PWM frequency library

Development Libraries
39

one question, how do I determine the PWM range fora a given pin with a modified frequency. Take for example my sketch below that uses 20khz on pin 9. Normally the values 0 256 work fine without the mod but I had to raise it to 256 to 512..it doesn't go as high as 512 more like in the 400 range..just want to know what I'm missing to understand that and if there is an easy calculation.

// simple move forward and backward one motor on the l298n using pins 12 13 9/ 9 being PWM
#include <PWM.h>
int ENA = 9;
int IN1 = 12;
int IN2 = 13;
int32_t frequency = 20000; //frequency (in Hz)

void setup ()
{
  Serial.begin(9600);
  InitTimersSafe(); 
  bool success = SetPinFrequencySafe(ENA, frequency);
  pinMode (ENA, OUTPUT);
  pinMode (IN1, OUTPUT);
  pinMode (IN2, OUTPUT);
}

void test_motor_1 ()
{
  for (int i = 256; i < 512; i++)
  {
    digitalWrite (IN1, HIGH);
    digitalWrite (IN2, LOW);
    analogWrite (ENA, i);
    delay (30);
    Serial.println(i);
  }

  delay (100);
  digitalWrite (IN1, LOW);
  
  for (int i = 256; i < 512; i++)
  {
    digitalWrite (IN1, LOW);
    digitalWrite (IN2, HIGH);
    pwmWrite  (ENA, i);  //was analogWrite
    delay (30);
   Serial.println(i);
  }

  delay (100);
  digitalWrite (IN2, LOW);
}

void loop()
{
  test_motor_1();
}