Computing Arc tangent only using the ratio y/x is not enough to determine the angle since there is an indeterminacy. Opposite quadrants are giving the same ratio.
ArcTan2 is not using y/x to compute the angle but y and x and use the sign of y and x to determine the quadrant. 
      
        
          | arm_status arm_atan2_f16 | ( | float16_t | y, | 
        
          |  |  | float16_t | x, | 
        
          |  |  | float16_t * | result | 
        
          |  | ) |  |  | 
      
 
Arc tangent in radian of y/x using sign of x and y to determine right quadrant.
- Parameters
- 
  
    | [in] | y | y coordinate |  | [in] | x | x coordinate |  | [out] | result | Result |  
 
- Returns
- error status.
- Compute the Arc tangent of y/x:
- The sign of y and x are used to determine the right quadrant and compute the right angle. 
 
 
Arc tangent in radian of y/x using sign of x and y to determine right quadrant.
- Parameters
- 
  
    | [in] | y | y coordinate |  | [in] | x | x coordinate |  | [out] | result | Result |  
 
- Returns
- error status.
- Compute the Arc tangent of y/x:
- The sign of y and x are used to determine the right quadrant and compute the right angle. 
 
 
Arc tangent in radian of y/x using sign of x and y to determine right quadrant.
- Parameters
- 
  
    | [in] | y | y coordinate |  | [in] | x | x coordinate |  | [out] | result | Result in Q2.13 |  
 
- Returns
- error status.
- Compute the Arc tangent of y/x:
- The sign of y and x are used to determine the right quadrant and compute the right angle. 
 
 
Arc tangent in radian of y/x using sign of x and y to determine right quadrant.
- Parameters
- 
  
    | [in] | y | y coordinate |  | [in] | x | x coordinate |  | [out] | result | Result in Q2.29 |  
 
- Returns
- error status.
- Compute the Arc tangent of y/x:
- The sign of y and x are used to determine the right quadrant and compute the right angle.