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Question about output_bit() function

 
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lighty



Joined: 29 Jan 2013
Posts: 1

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PostPosted: Tue Jan 29, 2013 6:45 am     Reply with quote
This question might seem odd but I'm coming from ANSI C compliant compiler and I'm trying to understand CCS syntax.

For example is this expression
Code:
output_bit(ONE_WIRE_PIN, shift_right(&data,1,0)); // set output bit on 1-wire


the equivalent of this one?

Code:
      data >>= 1;
      data &= 0x7F;
      ONE_WIRE_PIN = data;
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Tue Jan 29, 2013 4:33 pm     Reply with quote
The easiest way to see what the compiler is doing, is to make a small
test program, compile it and look at the .LST file. Then add comments
to the ASM code to describe what's happening:
Code:

..... output_bit(ONE_WIRE_PIN, shift_right(&data,1,0)); 
0032:  BCF    STATUS.C   // Clear the Carry bit   
0034:  RRCF   data,F     // Rotate data right, so Carry = LSB of data
0036:  BC     003C       // Jump to 003C if Carry bit = 1
0038:  BCF    LATB.LATB0 // If Carry = 0, set the pin = 0
003A:  BRA    003E
003C:  BSF    LATB.LATB0 // If Carry = 1, set the pin = 1
003E:  BCF    TRISB.RB0  // Set TRIS on pin = output

Test program (compiled with vs. 4.140):
Code:

#include <18F4520.h>
#fuses INTRC_IO,NOWDT,PUT,BROWNOUT,NOLVP
#use delay(clock=4M)
#use rs232(baud=9600, UART1, ERRORS)

#define ONE_WIRE_PIN  PIN_B0

//======================================
void main(void)
{
int8 data;

output_bit(ONE_WIRE_PIN, shift_right(&data,1,0));

while(1);
}
 
RF_Developer



Joined: 07 Feb 2011
Posts: 839

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PostPosted: Wed Jan 30, 2013 3:02 am     Reply with quote
lighty wrote:
This question might seem odd but I'm coming from ANSI C compliant compiler and I'm trying to understand CCS syntax.

For example is this expression
Code:
output_bit(ONE_WIRE_PIN, shift_right(&data,1,0)); // set output bit on 1-wire


the equivalent of this one?

Code:
      data >>= 1;
      data &= 0x7F;
      ONE_WIRE_PIN = data;


This is NOT an ANSI compliance issue. It's an understanding CCS library functions issue. CCS C can be set to be (mostly) ANSI compliant, and you'll still have those functions. They are simply library functions implemented by the compiler as are pretty much essential for all embedded programming. Realistically there cannot be any totally ANSI compliant and therefore fully portable embedded C code. There will always be some hardware dependence, which requires some sort of processor or hardware family-specific non-portable code of some kind; for example pin I/O. CCS C tries, generally successfully, to abstract all that and provide a set of library functions and the like that cover almost all PIC processors.

The answer to your question however is no. The equivalent, assuming data was a byte, which is what the shift function call suggests, would be:

Code:
     
      ONE_WIRE_PIN = (int1)(data & 0x01);
      data >>= 1;


This is because the function shift_right, shifts the single byte pointed to by the address of data by one bit, shifting in a zero - which is a don't care - and returning the old LSB. The reason this function is provided is that it maps far better onto the opcodes provided by the PIC hardware than any generic version such as ANSI C. In other words its a processor specific optimisation.

This code is unlikely to be on its own, it only makes sense as part of a loop, probably through all eight bits of a byte. Though you clearly are unlikely to code it that way as output bit with a variable is inefficent. Much better to do something like:
Code:

if (shift_right(&data,1,0))
{
   output_high(ONE_WIRE_PIN); // set output bit on 1-wire
}
else
{
   output_low(ONE_WIRE_PIN); // set output bit on 1-wire
}


RF Developer
Ttelmah



Joined: 11 Mar 2010
Posts: 19498

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PostPosted: Wed Jan 30, 2013 3:22 am     Reply with quote
The 'equivalence' question is also missing another thing. 'ONE_WIRE_PIN', is just a number. It is not physically the pin, and can't be written directly. If you read the manual for 'output_bit', it tells you:
"The actual number is a bit address. For example, port a (byte 5 ) bit 3 would have a value of 5*8+3 or 43"

So using the example number above, "ONE_WIRE_PIN = data;"

would translate as "43 = data;".

Duh......

Best Wishes
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