Introduction: 

Although the Port 1 bits can be controlled directly through BASIC using the PORT1 command, sometimes its easier to use assembly language calls. This method allows the individual bits to be changed instead of the whole port.

Background: 

The following table indicates the addresses for the Port 1 bits:

• P1.0 = 90h P1.4 = 94h P1.1 = 91h P1.5 = 95h 
• P1.2 = 92h P1.6 = 96h P1.3 = 93h P1.7 = 97h

If only a few of the bits are being controlled then the easiest way to implement this is by storing the machine code using data statements. The code to set, clear, or toggle a bit takes three bytes of memory (command, address, return).

The machine code commands are used as follows: 

NOTE: Assignments must begin with an integer, otherwise the processor will interpret them as a variable.

0D2h = set bit 0C2h = clear bit 0B2h = toggle bit (if set, clear it or if cleared, set it)

Program Listing: The sample program below shows how the commands are stored and then called:

10 DATA 0D2h, 94h, 22h : REM code to set bit 4 of port 1 
20 DATA 0C2h, 94h, 22h : REM code to clear bit 4 of port 1  
30 DATA 0B2h, 90h, 22h : REM code to toggle bit 0 of port 1 
40 FOR ADR = 4000h to 4008h : REM store the codes -this must be a free area of RAM!
50 READ DT 
60 XBY(ADR) = DT 
70 NEXT ADR 
80 REM all commands are now stored in RAM 
200 CALL 4000h : REM set bit 3 of port 1 
210 CALL 4003h : REM clear bit 3 of port 1 
220 CALL 4006h : REM toggle bit 0 of port 1 
230 GOTO 200 : REM continuous loop through the three commands

 This page is based on Micromint inc's application note AN103 Dated: 5/26/98