Correct PWM frequency prescale computation

Assuming this datasheet https://www.digikey.be/htmldatasheets/production/1640697/0/0/1/pca9685-datasheet.html#pf19 is correct, the prescale value computation is off by one. Prior to this change a PWM frequency of about 2 kHz could be set, and would be reported. After this change a maximum PWM frequency of 1526 Hz is accepted and reported, consistent with the datasheet.

Author: bdlucas1

adafruit_pca9685.py

@@ -166,11 +166,11 @@ def frequency(self) -> float:
             raise ValueError(
                 "The device pre_scale register (0xFE) was not read or returned a value < 3"
             )
-        return self.reference_clock_speed / 4096 / prescale_result
+        return self.reference_clock_speed / 4096 / (prescale_result + 1)
 
     @frequency.setter
     def frequency(self, freq: float) -> None:
-        prescale = int(self.reference_clock_speed / 4096.0 / freq + 0.5)
+        prescale = int(self.reference_clock_speed / 4096.0 / freq + 0.5) - 1
         if prescale < 3:
             raise ValueError("PCA9685 cannot output at the given frequency")
         old_mode = self.mode1_reg  # Mode 1