Uart loopback

I think a good place to start is to first make sure you can transmit a character and see it on an oscilloscope. Start out by bringing this pin to an output line and looking at it with an oscilloscope. You could also do this with Signaltap, but you may need to create a very deep memory b/c the serial transmission will likely be long relative to the clock frequency. My memory is that the UART tx line should be resting at a '1' normally, then toggling low during character times.

Good luck!

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Hello guys,

I have created a nios2 system with a uart port: uart_0

I have put the rx_uart and tx_uart in short circuit. So I expect that what I send is what I get.

Finally I have writtend this code:

#include <stdio.h># include <unistd.h># include <stdint.h># include <fcntl.h># include "sys/alt_stdio.h"# include "system.h"# include "altera_avalon_uart_regs.h"# include "altera_avalon_uart.h"
int main()
{
    uint8_t tx_var=0xFF;
    uint8_t rrdy=0x00;
    uint8_t trdy=0x00;
    uint8_t tmt=0x00;
    uint16_t status=0x00;
    uint8_t rx_var=0x00;
    //write the byte to be sent
    printf("Scrivo il byte da trasmettere.\n");
    IOWR_ALTERA_AVALON_UART_TXDATA(UART_0_BASE, tx_var);
    status=IORD_ALTERA_AVALON_UART_STATUS(UART_0_BASE);
    tmt=(status & ALTERA_AVALON_UART_STATUS_TMT_MSK)>>5;
    while(tmt != 1){ //Untill the byte is not trasmitted
        status=IORD_ALTERA_AVALON_UART_STATUS(UART_0_BASE);
        tmt=(status & ALTERA_AVALON_UART_STATUS_TMT_MSK)>>5;
    }
    rrdy=(status & ALTERA_AVALON_UART_STATUS_RRDY_MSK ) >> ALTERA_AVALON_UART_STATUS_RRDY_OFST;
    while(rrdy !=1){//until the byte is not received
        rrdy=(status & ALTERA_AVALON_UART_STATUS_RRDY_MSK ) >> ALTERA_AVALON_UART_STATUS_RRDY_OFST;
        printf("status RRDY bit= %x\n", rrdy);
    };
    rx_var = IORD_ALTERA_AVALON_UART_RXDATA(UART_0_BASE);
    printf("rx_data= %x\n\n", rx_var);
return 0;
}

The problem is that nothing is received. After i have executed the iowr_altera_avalon_uart_txdata(uart_0_base, tx_var); i can see the tmt bit of the status register change from 0 to 1 and from 1 to 0. So that it seems that the process of transmitting is going well. By the way I have noticed that the tx_uart pin i s always at 0 V. May be a bad pin assignment or I have to do something else to have the byte transmitted ?

I hope to have been sufficiently clear.

Have a nice day.

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