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 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"

This page is based on Micromint inc's AN101 Dated: 7/28/99