7.5.2. Test Program to Run on the Or1ksim ISS

The test program, uart-loop.c is a simple polling loop back driver of the UART. Characters are read and immediately echoed back.

A volatile structure is declared for the UART registers, with #defined constants for the base address and the register bits of interest.

#define BASEADDR   0x90000000
#define BAUD_RATE        9600
#define CLOCK_RATE  100000000           // 100 Mhz

struct uart16450
{
  volatile unsigned char  buf;          // R/W: Rx & Tx buffer when DLAB=0  
  volatile unsigned char  ier;          // R/W: Interrupt Enable Register   
  volatile unsigned char  iir;          // R: Interrupt ID Register         
  volatile unsigned char  lcr;          // R/W: Line Control Register       
  volatile unsigned char  mcr;          // W: Modem Control Register        
  volatile unsigned char  lsr;          // R: Line Status Register          
  volatile unsigned char  msr;          // R: Modem Status Register         
  volatile unsigned char  scr;          // R/W: Scratch Register            
};

#define UART_LSR_TEMT   0x40            // Transmitter serial register empty
#define UART_LSR_THRE   0x20            // Transmitter holding register empty
#define UART_LSR_DR     0x01            // Receiver data ready

#define UART_LCR_DLAB   0x80            // Divisor latch access bit
#define UART_LCR_8BITS  0x03		// 8 bit data bits

The utility functions to set and clear flags in the UART (see Section 6.5) are reused here, modified for C rather than C++ and volatile register arguments. They are included from the file binutils.c

#include "bitutils.c"

The main program declares a pointer to the UART register structure, uart, at the base address. Initialization requires setting the divisor latch, to divide the main clock down to 16 x the baud rate and setting 8-bit data.

  volatile struct uart16450 *uart = (struct uart16450 *)BASEADDR;
  unsigned short int         divisor;

  divisor = CLOCK_RATE/16/BAUD_RATE;            // DL is for 16x baud rate

  set( &(uart->lcr), UART_LCR_DLAB );           // Set the divisor latch
  uart->buf  = (unsigned char)( divisor       & 0x00ff);
  uart->ier  = (unsigned char)((divisor >> 8) & 0x00ff);
  clr( &(uart->lcr), UART_LCR_DLAB );

  set( &(uart->lcr), UART_LCR_8BITS );		// Set 8 bit data
  packet

The remainder of the program is a perpetual loop:

Wait for a character in the read buffer (flag DR of the line status register is set).
Read the character from the buffer and print it.
Wait for the transmit buffer to clear (flags TEMT and THRE of the line status register are set).
Write the character back.

  while( 1 ) {
    unsigned char  ch;

    do {                        // Loop until a char is available
      ;
    } while( is_clr(uart->lsr, UART_LSR_DR) );

    ch = uart->buf;

    simputs( "Read: '" );       // Log what was read
    simputc( ch );
    simputs( "'\n" );

    do {                        // Loop until the transmit register is free
      ;
    } while( is_clr( uart->lsr, UART_LSR_TEMT | UART_LSR_THRE ) );
      
    uart->buf = ch;
  }

The source code for the UART loopback program may be found in progs-or32/uart-loop.c in the distribution. It will be built using the OpenRISC 1000 tool chain as part of the overall system build.