Addressing pins of Register in microcontrollers - embedded

Question

I'm working on Keil software and using LM3S316 microcontroller. Usually we address registers in microcontrollers in form of:
#define GPIO_PORTC_DATA_R (*((volatile uint32_t *)0x400063FC))
My question is how can I access to single pin of register for example, if I have this method:
char process_key(int a)
{ PC_0 = a ;}
How can I get PC_0 and how to define it?
Thank you

Answer 1

Given say:
#define PIN0 (1u<<0)
#define PIN1 (1u<<1)
#define PIN2 (1u<<2)
// etc...
Then:
char process_key(int a)
{
if( a != 0 )
{
// Set bit
GPIO_PORTC_DATA_R |= PIN0 ;
}
else
{
// Clear bit
GPIO_PORTC_DATA_R &= ~PIN0 ;
}
}
A generalisation of this idiomatic technique is presented at How do you set, clear, and toggle a single bit?
However the read-modify-write implied by |= / &= can be problematic if the register might be accessed in different thread/interrupt contexts, as well as adding a possibly undesirable overhead. Cortex-M3/4 parts have a feature known as bit-banding that allows individual bits to be addressed directly and atomically. Given:
volatile uint32_t* getBitBandAddress( volatile const void* address, int bit )
{
__IO uint32_t* bit_address = 0;
uint32_t addr = reinterpret_cast<uint32_t>(address);
// This bit maniplation makes the function valid for RAM
// and Peripheral bitband regions
uint32_t word_band_base = addr & 0xf0000000u;
uint32_t bit_band_base = word_band_base | 0x02000000u;
uint32_t offset = addr - word_band_base;
// Calculate bit band address
bit_address = reinterpret_cast<__IO uint32_t*>(bit_band_base + (offset * 32u) + (static_cast<uint32_t>(bit) * 4u));
return bit_address ;
}
Then you can have:
char process_key(int a)
{
static volatile uint32_t* PC0_BB_ADDR = getBitBandAddress( &GPIO_PORTC_DATA_R, 0 ) ;
*PC0_BB_ADDR = a ;
}
You could of course determine and hard-code the bit-band address; for example:
#define PC0 (*((volatile uint32_t *)0x420C7F88u))
Then:
char process_key(int a)
{
PC0 = a ;
}
Details of the bit-band address calculation can be found ARM Cortex-M Technical Reference Manual, and there is an on-line calculator here.