Debugger "Port 1 Error"

Using uVision 4 on a C51 project (SiLabs F120 processor): I have a mixed use port (Port3) that has some bits defined as inputs and others as outputs. How can I stimulate the input pins without generating the "Port 1 Error - You have tried to modify a port pin which is configured as an output"?

FYI, my signal function is defined as follows:

signal void Toggle_ACCS (void)  {
  twatch(10000);
  PORT3 = (PORT3 ^ 0x01);
}