I need to do a linear interpolation over time between two values on an 8 bit PIC microcontroller (Specifically 16F627A but that shouldn’t matter) using PIC assembly language. Although I’m looking for an algorithm here as much as actual code.
I need to take an 8 bit starting value, an 8 bit ending value and a position between the two (Currently represented as an 8 bit number 0-255 where 0 means the output should be the starting value and 255 means it should be the final value but that can change if there is a better way to represent this) and calculate the interpolated value.
Now PIC doesn’t have a divide instruction so I could code up a general purpose divide routine and effectivly calculate (B-A)/(x/255)+A at each step but I feel there is probably a much better way to do this on a microcontroller than the way I’d do it on a PC in c++
Has anyone got any suggestions for implementing this efficiently on this hardware?
The value you are looking for is
(A*(255-x)+B*x)/255. It requires only 8×8 multiplication, and a final division by 255, which can be approximated by simply taking the high byte of the sum.Choosing x in range 0..128, no approximation is needed: take the high byte of
(A*(128-x)+B*x)<<1.