The Startup File startup_<Device>.c is executed after reset and calls SystemInit() in the reset hander. After the system initialization the control is transferred to the C/C++ run-time library which performs initialization and calls the main function in the user code. In addition the Startup File startup_<Device>.c contains all exception and interrupt vectors and implements a default function for every interrupt. It may also contain stack and heap configurations for the user application.

The System Configuration Files system_<Device>.c and system_<Device>.h performs the setup for the processor clock. The variable SystemCoreClock indicates the CPU clock speed. In addition the file may contain functions for the memory BUS setup and clock re-configuration.

‍Note

The files Startup File startup_<Device>.c and System Configuration Files system_<Device>.c and system_<Device>.h may require application specific adaptations and therefore should be copied into the application project folder prior configuration.

The Device Header File <Device.h> provides access to the following device-specific functionalities:

Peripheral Access provides a standardized register layout for all peripherals. Optionally functions for device-specific peripherals may be available.
Interrupts and Exceptions (NVIC) can be accessed with standardized symbols and functions for the Nested Interrupt Vector Controller (NVIC) are provided.
Intrinsic Functions for CPU Instructions allow to access special instructions, for example for activating sleep mode or the NOP instruction.
Intrinsic Functions for SIMD Instructions provide access to the DSP-oriented instructions.
Systick Timer (SYSTICK) function to configure and start a periodic timer interrupt.
Debug Access are functions that allow printf-style I/O via the CoreSight Debug Unit and ITM communication.

Usage in CMSIS-Packs

The easiest way to use CMSIS-Core in a project is with CMSIS Packs.

The CMSIS-Core Device Files are typically provided in a CMSIS Device Family Pack (DFP) that is maintained by the chip vendor for the target device family. The list of public CMSIS packs (including DFPs) can be found at keil.arm.com/packs.

A Device Family Pack (DFP) usually has a requirement for using the CMSIS:CORE component from the CMSIS Software pack that contains the CMSIS-Core Standard Files. In such case the CMSIS Software pack needs to be installed as well.

The files Startup File startup_<Device>.c and System Configuration Files system_<Device>.c and system_<Device>.h are typically provided in the DFP as part of Device class in the Startup group and are defined as configuration files, meaning they are copied from the pack into a project folder and can be modifed there if necessary.

The use of Device Header File <Device.h> can be abstracted with the #define CMSIS_header_file provided in RTE_Components.h. This allows to have uniform include code in the application independent of the target device.

#include "RTE_Components.h" // include information about project configuration

#include CMSIS_device_header // include <Device>.h file

Thereafter, the functions described under API Reference can be used in the application.

For example, the following files are provided by the STM32F10x device family pack:

File	Description
".\Device\Source\ARM\startup_stm32f10x_cl.s"	Startup File startup_<Device>.c for the STM32F10x device variants
".\Device\Source\system_stmf10x.c"	System Configuration Files system_<Device>.c and system_<Device>.h for the STM32F10x device families
".\Device\Include\stm32f10x.h"	Device Header File <Device.h> for the STM32F10x device families

Delivery in CMSIS-Packs provides more information on how CMSIS-Core files can be delivered in CMSIS Packs.

Usage Examples

Examples

Basic CMSIS Example is a simple example that shows the usage of the CMSIS layer.
Using Interrupt Vector Remap shows how to remap the interrupt vector table.
Use generic Arm Devices explains how to use CMSIS-Core (Cortex-M) for Arm processors.
Create generic libraries explains how to create libraries that support various Cortex-M cores.

Also see Using TrustZone for Armv8-M that details CMSIS-Core support for Arm TrusZone operation on Cortex-M.

Basic CMSIS Example

A typical example for using the CMSIS layer is provided below. The example is based on a STM32F10x Device.

#include <stm32f10x.h>                           // File name depends on device used
 
uint32_t volatile msTicks;                       // Counter for millisecond Interval
 
void SysTick_Handler (void) {                    // SysTick Interrupt Handler
  msTicks++;                                     // Increment Counter
}
 
void WaitForTick (void)  {
  uint32_t curTicks;
 
  curTicks = msTicks;                            // Save Current SysTick Value
  while (msTicks == curTicks)  {                 // Wait for next SysTick Interrupt
    __WFE ();                                    // Power-Down until next Event/Interrupt
  }
}
 
void TIM1_UP_IRQHandler (void) {                 // Timer Interrupt Handler
  ;                                              // Add user code here
}
 
void timer1_init(int frequency) {                // Set up Timer (device specific)
  NVIC_SetPriority (TIM1_UP_IRQn, 1);            // Set Timer priority
  NVIC_EnableIRQ (TIM1_UP_IRQn);                 // Enable Timer Interrupt
}
 
 
void Device_Initialization (void)  {             // Configure & Initialize MCU
  if (SysTick_Config (SystemCoreClock / 1000)) { // SysTick 1mSec
       : // Handle Error 
  }
  timer1_init ();                                // setup device-specific timer
}
 
 
// The processor clock is initialized by CMSIS startup + system file
void main (void) {                               // user application starts here
  Device_Initialization ();                      // Configure & Initialize MCU
  while (1)  {                                   // Endless Loop (the Super-Loop)
    __disable_irq ();                            // Disable all interrupts
    Get_InputValues ();                          // Read Values
    __enable_irq ();                             // Enable all interrupts 
    Calculation_Response ();                     // Calculate Results
    Output_Response ();                          // Output Results
    WaitForTick ();                              // Synchronize to SysTick Timer
  }
}

Using Interrupt Vector Remap

Most Cortex-M processors provide VTOR register for remapping interrupt vectors. The following example shows a typical use case where the interrupt vectors are copied to RAM and the SysTick_Handler is replaced.

#include "ARMCM3.h"                     // Device header
 
#define VECTORTABLE_SIZE        (240)    /* size of the used vector tables    */
                                         /* see startup file startup_ARMCM3.c */
#define VECTORTABLE_ALIGNMENT   (0x100U) /* 16 Cortex + 32 ARMCM3 = 48 words  */
                                         /* next power of 2 = 256             */
 
/* externals from startup_ARMCM3.c */
extern uint32_t __VECTOR_TABLE[VECTORTABLE_SIZE];        /* vector table ROM  */
 
/* new vector table in RAM, same size as vector table in ROM */
uint32_t vectorTable_RAM[VECTORTABLE_SIZE] __attribute__(( aligned (VECTORTABLE_ALIGNMENT) ));
 
 
/*----------------------------------------------------------------------------
  SysTick_Handler
 *----------------------------------------------------------------------------*/
volatile uint32_t msTicks = 0;                        /* counts 1ms timeTicks */
void SysTick_Handler(void) {
  msTicks++;                                             /* increment counter */
}
 
/*----------------------------------------------------------------------------
  SysTick_Handler (RAM)
 *----------------------------------------------------------------------------*/
volatile uint32_t msTicks_RAM = 0;                    /* counts 1ms timeTicks */
void SysTick_Handler_RAM(void) {
  msTicks_RAM++;                                      /* increment counter */
}
 
/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
  uint32_t i;
   
  for (i = 0; i < VECTORTABLE_SIZE; i++) {
    vectorTable_RAM[i] = __VECTOR_TABLE[i];       /* copy vector table to RAM */
  }
                                                   /* replace SysTick Handler */
  vectorTable_RAM[SysTick_IRQn + 16] = (uint32_t)SysTick_Handler_RAM;
  
  /* relocate vector table */ 
  __disable_irq();
    SCB->VTOR = (uint32_t)&vectorTable_RAM;
  __DSB();
  __enable_irq();
 
  SystemCoreClockUpdate();                        /* Get Core Clock Frequency */
  SysTick_Config(SystemCoreClock / 1000ul); /* Setup SysTick Timer for 1 msec */
   
  while(1);
}

Use generic Arm Devices

Test and example projects of many software components have a need for implementations that are independent from specific device vendors but still have adaptations for various Arm Cortex-M cores to benefit from their architectural differenceis.

The Cortex_DFP pack provides generic device definitions for standard Arm Cortex-M cores and contains corresponding CMSIS-Core Device Files. These generic Arm devices can be used as a target for embedded programs, with execution, for example, on processor simulation models.

Validation suits and example projects for such components as CMSIS-DSP, CMSIS-RTOS and CMSIS-Core itself use that approach already.

Create generic libraries

The CMSIS Processor and Core Peripheral files allow also to create generic libraries. The CMSIS-DSP libraries are an example for such a generic library.

To build a generic Library set the define __CMSIS_GENERIC and include the relevant core_<cpu>.h CMSIS CPU & Core Access header file for the processor.

The define __CMSIS_GENERIC disables device-dependent features such as the SysTick timer and the Interrupt System.

Refer to Configuration of the Processor and Core Peripherals for a list of the available core_<cpu>.h header files.

Example:

The following code section shows the usage of the core_<cpu>.h header files to build a generic library for Cortex-M0, Cortex-M3, Cortex-M4, or Cortex-M7 devices.

To select the processor, the source code uses the defines CORTEX_M7, CORTEX_M4, CORTEX_M3, CORTEX_M0, or CORTEX_M0PLUS. One of these defines needs to be provided on the compiler command line. By using this header file, the source code can access the correct implementations for Core Register Access, Intrinsic Functions for CPU Instructions, Intrinsic Functions for SIMD Instructions, and Debug Access.

#define __CMSIS_GENERIC              /* disable NVIC and Systick functions */
 
#if defined (CORTEX_M7)
  #include "core_cm7.h"
#elif defined (CORTEX_M4)
  #include "core_cm4.h"
#elif defined (CORTEX_M3)
  #include "core_cm3.h"
#elif defined (CORTEX_M0)
  #include "core_cm0.h"
#elif defined (CORTEX_M0PLUS)
  #include "core_cm0plus.h"
#else
  #error "Processor not specified or unsupported."
#endif