Configuration

This guide covers all configuration options available for the PCA9685 driver.

I2C Address Configuration

The PCA9685 I2C address is configured via hardware pins A0-A5. Each pin represents one bit of the 6-bit address field.

Pin	Address Bit	Description
A0	Bit 0	Least significant address bit
A1	Bit 1
A2	Bit 2
A3	Bit 3
A4	Bit 4
A5	Bit 5	Most significant address bit

Default I2C address: 0x40 (all address pins to GND)

Address Range: 0x40 to 0x7F (7-bit I2C addresses)

Example: To set address 0x41, connect A0 to VDD and A1-A5 to GND.

PWM Frequency Configuration

Setting Frequency

Use the SetPwmFreq() method to configure the PWM frequency:

pwm.SetPwmFreq(50.0f); // 50 Hz for servos
pwm.SetPwmFreq(1000.0f); // 1 kHz for LEDs

Valid Range: 24 Hz to 1526 Hz (typical)

Location: src/pca9685.ipp (template implementation)

Frequency Calculation

The driver automatically calculates the prescale value using the formula:

prescale = round(25,000,000 / (4096 * freq_hz)) - 1

Where:

• 25,000,000 is the internal oscillator frequency (25 MHz)
• 4096 is the PWM resolution (12 bits)
• freq_hz is the desired frequency

Implementation: src/pca9685.ipp (calcPrescale)

Common Frequencies

Application	Frequency	Prescale (approx)
Servos	50 Hz	121
LEDs (smooth dimming)	1000 Hz	5
Motors	100-500 Hz	11-61

Channel Configuration

Individual Channel Control

Set PWM for a single channel:

// Set channel 0: on at tick 0, off at tick 2048 (50% duty)
pwm.SetPwm(0, 0, 2048);
 
// Set duty cycle (0.0 = off, 1.0 = fully on)
pwm.SetDuty(0, 0.5f); // 50% duty

Channel Range: 0-15 (16 channels total)

PWM Value Range: 0-4095 (12-bit resolution)

Bulk Channel Control

Set all channels simultaneously:

// Set all channels to same PWM value
pwm.SetAllPwm(0, 2048); // All channels at 50% duty

Use Case: Synchronized control of all outputs

Register Map Reference

The driver abstracts register access, but understanding the register map helps with advanced usage:

Register	Address	Purpose
MODE1	0x00	Main configuration (reset, sleep, auto-increment)
MODE2	0x01	Output configuration (open-drain, totem-pole, inversion)
SUBADR1-3	0x02-0x04	Subaddress registers
ALLCALLADR	0x05	All-call I2C address
LED0_ON_L	0x06	Channel 0 ON time (low byte)
LED0_ON_H	0x07	Channel 0 ON time (high byte)
LED0_OFF_L	0x08	Channel 0 OFF time (low byte)
LED0_OFF_H	0x09	Channel 0 OFF time (high byte)
...	...	Channels 1-15 follow sequentially
ALL_LED_ON_L	0xFA	All channels ON time (low byte)
ALL_LED_ON_H	0xFB	All channels ON time (high byte)
ALL_LED_OFF_L	0xFC	All channels OFF time (low byte)
ALL_LED_OFF_H	0xFD	All channels OFF time (high byte)
PRE_SCALE	0xFE	PWM frequency prescaler
TESTMODE	0xFF	Test mode register

Register Definitions: inc/pca9685.hpp (enum Register)

Advanced Features

Output Enable (OE) Pin

The OE pin provides hardware control over all outputs:

• HIGH: All outputs disabled (high-impedance)
• LOW/Floating: All outputs enabled

Note: The driver does not control the OE pin. You must handle it externally via GPIO if needed.

Sleep Mode

The chip supports sleep mode for power saving. The driver handles sleep mode automatically when changing frequency (see SetPwmFreq() implementation).

Auto-Increment

The chip supports auto-increment for efficient register access. The driver uses this feature internally for multi-byte writes.

Runtime Configuration

Initialization Sequence

1. Reset: Call Reset() to put device in known state
2. Set Frequency: Call SetPwmFreq() before setting channels
3. Configure Channels: Use SetPwm() or SetDuty() to control outputs

Example:

pwm.Reset();              // Initialize
pwm.SetPwmFreq(50.0f);   // Set frequency
pwm.SetDuty(0, 0.5f);    // Configure channel

Recommended Settings

For Servo Control

pwm.Reset();
pwm.SetPwmFreq(50.0f); // Standard servo frequency
 
// Set servo to center position (1.5ms pulse)
pwm.SetDuty(0, 0.075f); // 7.5% duty = 1.5ms / 20ms

For LED Dimming

pwm.Reset();
pwm.SetPwmFreq(1000.0f); // High frequency for smooth dimming
 
// Fade LED on channel 0
for (float duty = 0.0f; duty <= 1.0f; duty += 0.01f) {
    pwm.SetDuty(0, duty);
    delay(10);
}

For Motor Control

pwm.Reset();
pwm.SetPwmFreq(200.0f); // Moderate frequency for motors
 
// Set motor speed
pwm.SetDuty(0, 0.75f); // 75% speed

Next Steps

• See Examples for configuration examples
• Review API Reference for all configuration methods
• Check Troubleshooting for configuration issues