DE1-SoC displaying LEDs


I am trying to use the DE1-SoC board to run this program. It is supposed to allow the user to input a character, and return that letter in binary on red LEDs on the board. It uses two functions that take in user input and displays the execution to the terminal. When I run the program, random characters (like å) get output and not regular characters.

Here's my code.

#include "JTAG_UART.h"
#include "address_map_arm.h"

int main(void) {
    /* Declare volatile pointers to I/O registers (volatile means that IO load
       and store instructions will be used to access these pointer locations,
       instead of regular memory loads and stores) */
    volatile int * JTAG_UART_ptr = (int *)JTAG_UART_BASE; // JTAG UART address
volatile int * LED_ptr = (int*)LED_BASE;
    char  text_string[] = "\nJTAG UART example code\n> \0";
    char *str, * c;
  //  char *c_ptr=c;

    /* print a text string */
    for (str = text_string; *str != 0; ++str)
        put_jtag(JTAG_UART_ptr, *str);

    /* read and echo characters */
    while (1) {
        c = get_jtag(JTAG_UART_ptr);
         if (c != 0 && c<123 && c>96){
         *LED_ptr = *c ;
         put_jtag(JTAG_UART_ptr, *c);
           // put_jtag(JTAG_UART_ptr, c);

This is the code for the functions I referenced.

#include "JTAG_UART.h"

 * Subroutine to send a character to the JTAG UART
void put_jtag(volatile int * JTAG_UART_ptr, char c) {
    int control;
    control = *(JTAG_UART_ptr + 1); // read the JTAG_UART control register
    if (control & 0xFFFF0000)       // if space, echo character, else ignore
        *(JTAG_UART_ptr) = c;

 * Subroutine to read a character from the JTAG UART
 * Returns \0 if no character, otherwise returns the character
char get_jtag(volatile int * JTAG_UART_ptr) {
    int data;
    data = *(JTAG_UART_ptr); // read the JTAG_UART data register
    if (data & 0x00008000)   // check RVALID to see if there is new data
        return ((char)data & 0xFF);
        return ('\0');

Inputting a character like 'a' which is the decimal number 97 in ASCII. Should display itself as 01100001 with each '1' representing itself lit up on the board. As I stated I'm having a logic error, while input is being read 'a' would appear as 00010000

asked on Stack Overflow May 18, 2020 by SS LBP • edited May 22, 2020 by Peter Cordes

1 Answer


You have defined c as a char* when it should clearly be a char.

char c ;

Then loose the *c de-references:

*LED_ptr = c ;
put_jtag(JTAG_UART_ptr, c);

The line:

 c = get_jtag(JTAG_UART_ptr);

should have issued a warning; GCC for example outputs:

warning: initialization makes pointer from integer without a cast [-Wint-conversion]

Do not ignore (or disable) warnings; at least don't ignore them then ask a question here without mentioning the warning.

answered on Stack Overflow May 18, 2020 by Clifford • edited May 18, 2020 by Clifford

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