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Using the SysTick Timer in PSoC® 4

Using the SysTick Timer in PSoC® 4

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What is the SysTick timer? How do you configure it and use it as a general-purpose timer in PSoC® 4?

The SysTick timer is an integral part of the ARM® Cortex™-M0 processor that powers the PSoC 4 family. The timer is a down counter with a 24-bit reload/tick value and clocked by the system clock (SysClk) reaching the Cortex-M0 from the PSoC 4 clocking system. The timer has the capability to generate an interrupt when the set number of ticks expires and the counter reloads. This interrupt is available as part of the Nested Vectored Interrupt Controller (NVIC) for service by the CPU and can be used for general purpose timing control in the user code. Since the timer is independent of the CPU (except for the clock), it can be handy in applications requiring precise timing but not having a dedicated timer/counter available for the job.

Configuring the timer:

The SysTick registers are part of the System Control Space (SCS) registers that are common to all Cortex-M0 CPU-based devices. Refer to Section 4.4, “Optional system timer, SysTick” of the Cortex-M0 Devices Generic User Guide for complete details on the registers and their usage. A brief summary of the registers is provided in Table 1 for quick reference.

                                                                                                                                                         

    Table 1. SysTick Timer Registers

    Address     Name     Type    
     Reset value
  
    Description
    0xE000E010     SYST_CSR     RW     -[a]     SysTick Control and Status Register
    0xE000E014     SYST_RVR     RW     Unknown     SysTick Reload Value Register
    0xE000E018     SYST_CVR     RW     Unknown     SysTick Current Value Register
    0xE000E01C     SYST_CALIB     RO     -[a]     SysTick Calibration Value Register
    [a]See the register description for more information

 

In the PSoC 4 family, these registers can be accessed using a pointer to the structure defined in the core_cm0.h file. This file also provides an API to configure and enable the SysTick timer to generate a periodic interrupt. The details of the structure and API are provided in Table 2 and Table 3.

Table 2. SysTick Structure details                                                                                       

    Structure Name     Member     Function/Register mapped
    SysTick            
          SysTick->CTRL     SYST_CSR (Control & Status Register)
          SysTick->LOAD     SYST_RVR (Reload value register)
          SysTick->VAL     SYST_CVR (Current value register)
          SysTick->CALIB     SYST_CALIB (Calibration value register)

 

Table 3. SysTick API Details                                             

    Function     Parameters     Return Value     Description
    uint32 SysTick_Config(uint32)     uint32 ticks –value to be written to the reload register. This defines the period of the timer.
   
    PERIOD = ticks/SYSCLK_FREQ
    uint32 status – status if the update was successful. The update fails if the passed parameter ‘ticks’ is more than 24-bits i.e. if the value cannot be written to the reload register     The function initializes the System Timer and its interrupt, and starts the System Tick Timer. Counter is in free running mode to generate periodic nterrupts.

 

Before the timer is configured and initialized with the desired period, the interrupt needs to be mapped to the proper function for servicing the periodic ISR that will be generated. For that, the CyIntSetSysVector() API can be used as explained in the code snippet below. The code snippet is a sample firmware that can be used to generate a SysTick ISR every 100 Sysclk ticks. The ISR is mapped to the USER_ISR routine, which you can rename to any valid name you choose.

 

#include #define NUMBER_OF_TICKS   100/* ISR prototype declaration */CY_ISR_PROTO(USER_ISR); /* User ISR function definition */CY_ISR(USER_ISR) {  /* User ISR Code*/}  int main(void) { /* Map systick ISR to the user defined ISR. SysTick_IRQn is already defined in core_cm0.h file */ CyIntSetSysVector((SysTick_IRQn + 16), USER_ISR);  /* Enable Systick timer with desired period/number of ticks */ SysTick_Config(NUMBER_OF_TICKS);  /* Enable global interrupts */ CyGlobalIntEnable;  /* infinite loop */ for(;;)  {  } }

 

Note: The SysTick timer is a CPU exception which cannot be masked in the NVIC. If enabled, it will generate an interrupt/exception and will be serviced.

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