AN101

Product: 80C52 BASIC

 Reading 80C52 Port 3 Bits

 Date: 7/28/99

 Introduction: While BASIC-52 allows you to access Port 1 bits directly through BASIC, it does not allow direct access of the Port 3 bits available on many of the Micromint boards and modules

Background: Port 3 is an 8 bit bi-directional I/O port with internal pullups.

                        Port Pin                   Alternaye Function

                           P3.0                      RXD (serial input port)

                           P3.1                      TXD (serial output port)

                           P3.2                      *INT0 (external interrupt 0)

                           P3.3                      *INT1 (external interrupt 1)

                           P3.4                      T0 (timer 0 external input)

                           P3.5                      T1 (timer 1 external input)

                           P3.6                      *WR (external Data Memory write strobe)

                           P3.7                       RD (external Data Memory read strobe)

Program Listing:

Example 1: Waiting for a bit to be set or cleared

BASIC-52 program
.
.
.
CALL xxxxh

Assembly routine

xxxxh Label1: JB P3.x, Label1 ;Loop until bit is set
RET ;Return to BASIC program

or

xxxxh Label1: JNB P3.x,Label1 ;Loop until bit is cleared
RET ;Return to BASIC program

Example 2: Return the state of a bit to BASIC-52

BASIC-52 program
.
.
.
CALL xxxxh

Assembly routine

Xxxxh Label1: JB P3.x, B_SET ;Test the bit, jump if set
B_CLR: MOV 20h, #0 ;Store 0 if cleared
RET

B_SET: MOV 20h, #1 ;Store 1 if set
RET

Once you are back in the BASIC program use the following line to read what was stored:

IF DBY(20h)=0 THEN PRINT "P3.x = Low" ELSE PRINT "P3.x = High"