Peripheral Access

Naming conventions and optional features for accessing peripherals. More...

Macros
#define	_VAL2FLD(field, value)
	Mask and shift a bit field value for assigning the result to a peripheral register.
#define	_FLD2VAL(field, value)
	Extract from a peripheral register value the a bit field value.

Description

Naming conventions and optional features for accessing peripherals.

The section below describes the naming conventions, requirements, and optional features for accessing device specific peripherals. Most of the rules also apply to the core peripherals. The Device Header File <device.h> contains typically these definition and also includes the core specific header files.

The definitions for Peripheral Access can be generated using the CMSIS-SVD System View Description for Peripherals.

Each peripheral provides a data type definition with a name that is composed of:

• an optional prefix <device abbreviation>_
• <peripheral name>
• postfix _Type or _TypeDef to identify a type definition.

Examples:

• UART_TypeDef for the peripheral UART.
• IMX_UART_TypeDef for the device family IMX and the peripheral UART.

The data type definition uses standard C data types defined by the ANSI C header file <stdint.h>.

• IO Type Qualifiers are used to specify the access to peripheral variables.

  IO Type Qualifier	Type	Description
  __IM	Struct member	Defines 'read only' permissions
  __OM	Struct member	Defines 'write only' permissions
  __IOM	Struct member	Defines 'read / write' permissions
  __I	Scalar variable	Defines 'read only' permissions
  __O	Scalar variable	Defines 'write only' permissions
  __IO	Scalar variable	Defines 'read / write' permissions

The typedef <device abbreviation>_UART_TypeDef shown below defines the generic register layout for all UART channels in a device.

typedef struct {
  __O  uint32_t UART_CR;            // Offset: 0x0000 ( /W) Control Register 
  __IO uint32_t UART_MR;            // Offset: 0x0004 (R/W) Mode Register 
  __O  uint32_t UART_IER;           // Offset: 0x0008 ( /W) Interrupt Enable Register 
  __O  uint32_t UART_IDR;           // Offset: 0x000C ( /W) Interrupt Disable Register 
  __I  uint32_t UART_IMR;           // Offset: 0x0010 (R/ ) Interrupt Mask Register 
  __I  uint32_t UART_SR;            // Offset: 0x0014 (R/ ) Status Register 
  __I  uint32_t UART_RHR;           // Offset: 0x0018 (R/ ) Receive Holding Register 
  __O  uint32_t UART_THR;           // Offset: 0x001C ( /W) Transmit Holding Register 
  __IO uint32_t UART_BRGR;          // Offset: 0x0020 (R/W) Baud Rate Generator Register 
  __IO uint32_t UART_CMPR;          // Offset: 0x0024 (R/W) Comparison Register 
  __IO uint32_t UART_RTOR;          // Offset: 0x0028 (R/W) Receiver Time-out Register 
  __I  uint32_t RESERVED[46];       // Offset: 0x002C (R/ ) Reserved                     
  __IO uint32_t UART_WPMR;          // Offset: 0x00E4 (R/W) Write Protection Mode Register 
} IMX_UART_TypeDef;

To access the registers of the UART defined above, pointers to this register structure are defined. If more instances of a peripheral exist, the variables have a postfix (digit or letter) that identifies the peripheral.

Example: In this example, IMX_UART2 and IMX_UART3 are two pointers to UARTs defined with above register structure.

#define IMX_UART2   ((IMX_UART_TypeDef *) IMX_UART2_BASE)
#define IMX_UART3   ((IMX_UART_TypeDef *) IMX_UART3_BASE)

Note

• The prefix IMX is optional.

The registers in the various UARTs can now be referred in the user code as shown below:

val = IMX_UART2->SR   // is the Status Register of UART2.

---

Minimal Requirements

To access the peripheral registers and related function in a device, the files device.h and core_ca.h define as a minimum:

• The Register Layout Typedef for each peripheral that defines all register names. RESERVED is used to introduce space into the structure for adjusting the addresses of the peripheral registers.
  
  Example:

  typedef struct
  {
    __IOM uint32_t C_CTLR;              // Offset: 0x0000 (R/W) CPU Interface Control Register 
    __IOM uint32_t C_PMR;               // Offset: 0x0004 (R/W) Interrupt Priority Mask Register 
    __IOM uint32_t C_BPR;               // Offset: 0x0008 (R/W) Binary Point Register 
    __IM  uint32_t C_IAR;               // Offset: 0x000C (R/ ) Interrupt Acknowledge Register 
    __OM  uint32_t C_EOIR;              // Offset: 0x0010 ( /W) End Of Interrupt Register 
    __IM  uint32_t C_RPR;               // Offset: 0x0014 (R/ ) Running Priority Register 
    __IM  uint32_t C_HPPIR;             // Offset: 0x0018 (R/ ) Highest Priority Pending Interrupt Register 
    __IOM uint32_t C_ABPR;              // Offset: 0x001C (R/W) Aliased Binary Point Register 
    __IM  uint32_t C_AIAR;              // Offset: 0x0020 (R/ ) Aliased Interrupt Acknowledge Register 
    __OM  uint32_t C_AEOIR;             // Offset: 0x0024 ( /W) Aliased End Of Interrupt Register 
    __IM  uint32_t C_AHPPIR;            // Offset: 0x0028 (R/ ) Aliased Highest Priority Pending Interrupt Register 
    __IOM uint32_t C_STATUSR;           // Offset: 0x002C (R/W) Error Reporting Status Register, optional 
    __I   uint32_t RESERVED1[40];       // Offset: 0x0030 (R/ ) Reserved
    __IOM uint32_t C_APR[4];            // Offset: 0x00D0 (R/W) Active Priority Register 
    __IOM uint32_t C_NSAPR[4];          // Offset: 0x00E0 (R/W) Non-secure Active Priority Register 
    __I   uint32_t RESERVED2[3];        // Offset: 0x00F6 (R/ ) Reserved
    __IM  uint32_t C_IIDR;              // Offset: 0x00FC (R/ ) CPU Interface Identification Register 
    __I   uint32_t RESERVED3[960];      // Offset: 0x0100 (R/ ) Reserved
    __OM  uint32_t C_DIR;               // Offset: 0x1000 ( /W) Deactivate Interrupt Register 
  }  GICInterface_Type;

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

  #define GIC_INTERFACE_BASE (0xe8202000UL)   // GIC Interface Base Address     

• Access Definitions for each peripheral. In case of multiple peripherals that are using the same register layout typedef, multiple access definitions exist.
  
  Example:

  #define GICInterface   ((GICInterface_Type *) GIC_INTERFACE_BASE)   // GIC Interface Access Definition 

These definitions allow accessing peripheral registers with simple assignments.

• Example:
  

  GICInterface->C_CTLR |= 1;   // Enable Interface

---

Optional Features

Optionally, the file device.h may define:

• Register Bit Fields and #define constants that simplify access to peripheral registers. These constants may define bit-positions or other specific patterns that are required for programming peripheral registers. The identifiers should start with <device abbreviation>_ and <peripheral name>_. It is recommended to use CAPITAL letters for #define constants.
• More complex functions (i.e. status query before a sending register is accessed). Again, these functions start with <device abbreviation>_ and <peripheral name>_.

---

Register Bit Fields

For Core Register, macros define the position and the mask value for a bit field.

Example:

Bit field definitions for register ACTLR in CP15.

// CP15 Register ACTLR
#define ACTLR_DDI_Pos                  28U                       
#define ACTLR_DDI_Msk                  (1UL << ACTLR_DDI_Pos)    
                                       
#define ACTLR_DDVM_Pos                 15U                       
#define ACTLR_DDVM_Msk                 (1UL << ACTLR_DDVM_Pos)   
                                       
#define ACTLR_L1PCTL_Pos               13U                       
#define ACTLR_L1PCTL_Msk               (3UL << ACTLR_L1PCTL_Pos) 
                                       
#define ACTLR_L1RADIS_Pos              12U                       
#define ACTLR_L1RADIS_Msk              (1UL << ACTLR_L1RADIS_Pos)
                                       
#define ACTLR_L2RADIS_Pos              11U                       
#define ACTLR_L2RADIS_Msk              (1UL << ACTLR_L2RADIS_Pos)
                                       
#define ACTLR_DODMBS_Pos               10U                       
#define ACTLR_DODMBS_Msk               (1UL << ACTLR_DODMBS_Pos) 
                                       
#define ACTLR_SMP_Pos                  6U                        
#define ACTLR_SMP_Msk                  (1UL << ACTLR_SMP_Pos)     

The macros _VAL2FLD(field, value) and _FLD2VAL(field, value) enable access to bit fields.

Macro Definition Documentation

_FLD2VAL

#define _FLD2VAL	(		field,
			value
	)

Extract from a peripheral register value the a bit field value.

Parameters

field	name of bit field.
value	value of the register. This parameter is interpreted as an uint32_t type.

The macro _FLD2VAL uses the #define's _Pos and _Msk of the related bit field to extract the value of a bit field from a register.

Example:

i = _FLD2VAL(ACTLR_SMP, ACTLR);

_VAL2FLD

#define _VAL2FLD	(		field,
			value
	)

Mask and shift a bit field value for assigning the result to a peripheral register.

Parameters

field	name of bit field.
value	value for the bit field. This parameter is interpreted as an uint32_t type.

The macro _VAL2FLD uses the #define's _Pos and _Msk of the related bit field to shift bit-field values for assigning to a register.

Example:

ACTLR = _VAL2FLD(ACTLR_SMP, 0x1)