How to create a variable of whose bits are mapped to certain locations?

1
votes

This is an embedded C question for 8-bit PIC microcontrollers.

Let's say there is a 7-Segment LED display that is connected to different pins of different ports and let's say they are defined as follows:

#define _7seg_A         PORTCbits.RC6
#define TRIS_7seg_A     TRISCbits.TRISC6
#define _7seg_B         PORTCbits.RC5
#define TRIS_7seg_B     TRISCbits.TRISC5
#define _7seg_C         PORTAbits.RA5
#define TRIS_7seg_C     TRISAbits.TRISA5
#define _7seg_D         PORTBbits.RB6
#define TRIS_7seg_D     TRISBbits.TRISB6
#define _7seg_E         PORTBbits.RB5
#define TRIS_7seg_E     TRISBbits.TRISB5
#define _7seg_F         PORTBbits.RB7
#define TRIS_7seg_F     TRISBbits.TRISB7
#define _7seg_G         PORTCbits.RC7
#define TRIS_7seg_G     TRISCbits.TRISC7
#define _7seg_DP        PORTAbits.RA4
#define TRIS_7seg_DP    TRISAbits.TRISA4

Also, here is a port definition from the microcontroller definitions header file of the compiler:

typedef union {
    struct {
        unsigned RA0                    :1;
        unsigned RA1                    :1;
        unsigned RA2                    :1;
        unsigned RA3                    :1;
        unsigned RA4                    :1;
        unsigned RA5                    :1;
    };
} PORTAbits_t;
extern volatile PORTAbits_t PORTAbits @ 0x005;

Now, I want to have something called _7seg_DATA which contains all eight of _7seg_X bits and when I write data to it, it will automatically put it into defined pins. Can this be done by unions?

For example, if I do _7seg_DATA = 0x00; it will shut down all the LEDs.. And if I do _7seg_DATA = 0xFF; it will light all of them.

3
cembeddedpic
As I have noted in my question, 7-Segment LED display is connected to different pins of different ports. – abdullah kahraman
Using a more advanced architecture, you could map the variable to an address that would trigger a bus error. You could then trap the bus error and decode the address requested and perform the action. This has tons of overhead and @kkrambo's answer is much simpler. – rjp

3 Answers

5
votes

No you cannot use a union or struct to map the individual bits of a single byte to multiple memory mapped register addresses. However, you could create a function that receives the 8-bit byte as a parameter and then the function maps each individual bit to the appropriate port's register.

2
votes

Just to add to what KKrambo said you can use #define to articulate where each bit should go for instance 

#define ledA 0x01
#define ledB 0x02

ect...

Then in the function (or before you call the function) you can write something like

u_int8 ledS  |= ledA;

and so on

0
votes

The best solution would be to redesign your hardware to combine all of the bits for the 7-segment display in one port. If this isn't possible, your best alternative is a function to do the mapping for you.