void setup ()
{
   //--- These lines set-up output at pins #9 and #10 for push-pull and are implemented in hardware ---//
   digitalWrite( 9, HIGH);  // set pin #9 to HIGH
   pinMode ( 9, OUTPUT);   // pin #9 in output mode
   digitalWrite(10, LOW );  // set pin #10 to LOW
   pinMode (10, OUTPUT);   // pin #10 in output mode

   TCCR1A = 0;             // Timer/Counter Control Register 1A set to 0
   TCCR1B = 0;             // Timer/Counter Control Register 1B set to 0
   TCNT1  = 0;             // Set counter for timer #1 to 0

   uint16_t freq = 9000;
   uint16_t tc = 8000000/freq;
   //    2 -> 4000 kHz
   //   20 ->  400 kHz
   //  200 ->   40 kHz
   //  800 ->   10 kHz
   // 1600 ->    5 kHz
   // 2000 ->    4 kHz
   
   ICR1 = tc;
   OCR1A = tc/2;              // number of ticks for counter 1A -  Output Compare Register for A
   OCR1B = tc/2;              // set number of ticks for counter 1B to the same value as for 1A -  Output Compare Register for B

   TCCR1A |= (0 << COM1A0);   // enable toggling output A - Compare Match Output A Mode bit set to 1/*
   TCCR1A |= (1 << COM1A1);
   TCCR1A |= (1 << COM1B0);   // enable toggling output B - Compare Match Output B Mode bit set to 1
   TCCR1A |= (1 << COM1B1);

   // Phase and Frequency correct PWM with TOP set by ICR1
   TCCR1B |= (1 << WGM13);       // WGM=8
   
   TCCR1B &= ~((1 << CS12) | (1 << CS11) | (1 << CS10)); // Clear the three clock select bits
   TCCR1B |= (1 << CS10); 
     
   // see https://ww1.microchip.com/downloads/aemDocuments/documents/MCU08/ProductDocuments/DataSheets/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061B.pdf
   // sections 16, 17, 18 for details

   // f = fclk/(2*N*(1+OCR1A))
   // N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024), set CS10/CS11/CS12 appropriately

   // find values of OCR1A for frequencies of interest
   // consider using timer #2!   

}

void loop() {
   // put your main code here, to run repeatedly:

}