Revision as of 10:59, 6 March 2012

const

const means that something is not modifiable, so a data object that is declared with const as a part of its type specification must not be assigned to in any way during the run of a program.

volatile

The use of volatile ensures that the compiler always carries out the memory accesses, rather than optimizing them out (for example if the access is in a loop).

Memorymapped access to a register

Example 1

In the example below the register 32 bit register located in memory at address 0x40000 can be accessed through the uartreg pointer.

In the example below uartreg is a pointer pointing to a unsigned int 0x40000. The pointer value in memory 0x40000 is a constant not what it points to in memory. Fx. a peripheral register.

unsigned int volatile * const uartreg = (unsigned int *) 0x40000;
*uartreg = 0x80000000; // Assign 80000000 to the 32 bit register located at memeory address 0x40000
...some code...
*uartreg |= 0x1  // *uartreg's contents is OR'et to 0x1. New contents = 0x80000001

Example 2

From the Cortex M STM32F107VCs Reference manual - See picture below - that the size of the embedded Flash can be seen in KB from the Flash size register at memory location 0x1FFFF7E0.

Method 1: Reading the Flash size register

 uint16_t volatile * const flashsize = (uint16_t *) 0x1ffff7e0;

 printf("Flashsize: %i KB\n\r", (int) *flashsize );

On the STM32F107VC the output to stdout would be Flashsize: 256 KB

Method 2: Reading the Flash size register

#define F_SIZE	((volatile uint16_t *) 0x1ffff7e0)		// Flash Size 

 printf("Flashsize: %i KB\n\r", (int) *F_SIZE );