Hi Everyone,
In this document I would like to go through a simple bare-metal example code I created for the recently released FRDMSTBC-A8471 development board with the NXP FXLS8471Q 3-axis linear accelerometer. This board is compatible with most NXP Freedom development boards and I decided to use one of the most popular - FRDM-KL25Z. The FreeMASTER tool is used to visualize the acceleration data that are read from the FXLS8471Q using an interrupt technique through the SPI interface. I will not cover the Sensor Toolbox software and Intelligent Sensing Framework (ISF) which also support this board.
This example illustrates:
1. Initialization of the MKL25Z128 MCU (mainly SPI and PORT modules).
2. SPI data write and read operations.
3. Initialization of the FXLS8471Q to achieve the highest resolution.
4. Simple offset calibration based on the AN4069.
5. Output data reading using an interrupt technique.
6. Conversion of the output values from registers 0x01 – 0x06 to real acceleration values in g’s.
7. Visualization of the output values in the FreeMASTER tool.
1. As you can see in the FRDMSTBC-A8471/FRDM-KL25Z schematics and the image below, SPI signals are routed to the SPI0 module of the KL25Z MCU and the INT1 output is connected to the PTD4 pin. The PTD0 pin (Chip Select) is not controlled automatically by SPI0 module, hence it is configured as a general-purpose output. The INT1 output of the FXLS8471Q is configured as a push-pull active-low output, so the corresponding PTD4 pin configuration is GPIO with an interrupt on falling edge.The core/system clock frequency is 20.97 MHz and SPI clock is 524.25 kHz.
The MCU is, therefore, configured as follows.
/******************************************************************************
* MCU initialization function
******************************************************************************/
void MCU_Init(void)
{
//SPI0 module initialization
SIM_SCGC4 |= SIM_SCGC4_SPI0_MASK; // Turn on clock to SPI0 module
SIM_SCGC5 |= SIM_SCGC5_PORTD_MASK; // Turn on clock to Port D module
PORTD_PCR1 = PORT_PCR_MUX(0x02); // PTD1 pin is SPI0 CLK line
PORTD_PCR2 = PORT_PCR_MUX(0x02); // PTD2 pin is SPI0 MOSI line
PORTD_PCR3 = PORT_PCR_MUX(0x02); // PTD3 pin is SPI0 MISO line
PORTD_PCR0 = PORT_PCR_MUX(0x01); // PTD0 pin is configured as GPIO (CS line driven manually)
GPIOD_PSOR |= GPIO_PSOR_PTSO(0x01); // PTD0 = 1 (CS inactive)
GPIOD_PDDR |= GPIO_PDDR_PDD(0x01); // PTD0 pin is GPIO output
SPI0_C1 = SPI_C1_SPE_MASK | SPI_C1_MSTR_MASK; // Enable SPI0 module, master mode
SPI0_BR = SPI_BR_SPPR(0x04) | SPI_BR_SPR(0x02); // BaudRate = BusClock / ((SPPR+1) * 2^(SPR+1)) = 20970000 / ((4+1) * 2^(2+1)) = 524.25 kHz
//Configure the PTD4 pin (connected to the INT1 of the FXLS8471Q) for falling edge interrupts
PORTD_PCR4 |= (0|PORT_PCR_ISF_MASK| // Clear the interrupt flag
PORT_PCR_MUX(0x1)| // PTD4 is configured as GPIO
PORT_PCR_IRQC(0xA)); // PTD4 is configured for falling edge interrupts
//Enable PORTD interrupt on NVIC
NVIC_ICPR |= 1 << ((INT_PORTD - 16)%32);
NVIC_ISER |= 1 << ((INT_PORTD - 16)%32);
}
2. The FXLS8471Q uses the ‘Mode 0′ SPI protocol, which means that an inactive state of clock signal is low and data are captured on the leading edge of clock signal and changed on the falling edge.
The falling edge on the CS pin starts the SPI communication. A write operation is initiated by transmitting a 1 for the R/W bit. Then the 8-bit register address, ADDR[7:0] is encoded in the first and second serialized bytes. Data to be written starts in the third serialized byte. The order of the bits is as follows:
Byte 0: R/W, ADDR[6], ADDR[5], ADDR[4], ADDR[3], ADDR[2], ADDR[1], ADDR[0]
Byte 1: ADDR[7], X, X, X, X, X, X, X
Byte 2: DATA[7], DATA[6], DATA[5], DATA[4], DATA[3], DATA[2], DATA[1], DATA[0]
The rising edge on the CS pin stops the SPI communication.
Below is the write operation which writes the value 0x3D to the CTRL_REG1 (0x3A).
Similarly a read operation is initiated by transmitting a 0 for the R/W bit. Then the 8-bit register address, ADDR[7:0] is encoded in the first and second serialized bytes. The data is read from the MISO pin (MSB first).
The screenshot below shows the read operation which reads the correct value 0x6A from the WHO_AM_I register (0x0D).
Multiple read operations are performed similar to single read except bytes are read in multiples of eight SCLK cycles. The register address is auto incremented so that every eighth next clock edges will latch the MSB of the next register.
A burst read of 6 bytes from registers 0x01 to 0x06 is shown below. It also shows how the INT1 pin is automatically cleared by reading the acceleration output data.
3. The dynamic range is set to ±2g and to achieve the highest resolution, the LNOISE bit is set and the lowest ODR (1.56Hz) and the High Resolution mode are selected (more details in AN4075). The DRDY interrupt is enabled and routed to the INT1 interrupt pin that is configured to be a push-pull, active-low output.
/******************************************************************************
* FXLS8471Q initialization function
******************************************************************************/
void FXLS8471Q_Init (void)
{
FXLS8471Q_WriteRegister(CTRL_REG2, 0x02); // High Resolution mode
FXLS8471Q_WriteRegister(CTRL_REG3, 0x00); // Push-pull, active low interrupt
FXLS8471Q_WriteRegister(CTRL_REG4, 0x01); // Enable DRDY interrupt
FXLS8471Q_WriteRegister(CTRL_REG5, 0x01); // DRDY interrupt routed to INT1-PTD4
FXLS8471Q_WriteRegister(CTRL_REG1, 0x3D); // ODR = 1.56Hz, Reduced noise, Active mode
}
4. A simple offset calibration method is implemented according to the AN4069.
/******************************************************************************
* Simple accelerometer offset calibration
******************************************************************************/
void FXLS8471Q_Calibrate (void)
{
unsigned char reg_val = 0;
while (!reg_val) // Wait for a first set of data
{
reg_val = FXLS8471Q_ReadRegister(STATUS_REG) & 0x08;
}
FXLS8471Q_ReadMultiRegisters(OUT_X_MSB_REG, 6, AccData); // Read data output registers 0x01-0x06
Xout_14_bit = ((short) (AccData[0]<<8 | AccData[1])) >> 2; // Compute 14-bit X-axis output value
Yout_14_bit = ((short) (AccData[2]<<8 | AccData[3])) >> 2; // Compute 14-bit Y-axis output value
Zout_14_bit = ((short) (AccData[4]<<8 | AccData[5])) >> 2; // Compute 14-bit Z-axis output value
Xoffset = Xout_14_bit / 8 * (-1); // Compute X-axis offset correction value
Yoffset = Yout_14_bit / 8 * (-1); // Compute Y-axis offset correction value
Zoffset = (Zout_14_bit - SENSITIVITY_2G) / 8 * (-1); // Compute Z-axis offset correction value
FXLS8471Q_WriteRegister(CTRL_REG1, 0x00); // Standby mode to allow writing to the offset registers
FXLS8471Q_WriteRegister(OFF_X_REG, Xoffset);
FXLS8471Q_WriteRegister(OFF_Y_REG, Yoffset);
FXLS8471Q_WriteRegister(OFF_Z_REG, Zoffset);
FXLS8471Q_WriteRegister(CTRL_REG1, 0x3D); // ODR = 1.56Hz, Reduced noise, Active mode
}
5. In the ISR, only the interrupt flag is cleared and the DataReady variable is set to indicate the arrival of new data.
/******************************************************************************
* PORT D Interrupt handler
******************************************************************************/
void PORTD_IRQHandler()
{
PORTD_PCR4 |= PORT_PCR_ISF_MASK; // Clear the interrupt flag
DataReady = 1;
}
6. The output values from accelerometer registers 0x01 – 0x06 are first converted to signed 14-bit values and afterwards to real values in g’s.
if (DataReady) // Is a new set of data ready?
{
DataReady = 0;
FXLS8471Q_ReadMultiRegisters(OUT_X_MSB_REG, 6, AccData); // Read data output registers 0x01-0x06
Xout_14_bit = ((short) (AccData[0]<<8 | AccData[1])) >> 2; // Compute 14-bit X-axis output value
Yout_14_bit = ((short) (AccData[2]<<8 | AccData[3])) >> 2; // Compute 14-bit Y-axis output value
Zout_14_bit = ((short) (AccData[4]<<8 | AccData[5])) >> 2; // Compute 14-bit Z-axis output value
Xout_g = ((float) Xout_14_bit) / SENSITIVITY_2G; // Compute X-axis output value in g's
Yout_g = ((float) Yout_14_bit) / SENSITIVITY_2G; // Compute Y-axis output value in g's
Zout_g = ((float) Zout_14_bit) / SENSITIVITY_2G; // Compute Z-axis output value in g's
}
7. The calculated values can be watched in the "Variables" window on the top right of the Debug perspective or in the FreeMASTER application. To open and run the FreeMASTER project, install the FreeMASTER 2.0 application and FreeMASTER Communication Driver.
Attached you can find the complete source code written in the CW for MCU's v10.6 including the FreeMASTER project.
If there are any questions regarding this simple application, please feel free to ask below. Your feedback or suggestions are also welcome.
Regards,
Tomas
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