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Passing #DEFINE values.

 
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beaker404



Joined: 24 Jul 2012
Posts: 163

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Passing #DEFINE values.
PostPosted: Thu Jun 03, 2021 8:29 am     Reply with quote
Question around passing variables in a function.

I have the following #DEFINE value:

Code:

#DEFINE ADS124S08_RATE         0x04      // Sample Rate Register


passing this information like below to a write register function causing a compiler error: Specifically, a 'identifier needed' error

Code:
ADS124S08_write_a_reg(ADS124S08_RATE,0x35);   


however, if I do the following, creating a variable and assigning the #DEFINE value to it, it compiles and runs fine and I am able to write the register and read back to verify.

Code:
register_pass_num = ADS124S08_RATE;
    ADS124S08_write_a_reg(register_pass_num,0x35);   


So my question is why cannot I pass the #DEFINE value in a function call?
I tried passing by value and reference, neither worked.

To me, the question is about the passing of the variable, so that is all the code I posted to keep the thread simple. If more code is needed, will be glad to post.

CCS: 5.094
MPLAB 8.92
Windows 10
processor: 18F23K22
Ttelmah



Joined: 11 Mar 2010
Posts: 19515

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PostPosted: Thu Jun 03, 2021 8:37 am     Reply with quote
This depends on how
ADS124S08_write_a_reg
declares it's first variable.

You can pass #defined values to a function fine, provided they are passed
to the function normally.
However if a value is passed 'by reference', then it needs to be an actual
variable.

So you need to show the actual declaration line of ADS124S08_write_a_reg
I'd guess it'll have something like:

ADS124S08_write_a_reg(int8 &reg, int8 val)

The '&' here says this first variable is to be passed by reference.
beaker404



Joined: 24 Jul 2012
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PostPosted: Thu Jun 03, 2021 8:49 am     Reply with quote
Here is the ADS124S08_write_a_reg code:
Code:
void ADS124S08_write_a_reg(int REG,int value) {
// test for writing a register

output_low(CS_ADC);
delay_ms(500);
SPI_XFER_124S08(ADS124S08_WREG | REG);   // write to reg
SPI_XFER_124S08(0x00);                              // write to only 1 reg
delay_ms(1000);

SPI_XFER_124S08(value);                              // write value to reg
delay_ms(1000);
output_high(CS_ADC);

} // end ADS124S08_write_reg()


This is working code and the ADS124S08 works properly, the extra delays are just so I can watch activity on the scope.

CS_ADC is defined as follows:

Code:
#DEFINE   CS_ADC  PIN_B4      


One side note, the #DEFINE for the CS_ADC occurs in my main .C file of the project. The #DEFINE for the ADS124S08 occurs in a .H included file housing all the ADS124S08 functions.

the SPI_XFER is renamed to the following in the same .H file for code readability and stream definition:

Code:
#DEFINE SPI_XFER_124S08(x)       SPI_XFER(STREAM_SPI1,x)    // SPI1 stream for 124S08 communication
allenhuffman



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PostPosted: Fri Jun 04, 2021 12:27 pm     Reply with quote
Unless there's a compiler quirk (I have stumbled into many, usually quickly fixed by CCS), these should be the same:

Code:
void function (int x);

#define VALUE 42

function (42);
function (VALUE);

_________________
Allen C. Huffman, Sub-Etha Software (est. 1990) http://www.subethasoftware.com
Embedded C, Arduino, MSP430, ESP8266/32, BASIC Stamp and PIC24 programmer.
http://www.whywouldyouwanttodothat.com ?
Ttelmah



Joined: 11 Mar 2010
Posts: 19515

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PostPosted: Sat Jun 05, 2021 12:18 am     Reply with quote
I've just tested with his compiler version, and as posted, it merrily works:
Code:

#include <18F23K22.h>
#device ADC=10

#FUSES NOWDT                    //No Watch Dog Timer

#use delay(internal=16000000)

#use spi (MASTER, SPI1, MODE=0, BITS=8, BAUD=1000000, STREAM=STREAM_SPI1) //hardware SPI

#DEFINE   CS_ADC  PIN_B4 

#DEFINE SPI_XFER_124S08(x)       SPI_XFER(STREAM_SPI1,x)    // SPI1 stream for 124S08 communication

#DEFINE ADS124S08_RATE         0x04      // Sample Rate Register
#DEFINE ADS124S08_WREG         0x00      // Just a value to make this work

void ADS124S08_write_a_reg(int REG,int value) {
// test for writing a register

output_low(CS_ADC);
delay_ms(500);
SPI_XFER_124S08(ADS124S08_WREG | REG);   // write to reg
SPI_XFER_124S08(0x00);                              // write to only 1 reg
delay_ms(1000);

SPI_XFER_124S08(value);                              // write value to reg
delay_ms(1000);
output_high(CS_ADC);

} // end ADS124S08_write_reg()   

void main()
{
   while(TRUE)
   {         
       ADS124S08_write_a_reg(ADS124S08_RATE,0x35);
       delay_ms(1000);
   }
}


I've just created a basic chip setup, and a hardware SPI setup.
This compiles correctly.

My guess would be that there is possibly a hidden typing error (there are
some invisible characters that can get typed into a line that will then stop
it working). I also don't 'like' using 'REG' in capitals for a variable. There is
an old standard in C, that 'ALL CAPITALS' should be reserved for defines,
and using 'REG', possibly brings the risk of it also being an already used
word.
This might depend on what else he is including, but a lot of the standard
libraries do use names like this internally. A 'caveat'...
This is why it is much better to make names 'informative', so something like

Reg_number
Jerson



Joined: 31 Jul 2009
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Re: Passing #DEFINE values.
PostPosted: Sat Jun 05, 2021 1:51 am     Reply with quote
beaker404 wrote:
Question around passing variables in a function.

I have the following #DEFINE value:

Code:

#DEFINE ADS124S08_RATE         0x04      // Sample Rate Register


passing this information like below to a write register function causing a compiler error: Specifically, a 'identifier needed' error

Code:
ADS124S08_write_a_reg(ADS124S08_RATE,0x35);   




A #define simply copies the string literally upto the end of line in a cut-paste type operation. So, when you call
Code:
ADS124S08_write_a_reg(ADS124S08_RATE,0x35);   
the compiler replaces this as
Code:
ADS124S08_write_a_reg(0x04    // Sample Rate Register ,0x35);   


Now, you see, the comment to end of line // causes the compler to complain about missing variable ,0x35 since it is supposed to comment till end of line.

You could possibly try this for the #define
Code:

#DEFINE ADS124S08_RATE         0x04      /* Sample Rate Register */

This should work correctly (not verified)
Ttelmah



Joined: 11 Mar 2010
Posts: 19515

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PostPosted: Sat Jun 05, 2021 8:23 am     Reply with quote
No, it doesn't.
Remarks are respected.
If you look, I show the code being compiled with the remarks there,
and if you look at the code generated, it respects the remarks, and only
passes the 0x4 to the function.
I tested with 5.094.
Compilers are not guaranteed to respect remarks after the #define, which is
why I tested with his compiler version.
However the compiler is meant to remove remarks as the very first
operation (so normally the 'pre-compile' phase).
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