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:

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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:

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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:

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// 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:

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// 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:

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

For Servo Control 

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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 

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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 

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pwm.Reset();
pwm.SetPwmFreq(200.0f); // Moderate frequency for motors

// Set motor speed
pwm.SetDuty(0, 0.75f); // 75% speed

Next Steps

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