µVISION DEBUGGER: Problems Simulating the Serial Interface

Information in this article applies to:

• C251 Version 3.20
• C51 Version 6.20
• µVision Version 2.20

QUESTION

I'm using the µVision Debugger to simulate my serial interface and I'm having problems receiving characters. My function to receive characters appears as follows:

char getch (void)  {
  while (!RI);
  RI = 0;
  return (SBUF);
}

This function always returns a value of zero when I run it in the simulator. It seems to work on the real hardware so why does the µVision simulator fail?

ANSWER

This function is potentially unsafe because it resets the RI bit before reading the SBUF register. In the real hardware, this code may work most of the time. However, the RI bit is being cleared too early. The µVision debugger always returns 0 in such cases. You should change the function to the following:

char getch (void)  {
  char ch;

  while (!RI);
  ch = SBUF;
  RI = 0;
  return (ch);
}

SEE ALSO

• µVision DEBUGGER: Using The SIN VTREG
• µVision DEBUGGER: Using the STIME VTReg

Last Reviewed: Tuesday, February 23, 2021