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Tips - I/O direction

How to program outputs?

Just remember that a pin is an input if the corresponding bit in the associated TRISx register is set.
PMP has no specifiec built-in instruction for manipulating such register, simply affect a value directly to the register.

An example with TRISB:

TRISB := $­­­­­0F; // PORTB 0..3 as inputs, 4..7 as outputs
PORTB := $­­­­­F0; // set PORTB 4..7 high
TRISB.0 := false; // PORTB.0 as output, immediately set low (PORTB.0 was 0)
// Remember that every output pin has a latch even if it is set as an input, the latch works...

Manipulating the TRISx bit is a way to do fast 0 output with an external pullup (ofen used in bidirectional i/o such as I2C).

Another example with the full PMP syntax possibilities:

program test;
var


MyInputPin1 : boolean @PORTB.1;
MyInputPin2 : boolean @PORTB.4;
MyOutputPin1 : boolean @PORTB.2;

begin

TRISB := [MyInputPin1, MyInputPin1]; // set mask to pins that are inputs
MyOutputPin1 := MyInputPin1 or MyInputPin2;

end.

Help! There is no TRISx in my device!

For old and small devices that had no TRISx register, PMP (V1.05 and later) provides a special built-in procedure TRIS(Port, Mask) that maps to the TRISx register if it exists or to a TRIS instruction otherwise. The problem with TRIS instruction (of such device in fact) is that it is write only, you cannot read what is the current mask.


Creation date : 2007.06.22 7:37 PM
Last update : 2013.08.05 11:24 AM
Category : Tips
Page read 8985 times


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