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Jump Table warning, code has no effect

 
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shson



Joined: 12 Feb 2016
Posts: 11

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Jump Table warning, code has no effect
PostPosted: Mon Feb 20, 2017 6:19 pm     Reply with quote

CCS V5.065, PIC18F44K22

Hello

I have a couple of 8:1 analogue multiplexers, and in my program depending on the value of unsigned int8 variable 'number' I need to use 'output_high()' and 'output_low()' to send output selector signal to the multiplexers. I first coded using switch statement like so:

Code:

unsigned int8 number = 0;

void MUX_Select(void)
{
  if (number > 16)
    number = 0;

  switch (number)
  {
    case 0:
      output_high(PIN_D1);
      output_low(PIN_D2);
      output_high(PIN_D3);
      break;
   
    // ... case 0 through to 16

    case 16:
      output_low(PIN_A3);
      output_low(PIN_B0);
      output_high(PIN_C1);
      break;

    default:
      break;
  }
  number++;
}


The code compiles and works on the breadboard as I wanted to. But I just learned about jump tables and since the variable 'number' only goes to 16, I wanted to try implement it using jump table like this:

Code:

typedef void (*FuncPtr)(void);

FuncPtr SWSenseSel[16] = { MUX_VS0, MUX_VS1, ... through to, MUX_VS15 };

void MUX_VS0(void)  // equivalent to case 0 using switch
{
      output_high(PIN_D1);
      output_low(PIN_D2);
      output_high(PIN_D3);
}

// ... case 0 through to 15

void MUX_VS15(void)  // equivalent to case 16 using switch
{
      output_low(PIN_A3);
      output_low(PIN_B0);
      output_high(PIN_C1);
}

void main(void)
{
  unsigned int8 number = 0;

  while(TRUE)
  {
    SWSenseSel[number]; //Code has no effect warning on this line

    //Now correct MUX channel is connected using selector, get ADC reading here
  }
}


I get "Code has no effect" warning on "SWSenseSel[number]", I can't seem to understand why it would have no effect. Should a jump table always return something?

Thank you
Eric
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Mon Feb 20, 2017 9:27 pm     Reply with quote

I made the necessary changes to make it work. See below. I also added
some printf's so I could test it in MPLAB vs. 8.92 simulator. It displays
the following in the UART1 output window. It's working:
Quote:

MUX_VS0
MUX_VS15

Test program:
Code:

#include <18F44K22.h>
#fuses INTRC_IO,NOWDT,PUT,BROWNOUT
#use delay(clock=4M)
#use rs232(baud=9600, UART1, ERRORS)

void MUX_VS0(void)   
{
printf("MUX_VS0 \r");
output_high(PIN_D1);
output_low(PIN_D2);
output_high(PIN_D3);
}

void MUX_VS15(void)   
{
printf("MUX_VS15 \r");
output_low(PIN_A3);
output_low(PIN_B0);
output_high(PIN_C1);
}

typedef void (*FuncPtr)(void);

FuncPtr SWSenseSel[2] = {MUX_VS0, MUX_VS15};

//======================================
void main(void)
{
int8 i = 0;

(*SWSenseSel[i])(); 

i++;   

(*SWSenseSel[i])(); 

while(TRUE);
}
 

 
Ttelmah



Joined: 11 Mar 2010
Posts: 19494

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PostPosted: Tue Feb 21, 2017 12:07 pm     Reply with quote

It's also worth just remarking that if you get rid of the default in the switch case code, the compiler will automatically generate a jump table for you. Given that you already test for a value outside the case range, there is no point in the default anyway...
shson



Joined: 12 Feb 2016
Posts: 11

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PostPosted: Wed Feb 22, 2017 3:58 am     Reply with quote

@PCM

Replacing SWSenseSel[i]; with (*SWSenseSel[i])(); made it work, rather simple oversight, the fact that it is a pointer itself rather than a straight array.

@Ttelmah

That makes sense, I'll try to compile switch w/o default case and now-working jump tables and compare the LST to better understand CCS's behaviour.

Thank you
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