Volatile and Constant usage

volatile uint32_t x;  // tells compiler not to optimize this. stored on SRAM in global section
const uint32_t x;  // stored on ROM. Part of code memory. Value cant be changed.
volatile const uint32_t x;  // Value cant be changed from the code inside the file where it is declared but can be changed from outside the file.

volatile uint32_t * TCNT;
// TCNT is a pointer to register whose value can be changed outside the scope of program flow.
Read write register. Pointer value (i.e. address of register) can be changed.

const uint32_t * TCNT;
// TCNT is pointer to register whose value cant be changed using pointer dereferencing.
Readonly register. Pointer value (i.e. address of register) can be changed.

volatile uint32_t * const TCNT;
// TCNT is a pointer whose value cant be changed. It point to register whose value can get changed out of flow of program.
Read or Write register.  Pointer value (i.e. address of register) cannot be changed.

const uint32_t * volatile TCNT;
// TCNT is a pointer whose value can be get changed out of flow of code execution. It points to register whose value cant be changed through the code. Its constant and cant be changed by pointer deferencing.
Read only memory.

volatile const uint32_t * const TCNT;
// TCNT is a pointer whose value is constant. It points to register whose value is constant for code, but can get changed out of flow of program execution.
Read only register accessing.

Using pointers, when accessing
- Read only register, whose value is not going to be changed at all use
const uint32_t * ptr;
- Read only register, whose value can change outside the flow of execution
volatile const uint32_t * ptr
- Write register
uint32_t *ptr