Page 223 - Programming Microcontrollers in C

208 Chapter 4 Small 8-Bit Systems

return result;
}
Listing 4-8: First BCD Conversion

This small function takes an 8-bit character n that is less than 99
and converts the number into one 8-bit result. The upper nibble is the
number of tens in the number and the lower nibble is the number of
units. A hexadecimal number 0xff is returned if the number is greater
than 99. Note that the calculation requires an integer divide opera­
tion and a modulus operation which is equivalent to a divide operation.
The code to execute the divide and modulus operations must be asso­
ciated with this function to complete its task.
Another approach is shown below. This approach avoids any exter­
nal function calls and actually requires less total code than the version in
Listing 4-8, even though the C code is somewhat longer. In this case, the
while loop essentially divides the input number by 10 and places the
result in the most significant nibble of the result. After the tens have been
removed from the number, all that is left is the units. This value—the
number of units—is ORed on the result before it is returned.
/* convert a binary number less than 100 to BCD */

unsigned char convert_bcd(unsigned char n)
{
unsigned int result;

if(n>99) return 0xff;
result=0;
while((n-10)>=0)
result+=0x10;
n+=10;
result += n;
return result;
}
Listing 4-9: Second BCD Conversion

The function shown below was originally written for use on a
M68HC05, but was later used on a M68HC11 as well. A two-digit
number must be sent to a seven-segment LED display. The number
to be shown is contained in the memory locations for tens and units.