hello:
I have two bytes say ADC, & OLD_ADC. I'd like to compare them to see if they differ by more than value HYST (a constant) & take action if the difference (+/-) is more than HYST. Thus I need to take an absolute value of the difference. Any thoughts on an efficient method?
Hoyt
Just do it one step at a time:
Subtract one ADC value from the other.
Swap sign if the result is negative.
Compare with HYST.
mneary Please select a language and reveal your choice to us.
C? ASM? 4th?.....................
tpappano I vote for FORTH!
adc C@ old_adc C@ - abs hys - 0>
Tom Pappano
Tulsa, Oklahoma
mneary Now I'm gonna have nightmares......
draymond 
If the values are a single byte, it requires more than a simple subtraction of bytes to determine the difference. One way is to extend the subtraction to integer size. Here is another (in C):
if (ADC > OLD_ADC) return HYST > (ADC - OLD_ADC); return HYST > (OLD_ADC - ADC);
Dave Raymond
mneary 
cp old_adc, new_adc ; which is bigger? carry knows!
mov temp, new_adc ; don't mess with the adc value
sub temp, old_adc ; preserves carry
brcs done ; correct answer if new > old
neg temp ; if new < old flip result
done: cp temp, hyst
What do I win?
:P
mneary return((abs(old_adc - new_adc))>hist) ;
cp old_adc, new_adc ; which is bigger? carry knows! mov temp, new_adc ; don't mess with the adc value sub temp, old_adc ; preserves carry brcs done ; correct answer if new > old neg temp ; if new < old flip result done: cp temp, hystWhat do I win?
:P
I would say nothing :D
You are making temp negative no matter what, as you are branching on carry set. And you don't need the CP instruction at all, as SUB changes carry anyway, despite what you wrote in the comment.
mneary :oops: I knew I should have simulated it. :oops:
Erm !!! Where was the challenge ? That was too easy.
Nigel
BTW I vote Pascal because I had to learn it at university and after that; ADA because everybody thought it was going to rule the programming world. (We didn't even have an ADA compiler though)