read/write bit from a structure

Question

in C if we have a structure defined as

struct PortBreg { 
unsigned int B0 :1; 
unsigned int B1 :1; 
unsigned int B2 :1; 
unsigned int B3 :1; 
unsigned int B4 :1; 
unsigned int B5 :1; 
unsigned int B6 :1; 
unsigned int B7 :1; 
};

#define Breg (*(volatile struct PortBreg *)(0x38)), 

If I want to read value from port B bit B3 and write value to port B bit B2, can I do like

int i=Breg.B3;      //to read
Breg.B2=i;          //to write ?

Answer 1

That might depend if the port is readable and writable, but for a plain variable this code works: copying B3 to B2. Please be consistent with your types, int i is not unsigned int i. Note too that I print each member in the usual sequence, but the struct definition of an actual port read might need to be reversed, so that B7 is first.

#include <stdio.h>

struct PortBreg { 
    unsigned int B0 :1; 
    unsigned int B1 :1; 
    unsigned int B2 :1; 
    unsigned int B3 :1; 
    unsigned int B4 :1; 
    unsigned int B5 :1; 
    unsigned int B6 :1; 
    unsigned int B7 :1; 
};

void show(struct PortBreg Qreg)
// bit order MSB -> LSB
{
    printf("%u%u%u%u%u%u%u%u\n", 
       Qreg.B7, Qreg.B6, Qreg.B5, Qreg.B4, Qreg.B3, Qreg.B2, Qreg.B1, Qreg.B0);
}

int main(void)
{
    struct PortBreg Breg = {0, 0, 0, 1,};
    unsigned i;
    show(Breg);
    i = Breg.B3;
    Breg.B2 = i;
    show(Breg);
    return 0;
}

Program output:

00001000
00001100

Answer 2

Not always. If the port is memory-mapped, the simple act of reading and writing to the right bit from C may be enough to trigger a signal on the wires. But you might have to do other things. Often you have to disable interrupts. And reading often also clears the port, often you have to test a bit to see if there is data, then read and clear.

So you need to take a look at the hardware docs. Fundamentally the port is controlled by writing to bits as though they were plain memory, however.