Problem with printing function return value

avrcmicrocontrollerprogramming

I am new to programming. I have small doubt, I know this is simple question but I am confused. I have the following function:

void ReadAdConfReg(void)
{              
    SPCR = 0x5D;
    ADC_CS = 0;      
    while (MISO_PIN != 0) ;
    spi(0x50);
    adcConfig = spi(0xFF);    
    adcConfig = (adcConfig << 8) | spi(0xFF);  
    ADC_CS = 1;    
}

I have declared adcConfig as global variable: unsigned int adcConfig.

How can I print the adcConfig value. Because this is a void function, it doesn't return any value.

I have tried like this:

ReadAdConfReg();
printf("configreg:%d",adcConfig);

Is it wrong? Or how can I print adcConfig value.

Controller is ATMega32A, compiler CodeVisionAVR.

Best Answer

If need the function to return a value it can't be declared void. As you have it now you have to call the function first, then pass the global variable into the printf function. So what you are doing should work.

To use the function with a return value, do something like this:

unsigned int ReadAdConfReg(void)
{        
    unsigned int retVal;
    SPCR = 0x5D;
    ADC_CS = 0;      
    while (MISO_PIN != 0) ;
    spi(0x50);
    adcConfig = spi(0xFF);    
    adcConfig = (adcConfig << 8) | spi(0xFF);  
    ADC_CS = 1;
    return acdConfig;    
}

(note - the previous edit used a local retVal, whilst leaving the adcConfig variable present which may have been confusing. You can pass the adcConfig directly as it's the correct type. If you want the function to be independent of the adcConfig though, use the local retVal instead in place of the adcConfig - this is probably better than using a global variable)

Then you can do:

printf("configreg:%d", ReadAdcConfigReg());

Related Solutions

Electronic – Creating an ASM file that will return the characters located in 8 consecutive registers

Step back and think about the whole problem. No, you haven't done A, B, and D yet. There are so many things wrong here that demonstrate such major misunderstanding that I don't know how to begin explaining. All I can do is enumerate some problems:

You haven't defined any variables at all. All EQU does is create a symbol of a particular value. It has nothing to do with allocating variables. Only the RES directive does that.
You seem to be writing this in absolute mode. You didn't say what compiler you are using, but most likely it only works in relocatable mode.
You don't need a label for every byte in the buffer. The MapNamex names are clutter that gives the impression these names will be used separately. In reality, you'll never use them because you will be indexing into the buffer at run time.
This code is written like it's the single stand-alone module of a project. Clearly this can't be if there is at least one C routine in the project.
The C runtime library needs to be initialized at startup, but you are preventing that by trying to own the reset vector.
You are running your routine directly from the reset vector. That makes no sense since it is intended to be called from C. You need to create a subroutine.
There is no place for the code to go two instructions after getMapChar. Your code just ends, so the machine will execute whatever happens to be in memory after the last instruction in this module. That makes no sense.

You need to step back and try to understand how compiled code gets run on the machine, what the subroutine linkage conventions are, and then how to implement them in assembler to make a C function that returns a byte. Try to understand the role of compiler, assembler, and linker and how they interact. You're nowhere near ready for this problem yet.

Electronic – bit banging

Bit banging is creating the whole series of pulses in software, instead of relying on a piece of hardware inside the microcontroller.

Many microcontrollers have a hardware SPI, and then all you have to do is write a byte to the output register, and the SPI controller will shift the data out, and at the same time receive data from the slave. You can get an interrupt when the transfer is complete, and then read the received data.

But some microcontrollers don't have the SPI hardware on board and then you have to simulate it by doing everything manually. SPI has a number of different modes, I'll use this pulse diagram as an example:

enter image description here

So while a dedicated SPI controller takes care of all the pulses, data shifting and timing, when bit-banging you have to take every action yourself:

Make Slave Select low  
Short delay
Do 8 times
  Make the SCK (Serial Clock) pin low 
  Make the MOSI (Master-Out-Slave-In) pin high or low depending on bit 7 of the data  
  Add brief delay  
  Make the SCK output high
  Read MISO (Master-In-Slave-Out) pin
  Shift received data left, and shift the bit just read in as bit 0   
  Add brief delay  
  Shift the data byte 1 bit left
Make Slave Select high again

Bit-banging SPI is relatively simple, the code for bit-banging I2C for instance will be more complex, and you'll need a timer somehow if you want to bit-bang the UART protocol.

Best Answer

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Electronic – bit banging

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