PSoC FreeRTOS & Kionix KXTJ2 Accelerometer


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

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.

In addition it has the the bit fields for the registers

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

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

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

After the accelerometer chip is reset, then initialize the chip

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

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