Some experience how to handle fixed point <libq.h>

• For full sine wave generation the Q15sinPI() function is needed.
  Libq library have Q15sin() function which is appropriate only for the tasks where the argument (the angle in radians) is limited by -1..1. For full four quadrant operation Q15sinPI() function should be used.
  Argument range of Q15sinPI() is -PI..+PI which is mapped from the range of Q0_15 fixed point number:
  -32768 is -Pi, 32767 is +0.999969482421875*Pi.
• _Q0_15 fixed point type is different from recently introduced XC16 1.20 fixed point type _Fract. Actually _Q0_15 is defined as signed short integer in libq.h: "typedef short _Q0_15;". There is no cast needed for handling _Q0_15 type as signed short integer.
• <libq.h> library header should be included.
• -lq parameter should be added in "Additional options" field of xc16-ld part of XC16 in MPLABX project parameters. As I've understood this is a linker option for libq library support.
• Small test loop of argument incrementing and _Q15sinePI() computing takes 69 instruction cycles  for every time except 13 cycles for -PI and 0 argument values.
• The sine is symmetric both in time and in amplitude.
• The sine is almost perfect. Artifacts on the spectrum are below -97dB form full sine power. See attachments.
• PIC24HJ_Start.zip in attachments is packaged MPLABX XC16 _Q15sinePI() example project for PIC24HJ64GP202 chip. The code starts the transmission of _Q15sinePI() output through an UART in forever loop: 115200-odd-2, little endian.
• PIC_Output.zip is twice (7Zip and ZIP) packed archive of 109 sine cycles (above 2^22 samples) recorded from hardware UART output.
• The spectrum comparison on PNG-image attached. The white is _Q15sinePI() output spectrum. The green is a spectrum of "ideal" 16bit per sample wave generated by Audacity 2.0.5 without a dithering. The frequency of sine on diagram is 192000/65536=2.9296875Hz because of 65536 samples period.

Attached Image(s)

// Transmitting the sine function results over UART

#include <xc.h> //The base for compiler
#include "custom_header.h" //Custom project-specific functions and definitions
#include <libq.h> //Fixed point math

int main(void) {
    _Q0_15 X,Y; //Argument and Result
    unsigned short int Z; //Transmitted symbol counter
    
    Initialization(); //Setitng up default pins states and configuring the clock
    UART1_initialization(); //Configuring and enabling UART1

    X=0; //X rolls over. -32768 is -Pi, 32767 is +0.999969482421875*Pi
    Z=0; //Symbol counter for TX LED indication

    do {
        Y=_Q15sinPI(X++); //Computing sine value then incrementing X
        UART1_wait_and_indicate(&Z); //Waiting for TX and flashing by LED
        U1TXREG = Y && 0xFF; //Transmitting low byte
        UART1_wait_and_indicate(&Z); //Waiting for TX and flashing by LED
        U1TXREG = Y >> 8; //Transmitting high byte
    }while(1);
}

cat /dev/ttyUSB0 >DataStream.csv