Good BCD to Decimal code?

Ravenkallen

I am looking for an efficient way to make a routine that will be able to convert a bcd value(0-255) to decimal. My current coding uses a ton of IF statements to get the job done and it is slow. Does anybody have a simple snippet of a method that can convert between the two? Just a little something to get me back on track. It is probably really trivial, but i am just having a hard time visualizing it..

kuroneko

Provided I understood your requirement correctly (3 digit BCD):

PUB BCD2DEC(bcd)

  return (bcd >> 8) * 100 + ((bcd >> 4) & %1111) * 10 + (bcd & %1111)

Ravenkallen

Oh, sorry. Forgot the details
It is 1 digit for every four bits(So each byte contains 2 digits) . It is the format commonly used in RTC's . So i need to convert a number that holds 2 bcd digits to a single byte value

MagIO2

These details do not describe anything which is not normal for BCD. This is what BCD means in common sense.

And Kuronekos code is correct if you call the function correctly. It only allows you to call it with $0000_00_00 - $0000_09_99. In your special case you only want to call it with $000-$255, so that should not be a problem.

What kuronekos code is doing for example with number $128:

$128 >> 8 = 1
1*100=100
$128>>4 = $12
$12 & %1111 = 2
2*10=20
$128 & %1111 = 8
and finally 100 + 20 + 8 = 128 = $80

Roy Eltham

Here is the 2 digit version:

PUB BCD2DEC(bcd)
  return ((bcd >> 4) & %1111) * 10 + (bcd & %1111)

Ravenkallen

Thanks for the help guys. I will try out the code once i can get my rtc back up(It's acting weird). I think i understand the principle behind it, but bcd numbers have always confused me. So basically you are shifting some of the number and adding %1111 in binary, and depending on the digit, multiplying it?

JonnyMac

The ANDing with %1111 is to isolate the nibbles. The upper nibble is the 10s digit so it gets multiplied by 10. The lower nibble is the 1s digit and only needs isolation from the other bits in the byte.

Ravenkallen

I think i got it know. I will give it a go soon!