Struggling with basic maths!

J

John O

Senior Member
Hi Guys,

Maths isn't my strong point so I'd be grateful if someone could talk me through this.

I'm reading a readadc10 and trying to convert the result into 0 to 359

Code: 
do
 readadc10  C.1, w0
 w1 = 360 / 32 * w0 / 32   ; saw that trick somewhere :)
 debug
 pause 100
loop
Although w0 reaches 1023, w1 never gets above 351. Doing the maths on an integer-only calcualtor is the same. I'm hoping that, as 1023 represents over 2.8 "units" per degree (the 360), I'd get better resolution than that.


John.
 
hippy

hippy

Ex-Staff (retired)
degrees = adc10 / K

so for 359 = 1023 / K

K = 1023/359

K= 2.8495821727019498607242339832869

Now you need to make the integer divisor as big as possible to reduce rounding errors, eg multiply by N, but you also need to multiply adc10 by N so the result is the same ...

degree = ( adc10 * N ) / (K * N )

Having N as big as possible reduces error but N cannot be bigger than 64 as (1023*65) is an overflow of 16-bit ( greater than 65535 ) so let's use N = 64, K*N = 182, so ...

degree = adc10 * 64 / 182

That's at least a good first pass. Someone better at maths may be able to work out a more accurate away of approaching it !
 
Last edited:
hippy

hippy

Ex-Staff (retired)
The other way is ...

degrees = adc10 / K

degrees = adc10 / ( 1023 / 359 )

degrees = adc10 * 359 / 1023

Then choose some N which makes adc10*(359/N) not overflow ...

degrees = adc10 * (359/N) / (1023/N)

N = 359 / (65535/1023) = 5.6039826047150377660791943236439

degrees = adc10 * (359/5.603) / (1023/5.603)

degrees = adc10 * 64 / 182
 
J

John O

Senior Member
Brilliant... many thanks :)

I understood your first method easiest so it's now duly bookmarked and printed out & put safe in my folder!

John.
 
M

Mike7081

New Member
Hippy you're quick at fixing your errors. I was working on a post showing the correct equation and just before I posted it I saw you had already edited your post. lol good job! You got the correct equation but the first part of your post should be:

degrees = adc10 / K

so for 360 = 1024 / K

K = 1024 / 360
K = 2.84444444444...
 
hippy

hippy

Ex-Staff (retired)
Welcome to the forum.

For "360 = 1024 / K" versus "359 = 1023 / K" ... That's an interesting point and I'm sure it will provoke some debate. Not sure my theoretical maths is up to that though ! In practice it's probably the same answer using PICAXE maths but there would be times where it does affect things for similar sitituations.
 
C

ckoehn

Member
Another way that decimal math has been used also works.

K= 2.8495821727019498607242339832869

Reduce to 2.85, no point going over to 10,000th

w1=w0*2
w1=w0*8/10+w1
w1=w0*5/100+w1



Clint
 
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