https://github.com/prestonsn/PCA9685-Multi-Channel-Servo-Controller-Driver-for-AVR-ATmega 
위에 올려준 링크의 코드에서 pca9685_servo() 함수를 보면
 
void pca9685_servo(uint8_t servoNum, float angle){
    if (angle > 90) { angle = 90; } else if (angle < -90) { angle = -90; }
    uint16_t pulse_us = 1500 + angle*10; 
us 변환을 여기에서 하고 있습니다.

함수를 복사해서 이름을 변경하고 내용을 조금 변경하면 되겠습니다.
void my_pca9685_servo(uint8_t servoNum, uint16_t usec){
    //if (angle > 90) { angle = 90; } else if (angle < -90) { angle = -90; } // 주석처리
    uint16_t pulse_us = usec; //1500 + angle*10;

    uint16_t period_us = 20000; // (float)1000000 / 50Hz;
    uint16_t count = (float)pulse_us/20000 * 4096; //((float)pulse_us / period_us) * 4096;

    uint8_t offLowCmnd = count;
    uint8_t offHighCmnd = count >> 8;

    i2c_tx_start(MASTER_TRANSMITTER); i2c_tx_address(0x40 + globalAddress); i2c_tx_byte(SERVO0 + (4 * servoNum));
    i2c_tx_byte(0x00); i2c_tx_byte(0x00); i2c_tx_byte(offLowCmnd); i2c_tx_byte(offHighCmnd); i2c_tx_stop();
}

usec의 범위는 600~2400 범위를 입력해주면 됩니다.
//


#include <avr/io.h>
#include <util/delay.h>
#include "pca9685.h"
//
void my_pca9685_servo(uint8_t servoNum, uint16_t usec){
    //if (angle > 90) { angle = 90; } else if (angle < -90) { angle = -90; } // 주석처리
    uint16_t pulse_us = usec; //1500 + angle*10;

    uint16_t period_us = 20000; // (float)1000000 / 50Hz;
    uint16_t count = (float)pulse_us/20000 * 4096; //((float)pulse_us / period_us) * 4096;

    uint8_t offLowCmnd = count;
    uint8_t offHighCmnd = count >> 8;

    i2c_tx_start(MASTER_TRANSMITTER); i2c_tx_address(0x40 + globalAddress); i2c_tx_byte(SERVO0 + (4 * servoNum));
    i2c_tx_byte(0x00); i2c_tx_byte(0x00); i2c_tx_byte(offLowCmnd); i2c_tx_byte(offHighCmnd); i2c_tx_stop();
}
//
int main(void){
    uint16_t usec = 0;
    pca9685_init(0x00, 50); // start PCA9685 device 0x00 at 50 Hz output
    _delay_ms(5);
    pca9685_servo(0, 0);
    while (1) {
        pca9685_servo(0, 0); // set servo0 position to 0 degrees
        for (usec= 600; usec<=2400; usec+=10){ my_pca9685_servo(0, usec); _delay_ms(25); }
        for (usec=2400; usec>= 600; usec-=10){ my_pca9685_servo(0, usec); _delay_ms(25); }
    }
    return 0;
}