Sin table in PIC....

[koolguy]

HI,

I am testing the sin function on simulator here is my code it is running but don't showing any value in PORTB register why???

#include <htc.h>
#include <math.h>
#include <stdio.h>

__CONFIG(LVP_OFF & BOREN_OFF & PWRTE_ON & WDTE_OFF & FOSC_HS);
#define _XTAL_FREQ 20000000


void main (void){
TRISB=0x00;
{
int i, a;
while(1)
a = sin(20);
PORTB=a;
}
}

 

[Ian Rogers]

Sometimes I wonder about you..... If sin(20) = 0.9294 radians or 0.34202 degrees... How the hell is an int going to display it...

 

[kubeek]

sin(x) is not in radians nor degrees, it is just a number between -1.0 and 1.0
Anyway the math.h defines the function as double sin(double); so unless you do a sin of pi/4 you will allways get 0 on the output.

 

[kubeek]

Also, didnt the compile give you some warning like possible loss of precision?

 

[Ian Rogers]

sin(x) is not in radians nor degrees, it is just a number between -1.0 and 1.0
Anyway the math.h defines the function as double sin(double); so unless you do a sin of pi/4 you will allways get 0 on the output.

Point being!! He has variable a as an int!!!

 

[Ian Rogers]

Just checked it out..... sin() returns radians....

sin(20) returns.. 0.9131
sin(30) returns..-0.9880
sin(45) returns.. 0.8513
sin(60) returns..-0.3049

 

[kubeek]

Sin() doesn´t return radians, it allways returns a fraction without any units, but it is true that the parameter is in radians.
(sinus of an angle is equal to a ratio of two sides in a triangle, so it can´t have any units)

 

[Ian Rogers]

Sin() doesn´t return radians, it allways returns a fraction without any units, but it is true that the parameter is in radians.
(sinus of an angle is equal to a ratio of two sides in a triangle, so it can´t have any units)

What am I on...... Don't know my head from my arse today.....Of course you are correct... Sorry for the argument Kubeek......

I need a lie down or something

 

[DerStrom8]

Hahaha, don't worry about it Ian. I've had days like that

But yeah, the value that is put into sin() has units of radians or degrees. i.e. sin(35 degrees) or sin(pi/2 radians).

[MODNOTE]Unhelpfull Post.[/MODNOTE]

 

[misterT]

The code never reach the line "PORTB=a;" because of this:

while(1)
a = sin(20);

 

[koolguy]

The code never reach the line "PORTB=a;" because of this:

while(1)
a = sin(20);

Thanks that what i want to know.....

 

[misterT]

Thanks that what i want to know.....

That is not what you need to know.

 

[WTP Pepper]

Why can't you just multiply by a scaling factor, converting to a uchar and passing that to the port?

 

[koolguy]

Why can't you just multiply by a scaling factor, converting to a uchar and passing that to the port?

yes i will do that..
but how to make look up table for sin wave 180 degree.....

 

[Ian Rogers]

yes i will do that..
but how to make look up table for sin wave 180 degree.....

Remember the math library I gave you in another post!! This has a sine wave table ( actually cosine, but its the same ).

 

[koolguy]

const int cosTable[] = {0x0037, 0x00A1, 0x0114, 0x017C, 0x01E8,
0x0253, 0x02BB, 0x0321, 0x0386, 0x03E7,
0x0446, 0x04A3, 0x04FA, 0x0550, 0x05A2,
0x05EF, 0x0638, 0x067E, 0x06BD, 0x06FA,
0x0730, 0x0763, 0x078E, 0x07B6, 0x07D8,
0x07F5, 0x080B, 0x081C, 0x0828};
const int tanTable[] = {0x0418, 0x041E, 0x042A, 0x043C, 0x0452,
0x0474, 0x049C, 0x04CA, 0x0506, 0x054E,
0x059E, 0x0604, 0x0682, 0x0710, 0x07C6};

Can you explain what r u doing and how to make discrete value?

 

[Ian Rogers]

How fast is your sine wave???

 

[misterT]

Here is few methods and theory. I hope you at least try to read it.
**broken link removed**

 

[MrAl]

Hello,

If you already have the sin(a) function you can easily generate it using that. Just multiply by a constant to form the table value. The highest value is 1 and that is at 90 degree, and the lowest value is -1 and that is at 270 degrees (convert degrees to radians as needed) but we usually only need the values for 0 to 90 degrees which returns a float value 0 to 1. So if 0x0FFF is the highest value you need at 90 degrees, then multiply all the return values of sin(a) by 0x0FFF.

For example in a simplified language:

for k=0 to 19 do
a=sin(k/19*pi/4)
y=sin(a)
Y=y*0x0FFF
Table[k]=Y
end for

That creates a table that is entered into ram that covers 0 to 90 degrees, and that provides enough values to cover the full range of 0 to 360 degrees.

 

[Brian Griffin]

A sine table in a microcontroller is straightforward.

Since microcontrollers don't read or store decimal numbers, these sine values must be scaled. Precalculate these sine wave values as a microcontroller will have a tough time working on it.

Then discretize the sine values for one period. Example, for 256-value sine table, sin(x/256*2pi*127) + 127 and rounded. So after getting these 256 values, dump them inside the array. The values should be 0 - 255 and plotting this in excel will get you a sine wave.

For a controllable synthesis of sine wave, it's advisable to read up on Digital Direct Synthesis (DDS). It is a simple algorithm to generate a sine wave on a microcontroller. However, some filtering is required on the output.

I made my own sine wave generators (it's for the toy music boxes) all using this method, without involving math.h or whatsoever.