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Pass a variable to Output_High ?

 
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laser47
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Pass a variable to Output_High ?
PostPosted: Sat May 17, 2003 2:33 am     Reply with quote

I'm trying to set up a sequencer where I have an array of events, and wat to pass a variable to Output_High/Output_Low. I've got an array of a structure that has time, pin, and high/low. I'm trying something like:

if (EventArray[CurrentEvent].Value = 1)
Output_High (EventArray[CurrentEvent].Pin);
else
Output_Low (EventArray[CurrentEvent].Pin);

the .pin element is equated to PIN_B5, but the compiler complains with "Must Equate to a Constant"

I could accomplish this with:

if (EventArray[CurrentEvent].Pin = PIN_B5)
if (EventArray[CurrentEvent].Value = 1)
Output_High (PIN_B5);
else
Output_Low (PIN_B5);
else if (EventArray[CurrentEvent].Pin = PIN_B6)
if (EventArray[CurrentEvent].Value = 1)
Output_High (PIN_B6);
else
Output_Low (PIN_B6);
.... Etc...

This seems counter-intuative, and very poor for expandability. Any suggestions?

Thanx!

Laser
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Original Post ID: 144514521
R.J.Hamlett
Guest







Re: Pass a variable to Output_High ?
PostPosted: Mon May 19, 2003 5:20 am     Reply with quote

:=I'm trying to set up a sequencer where I have an array of events, and wat to pass a variable to Output_High/Output_Low. I've got an array of a structure that has time, pin, and high/low. I'm trying something like:
:=
:=if (EventArray[CurrentEvent].Value = 1)
:= Output_High (EventArray[CurrentEvent].Pin);
:=else
:= Output_Low (EventArray[CurrentEvent].Pin);
:=
:=the .pin element is equated to PIN_B5, but the compiler complains with "Must Equate to a Constant"
:=
:=I could accomplish this with:
:=
:=if (EventArray[CurrentEvent].Pin = PIN_B5)
:= if (EventArray[CurrentEvent].Value = 1)
:= Output_High (PIN_B5);
:= else
:= Output_Low (PIN_B5);
:=else if (EventArray[CurrentEvent].Pin = PIN_B6)
:= if (EventArray[CurrentEvent].Value = 1)
:= Output_High (PIN_B6);
:= else
:= Output_Low (PIN_B6);
:=.... Etc...
:=
:=This seems counter-intuative, and very poor for expandability. Any suggestions?
:=
:=Thanx!
:=
:=Laser
These functions require constants that are present at compile time. They do not support dynamic modification of the bit to be changed, using a variable. However, there are a lot of other ways you can approach this.
For instance, deal with the port as a byte (using the output_b function), and maintain your own byte mask, and output this. to the byte. You can then use the standard bit set/reset commands on your byte.
You can also use the bit commands directly on the port itself (but with the normal caveats about read-modify-write operations, and assuming you are using 'fast_io').
So something like:

#byte PORTB=0x106 //This will depend on the chip involved.

if (EventArray[CurrentEvent].Value)
//Warning, your code here would not work, '=' means to set
//a value. '==' is the test function...
bit_set(PORTB,EventArray[CurrentEvent].Pin);
else
bit_clear(PORTB,EventArray[CurrentEvent].Pin);

With the .Pin values being from 0 to 7.

Best Wishes
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Original Post ID: 144514545
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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Re: Pass a variable to Output_High ?
PostPosted: Mon May 19, 2003 12:31 pm     Reply with quote

Here is a general purpose bit setting function.
The problem is that if the compiler knows the bit
value at compile-time, it can generate really efficient
code. But if it can only learn the bit value at run-time,
then it's not very efficient. If you look at the
.LST file, there's a lot of code, and it's going
to be slow. I don't know if that's an issue for you.

This issue is heavily discussed on a Piclist page:
<a href="http://www.piclist.com/techref/microchip/math/bit/setbit.htm" TARGET="_blank">http://www.piclist.com/techref/microchip/math/bit/setbit.htm</a>
CCS appears to use a method of generating the bitmask
that's somewhat similar to Kevin Blain's code.

<PRE>
#include "c:\Program Files\Picc\Devices\16F877.H"
#fuses HS, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 8000000)
#use rs232(baud = 9600, xmit=PIN_C6, rcv = PIN_C7, ERRORS)
<BR>
#define PORT_B 6 // Note this is a define, not a #byte statement
<BR>
void set_port_bit(char *port, char bit, char value);
<BR>
//=====================================
void main()
{
char port;
char bit;
char value;
<BR>
// Define the pin as an output and initialize
// it to a low level. (Or, do this with a
// set_tris_b() statement, etc.).
output_low(PIN_B7);
<BR>
// Initialize the port and bit values for this test.
port = PORT_B;
bit = 7;
<BR>
// Toggle the specified pin. Watch it on the oscilloscope.
while(1)
{
value = 1;
set_port_bit(port, bit, value);
delay_us(500);
<BR>
value = 0;
set_port_bit(port, bit, value);
delay_us(500);
}
<BR>
}
<BR>
//========================================================
// This function will set a specified bit (0-7) of an i/o
// port to the specified value (0 or 1). The port number
// must be 5 for Port A, 6 for Port B, etc. (Per the data
// sheet for a 16F877).
void set_port_bit(char *port, char bit, char value)
{
if(value)
bit_set(*port, bit);
else
bit_clear(*port, bit);
}
</PRE>
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