to be initialized
  // wait for the clock at Port D to be initialized
  // enable the GPIO pins PD0, PD1, PD2, PD3 as alternate function of SSI
  // enable PD0-PD3 as SSI pins
  // enable PD0-PD3 as digital output
  // enable PD2 as digital input
  // enable PD0 with a pull-up resistor
  // disable the SSI3 module before editing
  // set Tiva to master and set TXRIS interrupt to EOT
  // set SSI clock source as system clock
  // set CPSDVSR and SCR to set the SSI baud clock rate
  // set SSI to Mode 3 (SPO = 1, SPH = 1)
  // set data transfer size to 8 bits
  // enable NVIC
  // global enable
  // re-enable the SSI3 module

static void sendCommand(uint8_t regAddr, uint8_t data) ------------------------
  // send register address byte
  // send value to be set in register
  // allows for the interrupt to be triggered

static void startQuery(void) --------------------------------------------------
  // send first address to read from (0x32 = DATAX0)
  // send 6 garbage writes for multi-byte read
  // mask TXMIS for interrupt to be triggered

static void readQuery(void) ---------------------------------------------------
  // read from accelerometer 7 times and save to ReadBuffer
  // combine DATAX0 and DATAX1 to get HeadingX
  // combine DATAY0 and DATAY1 to get HeadingY
  // combine DATAZ0 and DATAZ1 to get HeadingZ

uint16_t getHeading(void) -----------------------------------------------------
  // initialize return value to 0
  // initialize previous norm to random max value
  // calculate average X and average Y (must be called after waiting 1.2 sec)
  // calculate difference between average and current x value 24 times
  // compute norm of differences in x and y
  // if norm is less than previous norm, set heading to alpha for these x and y
  // set previous norm to norm

void AddToBufferX(int8_t NewItem) ---------------------------------------------
  // add NewItem to bufferX
  // increment buffer_index
  // if buffer_index reaches SIZEOFBUFFER
    // set buffer_index to 0

int16_t GetAvgX(void) ---------------------------------------------------------
  // set sum to 0
  // go through all values in bufferX and add to sum
  // calculate mean - divide sum by SIZEOFBUFFER

void AddToBufferY(int8_t NewItem) ---------------------------------------------
  // add NewItem to bufferY
  // increment buffer_index
  // if buffer_index reaches SIZEOFBUFFER
    // set buffer_index to 0

int16_t GetAvgY(void) ---------------------------------------------------------
  // set sum to 0
  // go through all values in bufferY and add to sum
  // calculate mean - divide sum by SIZEOFBUFFER

void AddToBufferZ(int8_t NewItem) ---------------------------------------------
  // add NewItem to bufferZ
  // increment buffer_index
  // if buffer_index reaches SIZEOFBUFFER
    // set buffer_index to 0

int16_t GetAvgZ(void) ---------------------------------------------------------
  // set sum to 0
  // go through all values in bufferZ and add to sum
  // calculate mean - divide sum by SIZEOFBUFFER