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.