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Defining address in microcontroller using preprocessor (#define)

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nikhilsigma

nikhilsigma

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Hello Everyone,

I came across following statement, but I am unable to understand it.
Kindly explain in detail about what exactly is happening.

Code: 
[COLOR=#000000][FONT=monospace]#define GPIO_PORTA_DATA_R       (*((volatile unsigned long *)0x400043FC))


My guess is that some pointer of unsigned long type is being created with address 0x400043FC.
My understanding of pointer is as follows:[/FONT][/COLOR]
Code: 
[COLOR=#000000][FONT=monospace]unsigned long my_variable; // Creating a variable[/FONT][/COLOR][COLOR=#000000][FONT=monospace]
unsigned long *my_pointer; // Creating a pointer
my_pointer=&my_variable;   // Assigning the address of my variable to my_pointer
                           //Now my_pointer contains the address of my_variable and [B]*my_pointer[/B] can be used to directly access the value of my_variable. [/FONT][/COLOR]
 

Your long-winded translation describes the meaning of the expression correctly, but other than in your code, neither a variable or a pointer are created. The purpose the #define is to give direct access to special function registers, e.g. 
Code C - [expand]
1

port_input = GPIO_PORTA_DATA_R;

You are reading or writing it like a variable, but it's actually a hardware register. Similar #defines are often found in a processor specific include file.

Review your C text book about the purpose of volatile attribute in the define.
 

Thank you very much for the explanation.
But I want to know how "port_input = GPIO_PORTA_DATA_R" lets us access the register directly?

I understand that the address of a register named GPIO_PORTA_DATA_R is 0x400043FC.

I try to decifer "port_input = GPIO_PORTA_DATA_R" statement as follows:

port_input = GPIO_PORTA_DATA_R
above can be written as below since pre processor replaces GPIO_PORTA_DATA_R with its defined value.
port_input = (*((volatile unsigned long *)0x400043FC))
now i want to understand the above statement.

I think port_input will get a value written at address = ((volatile unsigned long *)0x400043FC)

I dont understand how the C compiler processes following:
((volatile unsigned long *)0x400043FC)
 

Code: 
port_input = GPIO_PORTA_DATA_R;

port_input is your variable for reading the DATA port and GPIO_PORTA_DATA_R is the DATA register associated with PORTA here.

In the above code, you are reading PORTA data register into the variable port_input.

Yes, it can be written like

Code: 
port_input = (*((volatile unsigned long *)0x400043FC))

It is easier to remember PORTA data register with name like GPIO_PORTA_DATA_R once it is defined in device header or in your code.
 

It's just common C syntax, required to specify the size and data type of the memory object.

Volatile instructs the compiler to skip possible optimizations and perform the accesses unconditionally and in the given order.
 

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