In the previous PSoC FreeRTOS article I showed you a scheme for sharing the I2C between multiple tasks by sending “I2C Transactions” to a single I2C Master thread.  In this article I will modify the Accelerometer task to use the hardware interrupt pin from the KXTJ2-1009 to only read data when there is new data to be read (i.e. stop polling).  I will also modify the project to use “kxtj2.h” a header file which I downloaded from Rohm’s website with all of the register definitions.

Updates to the PSoC FreeRTOS Project Schematics

When I was looking at the schematics for the CY8CKIT-044 I noticed that the KXTJ2-1009 had one of the pins attached to the PSoC P1[6].  Even better that pin was labeled “INT”.  In the original PSoC FreeRTOS project I polled the values from the accelerometer, which is almost always a bad idea.


CY8CKIT-044 Schematic

Then when you look in the data sheet you see that you ask for an interrupt when the data is ready “DRDYE”

KTXJ2-1009 Datasheet

And you can program the INT pin to behave how you want.  I chose:

  • IEN=1 (Enable interrupts)
  • IEA=1 (Make the interrupt active high)
  • IEL=1 (Pulse the pin)

KTXJ2-1009 Datasheet

After I figured all of that out, I modify the PSoC FreeRTOS Creator schematic to have the accel_int_pin (aka P1[6]) attached to an interrupt.

PSoC FreeRTOS Schematic

Then I write an ISR to handle the interrupt, which just resets the pin interrupt and sets a semaphore.


Finally register the interrupt and the start the semaphore

   accelIntSemaphore = xSemaphoreCreateBinary(); // Signal from the Accel that it is ready

Using the “kxtj2.h” Header File

When I looked on the Kionix website for the KXTJ2-1009 datasheet I noticed that there was a download link… which I assumed would take me to a place where I could download the datasheet.

KTXJ2-1009 Datasheet

But when I go to the download page I got the  pleasant surprise of finding a bunch of driver files… but even better there was a “Register Definitions .h/.py”

KTXJ2-1009 Datasheet Download

This file had a bunch of #defines for the registers in the chip.

/* registers */
// output register x
#define KXTJ2_OUTX_L 0x06
#define KXTJ2_OUTX_H 0x07
// output register y
#define KXTJ2_OUTY_L 0x08
#define KXTJ2_OUTY_H 0x09
// output register z
#define KXTJ2_OUTZ_L 0x0A
#define KXTJ2_OUTZ_H 0x0B
// This register can be used to verify proper integrated circuit functionality
#define KXTJ2_DCST_RESP 0x0C
// This register can be used for supplier recognition, as it can be factory written to a known byte value.
#define KXTJ2_WHO_AM_I 0x0F
// This register reports which function caused an interrupt.
#define KXTJ2_INT_SOURCE1 0x16
// This register reports the axis and direction of detected motion
#define KXTJ2_INT_SOURCE2 0x17
// This register reports the status of the interrupt

In addition it has the the bit fields for the registers

// 12.5Hz
#define KXTJ2_DATA_CTRL_REG_OSA_12P5 (0x00 << 0)
// 25Hz
#define KXTJ2_DATA_CTRL_REG_OSA_25 (0x01 << 0)
// 50Hz
#define KXTJ2_DATA_CTRL_REG_OSA_50 (0x02 << 0)
// 100Hz
#define KXTJ2_DATA_CTRL_REG_OSA_100 (0x03 << 0)
// 200Hz
#define KXTJ2_DATA_CTRL_REG_OSA_200 (0x04 << 0)
// 400Hz
#define KXTJ2_DATA_CTRL_REG_OSA_400 (0x05 << 0)
// 800Hz
#define KXTJ2_DATA_CTRL_REG_OSA_800 (0x06 << 0)
// 1600Hz
#define KXTJ2_DATA_CTRL_REG_OSA_1600 (0x07 << 0)
// 0.78Hz
#define KXTJ2_DATA_CTRL_REG_OSA_0P781 (0x08 << 0)
// 1.563Hz
#define KXTJ2_DATA_CTRL_REG_OSA_1P563 (0x09 << 0)
// 3.125Hz
#define KXTJ2_DATA_CTRL_REG_OSA_3P125 (0x0A << 0)
// 6.25Hz
#define KXTJ2_DATA_CTRL_REG_OSA_6P25 (0x0B << 0)

To use it all I did was add it to my project by right clicking on Header Files and selecting “Add–>Existing Item”

Add header file to PSoC FreeRTOS Project

Then I add the include “ktxk2.h” to the includes

#include "kxtj2.h"

Finally I can use the #define names instead of the hardcoded values for example to reset the chip you can write to the KXTJ2_CTRL_REG with the value  KXTJ2_CTRL_REG _SRST

    setupData[0] = KXTJ2_CTRL_REG2_SRST; // Software reset
    mytransaction.i2cm_register = KXTJ2_CTRL_REG2; 

Updates to the Accelerometer Task

With all of the updates in place I can now make the modifications to the PSoC FreeRTOS accel_Task to use the new interrupt structure.  The first thing to do is write into the reset register to force a software reboot.  The datasheet says that you need to:

  • Write 0x00 into the CTRL_REG1
  • Then WAIT until the value in KXTJ2_CTRL_REG2 is 0x0
    // Setup for initialization
    uint8_t setupData[1];
    mytransaction.i2cm_method = I2CM_WRITE;
    mytransaction.i2cm_byteNum = 1;
    mytransaction.i2cm_doneSemaphore = mySemaphore;
    mytransaction.i2cm_bytes = setupData;
    // reset the accelerometer
    setupData[0] = 0b00000000; 
    mytransaction.i2cm_register = KXTJ2_CTRL_REG1; 
    setupData[0] = KXTJ2_CTRL_REG2_SRST; // Software reset
    mytransaction.i2cm_register = KXTJ2_CTRL_REG2; 

    vTaskDelay(10); // From the datasheet power up time

    while(1) // delay until the accelerometer is reset
        mytransaction.i2cm_method = I2CM_READ;  
        mytransaction.i2cm_register = KXTJ2_CTRL_REG2;
        if(setupData[0] & KXTJ2_CTRL_REG2_SRST)
            vTaskDelay(10); // backup 10 ms

After the accelerometer chip is reset, then initialize the chip

    // initialize the interrupt 
    mytransaction.i2cm_register = KXTJ2_INT_CTRL_REG1; 
    // initialize the  DATA_CTRL_REG
    setupData[0] = KXTJ2_DATA_CTRL_REG_OSA_12P5;      
    mytransaction.i2cm_register = KXTJ2_DATA_CTRL_REG; 
    // Initalize the acceleromter CTRL_REG_1
    mytransaction.i2cm_register = KXTJ2_CTRL_REG1; 
    // Setup for repeated reads
    mytransaction.i2cm_method = I2CM_READ;
    mytransaction.i2cm_register = 0x06;
    mytransaction.i2cm_byteNum = sizeof(myData);
    mytransaction.i2cm_doneSemaphore = mySemaphore;
    mytransaction.i2cm_bytes = (uint8_t *)&myData;

After that you can wait for the semaphore, then get the data

    while(1)  // Run the same transaction over and over
        myData.x /= 16;  // The KXTJ2 Datasheet say lower 4 bits are X
        myData.y /= 16;  // The KXTJ2 Datasheet say lower 4 bits are X
        myData.z /= 16;  // The KXTJ2 Datasheet say lower 4 bits are X
        xs = (myData.x < 0? -1:1);  // Find the XSign
        ys = (myData.y < 0? -1:1);  // Find the YSign
        zs = (myData.z < 0? -1:1);  // Find the ZSign
        // The 1024 is a hardcode based on the range of the acceleromter
        x = (int)((float)myData.x / 1024.0 * 10.0) * xs; // Turn it into integer G * 10
        y = (int)((float)myData.y / 1024.0 * 10.0) * ys; // Turn it into integer G * 10
        z = (int)((float)myData.z / 1024.0 * 10.0) * zs; // Turn it into integer G * 10
        // Format data in x.x  

As always you can git this PSoC FreeRTOS project at the IoT Expert GitHub site or The project name is “8-Shared-I2C_Interrupt”

Topic Description
FreeRTOS: A PSoC4 FreeRTOS Port An introduction to making FreeRTOS work on PSoC 4
FreeRTOS PSoC Examples Using multiple tasks in FreeRTOS
FreeRTOS Queue Example Using a queue to communicate between tasks
PSoC 6 FreeRTOS - The First Example Booting FreeRTOS on PSoC 6
FreeRTOS Binary Semaphore An first example of a binary semaphore
FreeRTOS Binary Semaphore (Part 2) Removing polling in the UART Task
FreeRTOS Counting Semaphore An example using a counting semaphore
PSoC FreeRTOS Reading I2C Sensors with a shared I2C Bus
PSoC FreeRTOS Task Notify A light weight scheme to replace Semaphores
PSoC FreeRTOS Task Notification Values A very light weight method to transfer one word of information into a task

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  1. Perfect job, Alan! but one note – it’s needed to change “\n\r’ with “\r\n” through whole main.c…

    • Thanks… I guess there are some terminal emulators that care… but Putty (the terminal emulator that I use) can do a carriage return then a new line… or visa-versa it doesnt care.

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