typecasting char variable into unsigned int - c

Question

I am trying to convert char variables into Unsigned int. my code is
char spi(char data)
{
//Start transmision
SPDR = data;
//Wait for transmision complete
while(!(SPSR & 0x80));
return SPDR;
}
unsigned int ReadAd(void)
{
unsigned int data;
ChipSelectAd(1);
//Read data
CheckStatus();
spi(0x58);
data = (spi(0xFF)<< 8);
data |= spi(0xFF);
return data;
}
Actually my problem is The spi function return an 8bit char so the above code shifts left 8bits the char variable and then assigns it to a 16bit variable, the result will always be 0.
In order to actually shift the data to the left I need to typecast them first to a 16bit type variable. I have tried like this
char spi(char data)
{
//Start transmision
SPDR = data;
//Wait for transmision complete
while(!(SPSR & 0x80));
return SPDR;
}
unsigned int ReadAd(void)
{
unsigned int data;
ChipSelectAd(1);
//Read data
CheckStatus();
spi(0x58);
data = (unsigned int)((unsigned char)spi(0xFF)<< 8);
data |= (unsigned int)((unsigned char)spi(0xFF));
return data;
}
void CheckStatus(void)
{
//char adcStatus;
adcStatus = 0xFF;
//Read status
while(!(adcStatus & 0x80))
{
spi(0x40);
adcStatus = spi(0xFF);
}
}
void ChipSelectAd(char s)
{
if(s == 1){
PORTB.3 = 0; //Switch on ADC
//while(PINB.3); //Wait for chip select pin
}
else
PORTB.3 = 1; //Switch off ADC
}
its not working. please suggest me which function i have to use.
Thanks in advance.

Answer 1

Your problem is not the casts. It works the same without them, due to the integral promotion rules.
#include <stdio.h>
char spi ( char data )
{
char SPDR = data;
return SPDR;
}
unsigned int ReadAd ( void )
{
unsigned int data;
data = spi ( 0x81 ) << 8;
data |= spi ( 0x42 );
return data;
}
int main ( void )
{
printf ( "Result %x\n", ReadAd() );
return 0;
}
This outputs Result ffff8142 on a system where char is a signed type. To get to the real problem, try assinging the values of spi() calls to variables and then print their values. Please also show us the declaration/definition of SPDR.

Answer 2

1) in embedded systems: get rid of the standard integer types, the default char type in particular. If you have a modern compiler, use uint8_t, uint16_t etc from stdint.h. If you have an ancient compiler, then typedef unsigned char uint8_t and so on. If you aren't using unsigned types of known size in embedded systems, you are asking for bugs bugs bugs.
2) Learn and understand integer promotion rules. It is frightening how many programmers there are that don't know about them or understand them.
Once you have the two above fundamentals sorted out, your code should look something like this code:
(I took the liberty to fix various suspicious, possible bugs and the crappy, inconsistent indention. Plus some style nitpicks.)
#include <stdint.h>
uint8_t spi (uint8_t data)
{
SPDR = data; //Start transmision
while((SPSR & 0x80) > 0) //Wait for transmision complete
;
return SPDR;
}
uint16_t ReadAd(void)
{
uint16_t data;
ChipSelectAd(true);
CheckStatus(); //Read data
(void) spi(0x58);
data = ((uint16_t)spi(0xFF)) << 8;
data |= (uint16_t)spi(0xFF);
return data;
}
void CheckStatus(void)
{
uint8_t adcStatus;
do
{
(void) spi(0x40);
adcStatus = spi(0xFF);
} while((adcStatus & 0x80) > 0);
}
void ChipSelectAd(bool on)
{
if(on)
{
PORTB.3 = 0; //Switch on ADC
}
else
{
PORTB.3 = 1; //Switch off ADC
}
}