Cortex Microcontroller Software Interface Standard

This file describes the Cortex Microcontroller Software Interface Standard (CMSIS).

Version: 1.20 - 22. May 2009

Information in this file, the accompany manuals, and software is
Copyright © ARM Ltd.
All rights reserved.

Revision History

• Version 1.00: initial release.
• Version 1.01: added __LDREXx, __STREXx, and __CLREX.
• Version 1.02: added Cortex-M0.
• Version 1.10: second review.
• Version 1.20: third review.

Contents

1. About
2. Coding Rules and Conventions
3. CMSIS Files
4. Core Peripheral Access Layer
5. CMSIS Example

About

The Cortex Microcontroller Software Interface Standard (CMSIS) answers the challenges that are faced when software components are deployed to physical microcontroller devices based on a Cortex-M0 / Cortex-M1 or Cortex-M3 processor. The CMSIS will be also expanded to future Cortex-M processor cores (the term Cortex-Mx is used to indicate that). The CMSIS is defined in close co-operation with various silicon and software vendors and provides a common approach to interface to peripherals, real-time operating systems, and middleware components.

ARM provides as part of the CMSIS the following software layers that are available for various compiler implementations:

• Core Peripheral Access Layer: contains name definitions, address definitions and helper functions to access core registers and peripherals. It defines also an device independent interface for RTOS Kernels that includes debug channel definitions.
• Middleware Access Layer: provides common methods to access peripherals for the software industry. The Middleware Access Layer is adapted by the Silicon Vendor for the device specific peripherals used by middleware components. The middleware access layer is currently in development and not yet part of this documentation

These software layers are expanded by Silicon partners with:

• Device Peripheral Access Layer: provides definitions for all device peripherals
• Access Functions for Peripherals (optional): provides additional helper functions for peripherals

CMSIS defines for a Cortex-Mx Microcontroller System:

• A common way to access peripheral registers and a common way to define exception vectors.
• The register names of the Core Peripherals and the names of the Core Exception Vectors.
• An device independent interface for RTOS Kernels including a debug channel.
• Interfaces for middleware components (TCP/IP Stack, Flash File System).

By using CMSIS compliant software components, the user can easier re-use template code. CMSIS is intended to enable the combination of software components from multiple middleware vendors.

Coding Rules and Conventions

The following section describes the coding rules and conventions used in the CMSIS implementation. It contains also information about data types and version number information.

Essentials

• The CMSIS C code conforms to MISRA 2004 rules. In case of MISRA violations, there are disable and enable sequences for PC-LINT inserted.
• ANSI standard data types defined in the ANSI C header file <stdint.h> are used.
• #define constants that include expressions must be enclosed by parenthesis.
• Variables and parameters have a complete data type.
• All functions in the Core Peripheral Access Layer are re-entrant.
• The Core Peripheral Access Layer has no blocking code (which means that wait/query loops are done at other software layers such as the Middleware Access Layer).
• For each exception/interrupt there is definition for:
  • an exception/interrupt handler with the postfix _Handler (for exceptions) or _IRQHandler (for interrupts).
  • a default exception/interrupt handler (weak definition) that contains an endless loop.
  • a #define of the interrupt number with the postfix _IRQn.

Recommendations

The CMSIS recommends the following conventions for identifiers.

• CAPITAL names to identify Core Registers, Peripheral Registers, and CPU Instructions.
• CamelCase names to identify peripherals access functions and interrupts.
• PERIPHERAL_ prefix to identify functions that belong to specify peripherals.
• Doxygen comments for all functions are included as described under Function Comments below.

• Comments use the ANSI C90 style (/* comment */) or C++ style (// comment). It is assumed that the programming tools support today consistently the C++ comment style.
• Function Comments provide for each function the following information:
  • one-line brief function overview.
  • detailed parameter explanation.
  • detailed information about return values.
  • detailed description of the actual function.

  Doxygen Example:

  /** 
   * @brief  Enable Interrupt in NVIC Interrupt Controller
   * @param  IRQn  interrupt number that specifies the interrupt
   * @return none.
   * Enable the specified interrupt in the NVIC Interrupt Controller.
   * Other settings of the interrupt such as priority are not affected.
   */

Data Types and IO Type Qualifiers

The Cortex-Mx HAL uses the standard types from the standard ANSI C header file <stdint.h>. IO Type Qualifiers are used to specify the access to peripheral variables. IO Type Qualifiers are indented to be used for automatic generation of debug information of peripheral registers.

CMSIS Version Number

File core_cm3.h contains the version number of the CMSIS with the following define:

#define __CM3_CMSIS_VERSION_MAIN  (0x00)      /* [31:16] main version       */
#define __CM3_CMSIS_VERSION_SUB   (0x03)      /* [15:0]  sub version        */
#define __CM3_CMSIS_VERSION       ((__CM3_CMSIS_VERSION_MAIN << 16) | __CM3_CMSIS_VERSION_SUB)

File core_cm0.h contains the version number of the CMSIS with the following define:

#define __CM0_CMSIS_VERSION_MAIN  (0x00)      /* [31:16] main version       */
#define __CM0_CMSIS_VERSION_SUB   (0x00)      /* [15:0]  sub version        */
#define __CM0_CMSIS_VERSION       ((__CM0_CMSIS_VERSION_MAIN << 16) | __CM0_CMSIS_VERSION_SUB)

CMSIS Cortex Core

File core_cm3.h contains the type of the CMSIS Cortex-Mx with the following define:

#define __CORTEX_M                (0x03)

File core_cm0.h contains the type of the CMSIS Cortex-Mx with the following define:

#define __CORTEX_M                (0x00)

CMSIS Files

This section describes the Files provided in context with the CMSIS to access the Cortex-Mx hardware and peripherals.

device.h

The file device.h is provided by the silicon vendor and is the central include file that the application programmer is using in the C source code. This file contains:

• Interrupt Number Definition: provides interrupt numbers (IRQn) for all core and device specific exceptions and interrupts.

• Configuration for core_cm0.h / core_cm3.h: reflects the actual configuration of the Cortex-Mx processor that is part of the actual device. As such the file core_cm0.h / core_cm3.h is included that implements access to processor registers and core peripherals.

• Device Peripheral Access Layer: provides definitions for all device peripherals. It contains all data structures and the address mapping for the device specific peripherals.

• Access Functions for Peripherals (optional): provides additional helper functions for peripherals that are useful for programming of these peripherals. Access Functions may be provided as inline functions or can be extern references to a device specific library provided by the silicon vendor.

Interrupt Number Definition

To access the device specific interrupts the device.h file defines IRQn numbers for the complete device using a enum typedef as shown below:

typedef enum IRQn
{
/******  Cortex-M3 Processor Exceptions/Interrupt Numbers ************************************************/
  NonMaskableInt_IRQn             = -14,      /*!< 2 Non Maskable Interrupt                              */
  HardFault_IRQn                  = -13,      /*!< 3 Cortex-M3 Hard Fault Interrupt                      */
  MemoryManagement_IRQn           = -12,      /*!< 4 Cortex-M3 Memory Management Interrupt               */
  BusFault_IRQn                   = -11,      /*!< 5 Cortex-M3 Bus Fault Interrupt                       */
  UsageFault_IRQn                 = -10,      /*!< 6 Cortex-M3 Usage Fault Interrupt                     */
  SVCall_IRQn                     = -5,       /*!< 11 Cortex-M3 SV Call Interrupt                        */
  DebugMonitor_IRQn               = -4,       /*!< 12 Cortex-M3 Debug Monitor Interrupt                  */
  PendSV_IRQn                     = -2,       /*!< 14 Cortex-M3 Pend SV Interrupt                        */
  SysTick_IRQn                    = -1,       /*!< 15 Cortex-M3 System Tick Interrupt                    */
/******  STM32 specific Interrupt Numbers ****************************************************************/
  WWDG_STM_IRQn                   = 0,        /*!< Window WatchDog Interrupt                             */
  PVD_STM_IRQn                    = 1,        /*!< PVD through EXTI Line detection Interrupt             */
  :
  :
  } IRQn_Type;

Configuration for core_cm0.h / core_cm3.h

The Cortex-Mx core configuration options which are defined for each device implementation. Some configuration options are reflected in the CMSIS layer using the #define settings described below.

To access core peripherals file device.h includes file core_cm0.h / core_cm3.h. Several features in core_cm0.h / core_cm3.h are configured by the following defines that must be defined before #include <core_cm0.h> / #include <core_cm3.h> preprocessor command.

Device Peripheral Access Layer

Each peripheral uses a PERIPHERAL_ prefix to identify peripheral registers and functions that access this specific peripheral. If more than one peripheral of the same type exists, identifiers have a postfix (digit or letter). For example:

• UART_Type: defines the generic register layout for all UART channels in a device.
• UART1: is a pointer to a register structure that refers to a specific UART. For example UART1->DR is the data register of UART1.
• UART_SendChar(UART1, c): is a generic function that works with all UART's in the device. To communicate the UART that it accesses the first parameter is a pointer to the actual UART register structure.
• UART1_SendChar(c): is an UART1 specific implementation (in this case the send function).

Minimal Requiements

To access the peripheral registers and related function in a device the files device.h and core_cm0.h / core_cm3.h defines as a minimum:

• The Register Layout Typedef for each peripheral that defines all register names. Names that start with RESERVE are used to introduce space into the structure to adjust the addresses of the peripheral registers. For example:

  typedef struct {
    __IO uint32_t CTRL;      /* SysTick Control and Status Register */
    __IO uint32_t LOAD;      /* SysTick Reload Value Register       */
    __IO uint32_t VAL;       /* SysTick Current Value Register      */
    __I  uint32_t CALIB;     /* SysTick Calibration Register        */
    } SysTick_Type;

• Base Address for each peripheral (in case of multiple peripherals that use the same register layout typedef multiple base addresses are defined). For example:

  #define SysTick_BASE (SCS_BASE + 0x0010)            /* SysTick Base Address */

• Access Definition for each peripheral (in case of multiple peripherals that use the same register layout typedef multiple access definitions exist, i.e. UART0, UART1). For Example:

  #define SysTick ((SysTick_Type *) SysTick_BASE)     /* SysTick access definition */

These definitions allow to access the peripheral registers from user code with simple assignments like:

SysTick->CTRL = 0;

Optional Features

In addition the device.h file may define:

• #define constants that simplify access to the peripheral registers. These constant define bit-positions or other specific patterns are that required for the programming of the peripheral registers. The identifiers used start with the name of the PERIPERHAL_. It is recommended to use CAPITAL letters for such #define constants.
• Functions that perform more complex functions with the peripheral (i.e. status query before a sending register is accessed). Again these function start with the name of the PERIPHERAL_.

core_cm0.h and core_cm0.c

File core_cm0.h describes the data structures for the Cortex-M0 core peripherals and does the address mapping of this structures. It also provides basic access to the Cortex-M0 core registers and core peripherals with efficient functions (defined as static inline).

File core_cm0.c defines several helper functions that access processor registers.

Together these files implement the Core Peripheral Access Layer for a Cortex-M0.

core_cm3.h and core_cm3.c

File core_cm3.h describes the data structures for the Cortex-M3 core peripherals and does the address mapping of this structures. It also provides basic access to the Cortex-M3 core registers and core peripherals with efficient functions (defined as static inline).

File core_cm3.c defines several helper functions that access processor registers.

Together these files implement the Core Peripheral Access Layer for a Cortex-M3.

startup_device

A template file for startup_device is provided by ARM for each supported compiler. It is adapted by the silicon vendor to include interrupt vectors for all device specific interrupt handlers. Each interrupt handler is defined as weak function to an dummy handler. Therefore the interrupt handler can be directly used in application software without any requirements to adapt the startup_device file.

The following exception names are fixed and define the start of the vector table for a Cortex-M0:

__Vectors       DCD     __initial_sp              ; Top of Stack
                DCD     Reset_Handler             ; Reset Handler
                DCD     NMI_Handler               ; NMI Handler
                DCD     HardFault_Handler         ; Hard Fault Handler
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     SVC_Handler               ; SVCall Handler
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     PendSV_Handler            ; PendSV Handler
                DCD     SysTick_Handler           ; SysTick Handler

The following exception names are fixed and define the start of the vector table for a Cortex-M3:

__Vectors       DCD     __initial_sp              ; Top of Stack
                DCD     Reset_Handler             ; Reset Handler
                DCD     NMI_Handler               ; NMI Handler
                DCD     HardFault_Handler         ; Hard Fault Handler
                DCD     MemManage_Handler         ; MPU Fault Handler
                DCD     BusFault_Handler          ; Bus Fault Handler
                DCD     UsageFault_Handler        ; Usage Fault Handler
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     0                         ; Reserved
                DCD     SVC_Handler               ; SVCall Handler
                DCD     DebugMon_Handler          ; Debug Monitor Handler
                DCD     0                         ; Reserved
                DCD     PendSV_Handler            ; PendSV Handler
                DCD     SysTick_Handler           ; SysTick Handler

In the following examples for device specific interrupts are shown:

; External Interrupts
                DCD     WWDG_IRQHandler           ; Window Watchdog
                DCD     PVD_IRQHandler            ; PVD through EXTI Line detect
                DCD     TAMPER_IRQHandler         ; Tamper

Device specific interrupts must have a dummy function that can be overwritten in user code. Below is an example for this dummy function.

Default_Handler PROC
                EXPORT WWDG_IRQHandler   [WEAK]
                EXPORT PVD_IRQHandler    [WEAK]
                EXPORT TAMPER_IRQHandler [WEAK]
                :
                :
                WWDG_IRQHandler
                PVD_IRQHandler
                TAMPER_IRQHandler
                :
                :
                B .
                ENDP

The user application may simply define an interrupt handler function by using the handler name as shown below.

void WWDG_IRQHandler(void)
{
  :
  :
}

system_device.c

A template file for system_device.c is provided by ARM but adapted by the silicon vendor to match their actual device. As a minimum requirement this file must provide a device specific system configuration function and a global variable that contains the system frequency. It configures the device and initializes typically the oscillator (PLL) that is part of the microcontroller device.

The file system_device.c must provide as a minimum requirement the SystemInit function as shown below.

Also part of the file system_device.c is the variable SystemFrequency which contains the current CPU clock speed shown below.

Note

• The above definitions are the minimum requirements for the file system_device.c. This file may export more functions or variables that provide a more flexible configuration of the microcontroller system.

Core Peripheral Access Layer

Cortex-Mx Core Register Access

The following functions are defined in core_cm0.h / core_cm3.h and provide access to Cortex-Mx core registers.

Cortex-Mx Instruction Access

The following functions are defined in core_cm0.h / core_cm3.hand generate specific Cortex-Mx instructions. The functions are implemented in the file core_cm0.c / core_cm3.c.

NVIC Access Functions

The CMSIS provides access to the NVIC via the register interface structure and several helper functions that simplify the setup of the NVIC. The CMSIS HAL uses IRQ numbers (IRQn) to identify the interrupts. The first device interrupt has the IRQn value 0. Therefore negative IRQn values are used for processor core exceptions.

For the IRQn values of core exceptions the file device.h provides the following enum names.

The following functions simplify the setup of the NVIC. The functions are defined as static inline.

Note

• The processor exceptions have negative enum values. Device specific interrupts have positive enum values and start with 0. The values are defined in device.h file.

• The values for PreemptPriority and SubPriority used in functions NVIC_EncodePriority and NVIC_DecodePriority depend on the available __NVIC_PRIO_BITS implemented in the NVIC.

SysTick Configuration Function

The following function is used to configure the SysTick timer and start the SysTick interrupt.

Cortex-M3 ITM Debug Access

The Cortex-M3 incorporates the Instrumented Trace Macrocell (ITM) that provides together with the Serial Viewer Output trace capabilities for the microcontroller system. The ITM has 32 communication channels; two ITM communication channels are used by CMSIS to output the following information:

• ITM Channel 0: implements the ITM_putchar function which can be used for printf-style output via the debug interface.
• ITM Channel 31: is reserved for the RTOS kernel and can be used for kernel awareness debugging.

Note

• The ITM channel 31 is selected for the RTOS kernel since some kernels may use the Privileged level for program execution. ITM channels have 4 groups with 8 channels each, whereby each group can be configured for access rights in the Unprivileged level. The ITM channel 0 may be therefore enabled for the user task whereas ITM channel 31 may be accessible only in Privileged level from the RTOS kernel itself.

The prototype of the ITM_putchar routine is shown in the table below.

Example for the usage of the ITM Channel 31 for RTOS Kernels:

  // check if debugger connected and ITM channel enabled for tracing
  if ((CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA) &&
  (ITM->TCR & ITM_TCR_ITMENA) &&
  (ITM->TER & (1UL << 31))) {
    // transmit trace data
    while (ITM->PORT31_U32 == 0);
    ITM->PORT[31].u8 = task_id;      // id of next task
    while (ITM->PORT[31].u32 == 0);
    ITM->PORT[31].u32 = task_status; // status information
  }

CMSIS Example

The following section shows a typical example for using the CMSIS layer in user applications.

#include <device.h>                              // file name depends on the device used.

void SysTick_Handler (void)  {                   // SysTick Interrupt Handler
  ;
}

void TIM1_UP_IRQHandler (void)  {                // Timer Interrupt Handler
  ;
}

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 main (void) {
  SystemInit ();

  if (SysTick_Config (SystemFrequency / 1000)) { // Setup SysTick Timer for 1 msec interrupts
    :                                            // Handle Error
    :
    while (1);
  }

  timer1_init ();                                // device specific timer

  while (1);
}