Our objective for this project was to wire together two raspberry pi's and have them send/receive data from one another. The program would ask whether the user was sending or receiving , to which the user would respond with the proper value. The program would then run either the receive portion of the switch block or the send.

The code:

  1 #include "rpi.h"
  2 
  3 int main()
  4 {
  5     int counter = 0;
  6     int option;
  7 
  8     int led_1, led_2, led_3, led_4; // variables to hold the value that is read from the GPIO_READ functions (control on/off state of receiving pins)
  9 
 10     if( map_peripheral(&gpio) == -1 )
 11     {
 12         printf("Failed to map the physical GPIO registers into the virtual memory space.\n");
 13         return -1;
 14     }
 15 
 16     //Define output pins
 17     OUT_GPIO(7);
 18     OUT_GPIO(8);
 19     OUT_GPIO(9);
 20     OUT_GPIO(10);
 21 
 22     printf("Are you the sender or receiver?\n");
 23     printf("1: Sender\n");
 24     printf("2: Receiver\n");
 25     scanf("%d", &option);
 26 
 27     switch ( option )
 28     {
 29 
 30
 31 
 32 
 33 
 34 
 35         case 1: // case 1 sends signals to the other pi to turn on/off the necessary pins
 36
 37             printf("Sending...\n");
 38
 39             //Define output pins
 40             OUT_GPIO(4);
 41             OUT_GPIO(17);
 42             OUT_GPIO(27);
 43             OUT_GPIO(22);
 44 
 45             while(1) // binary counter loop which sets output pins to other pi on/off
 46             {
 47                 if( (counter % 2) == 1 )
 48                     GPIO_SET = 1 << 4;
 49                 else
 50                     GPIO_CLR = 1 << 4;
 51 
 52                 if( (counter % 4) >= 2 )
 53                     GPIO_SET = 1 << 17;
 54                 else
 55                     GPIO_CLR = 1 << 17;
 56 
 57                 if( (counter % 8) >= 4 )
 58                     GPIO_SET = 1 << 27;
 59                 else
 60                     GPIO_CLR = 1 << 27;
 61 
 62                 if( (counter % 16) >= 8 )
 63                     GPIO_SET = 1 << 22;
 64                 else
 65                     GPIO_CLR = 1 << 22;
 66 
 67                 usleep(500000);
 68 
 69                 counter++;
 70             }
 71 
 72             break;
 73 
 74         case 2:
 75             printf("Receiving...\n");
 76 
 77             //Define input pins
 78             INP_GPIO(4);
 79             INP_GPIO(17);
 80             INP_GPIO(27);
 81             INP_GPIO(22);
 82 
 83             while(1) // loop which turns on corresponding pins depending on values of led_1-4 (which are directly related to on/off state of pins 4, 17, 27, 22)
 84             {
 85                 // setting led_1-4 variables equal to value of each pin 4, 17, 27, 22 (on/off state)
 86                 led_1 = GPIO_READ(4);
 87                 led_2 = GPIO_READ(17);
 88                 led_3 = GPIO_READ(27);
 89                 led_4 = GPIO_READ(22);
 90 
 91                 if( led_1 != 0 )
 92                     GPIO_SET = 1 << 7;
 93                 else
 94                     GPIO_CLR = 1 << 7;
 95 
 96                 if( led_2 != 0 )
 97                     GPIO_SET = 1 << 8;
 98                 else
 99                     GPIO_CLR = 1 << 8;
100 
101                 if( led_3 != 0 )
102                     GPIO_SET = 1 << 9;
103                 else
104                     GPIO_CLR = 1 << 9;
105 
106                 if( led_4 != 0 )
107                     GPIO_SET = 1 << 10;
108                 else
109                     GPIO_CLR = 1 << 10;
110             }
111 
112             printf("Finished!\n");
113             break;
114 
115         default:
116             printf("Error: Please try again.\n");
117             break;
118     }
119 
120     return 0;
121 }

The code, which is fully functional, asks the user whether he/she will be sending or receiving signals:

When sending, the program will run through a while loop and count from 0 to 15 (binary counter), which will then turn on/off the necessary pins for the value to be sent over to the other pi (the receiver). The the rest is left to the receiving pi.

When receiving, the pi will set pins 4, 17, 27, and 22 as input, since they are the pins that will be receiving the data from the other pi. A while loop will then be run which sets led_1 equal to the value of the read gpio pin #4, led_2 equal to the value of the read gpio pin #17, etc., until all four input pins have been read, then the program will run the binary counter and display the data through pins 7 - 10.

A few problems occurred in the writing of this program that, after many hours of frustration and headaches, we were able to overcome.

1. Correctly setting Input and Output properly is CRUCIAL! - On multiple occasions, incorrectly setting I/O or not setting I/O at all messed the program up pretty good. The problem would be that the receiving pins would pick up random values and set LED's on at random, which is not what we wanted at all.
2. Setting up IF statements correctly - I first set up the if statements to read led_1-4 as being equal to 0, when I wanted to check when it wasn't. That would give me the correct results.