PCA9685 Servo Driver

How to use the PCA9685 Servo Driver robot skill

A robot skill that turns any EZB-connected I2C bus into a 16-channel hobby servo controller by driving a low-cost NXP PCA9685 PWM chip.

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What is the PCA9685?

The PCA9685 is a popular 16-channel, 12-bit PWM controller from NXP. It is controlled entirely over I2C and can generate clean, hardware-timed PWM signals for LEDs, servos, or anything else that expects a pulse-width input. Boards built around this chip (such as the Adafruit 16-channel servo HAT/shield and many lower-cost generic clones available from Amazon and Alibaba) are inexpensive and widely available.

This robot skill acts as a bridge between ARC's servo subsystem and a PCA9685 sitting on your EZB's I2C bus. When ARC commands a servo to move, the skill converts that movement into the correct PWM pulse width and writes it to the PCA9685 over I2C. From ARC's point of view, the PCA9685 channels look like ordinary virtual servo ports - by default V0 through V15, or any other 16-port window (V16..V31, V32..V47, ...) that you select in the configuration.

In simpler terms: Think of this skill as a translator. ARC speaks "servo position" (like 1-180 degrees), while the PCA9685 speaks "PWM signals." This skill converts between the two so you don’t have to worry about low-level electronics.

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Why You'd Use It

• More servo channels. The PCA9685 adds 16 servo outputs to any EZB that has an I2C port, without using up the EZB's native digital servo pins. Great for large robots with many moving parts.

• Hardware-timed PWM. The PCA9685 generates pulses in hardware, so servo jitter is low even when the host is busy. Your servos move more smoothly and reliably.

• Wide voltage range. The chip operates from 2.3V to 5.5V on the logic side, and the V+ rail can be powered separately to match your servos. You can safely power stronger servos without stressing your EZB.

• Use existing ARC servo features. Because the skill exposes the channels as V0..V15, you can drive them from any existing ARC skill that commands servos - Auto Position, Inverse Kinematics, speech gestures, scripts, etc. No need to learn anything new - everything works like normal servo ports.

• Chain multiple boards. The PCA9685 has address-select pins, so you can put several boards on the same I2C bus at different addresses. Add one instance of this skill per board and give each instance a different Port Range (e.g. board 1 = V0..V15, board 2 = V16..V31, board 3 = V32..V47, ...). Scale up to dozens of servos without any port conflicts.

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How It Connects to the EZB

The PCA9685 is an I2C device, so you wire it to your EZB's I2C port:

PCA9685 pin	EZB I2C pin
VCC	3.3V or 5V
GND	GND
SDA	SDA (data)
SCL	SCL (clock)

Important Notes for Beginners

• I2C is a communication protocol that uses just two wires:

• SDA = data

• SCL = clock

• Servo power is separate! Servo power (the V+ rail on the PCA9685 board) must come from a separate supply sized for your servos - do not try to power servos from the EZB's logic rail.

• Shared ground is required. Always connect the PCA9685 ground to the EZB ground. Without a common ground, signals won’t work correctly.

• Addressing the board: Most PCA9685 breakout boards have solder-jumper address pads (A0..A5). By default, the 7-bit I2C address is 0x40, which becomes 0x80 in the 8-bit form that the EZB's I2C.Write API expects.

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How to Use It

1. Wire your PCA9685 board to the EZB's I2C port and connect V+ to a suitable servo power supply.
2. Plug your servos into channels 0..15 on the PCA9685 board, paying attention to signal/+/- orientation.
3. In ARC, add the PCA9685 servo Driver robot skill to your project (Project Add robot skill servo PCA9685 servo Driver).
4. Open the skill's configuration (gear/config button in the title bar) and set the I2C address, pulse-width limits, and Port Range to match your hardware - see Configuration below.
5. Connect to your EZB. On connection the skill runs the PCA9685 wake-up and prescale sequence automatically. You should see a log line similar to:

Initialized PCA9685 @ 0x80

6. From any other skill, command a servo on any port in the selected Port Range window (by default V0 through V15).

Channel Mapping

Whichever 16-port window you select in the configuration is mapped onto the PCA9685's 16 physical channels, in order. With the default V0..V15 window:

• V0 PCA9685 channel 0
• V1 PCA9685 channel 1
• ...
• V15 PCA9685 channel 15

If you select a different window (for example V16..V31 on a second board), the first port in the window maps to channel 0:

• V16 PCA9685 channel 0
• V17 PCA9685 channel 1
• ...
• V31 PCA9685 channel 15

These behave exactly like normal servo ports in ARC. Ports outside the selected window are ignored by this plugin instance, so multiple PCA9685 boards can share one project without stepping on each other.

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How a servo Command Becomes PWM

When ARC commands a move, the skill:

1. Takes the ARC servo position (1..SERVO_MAX). Configure the MAX servo position in the ARC -> Project -> Options menu.
2. Linearly maps it into the configured pulse-width range (Min s .. Max s).
3. Clamps the result to stay inside those limits.
4. Converts microseconds to a 12-bit tick count inside the PCA9685's 20 ms (50 Hz) frame.
5. Writes a single 4-byte I2C transaction to the PCA9685.

Why This Matters

You don’t need to manually calculate PWM signals - the skill handles everything automatically. You just tell ARC:

Servo(V3, 90)

and the hardware does the rest.

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Configuration

Click the gear/config button in the robot skill's title bar to open the configuration form.

I2C Address

The 8-bit I2C address of the PCA9685 on the EZB's bus.

• Default: 0x80
• Required when using multiple boards

Tip: If your board isn’t responding, this is the first thing to check.

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Min s

The pulse width that corresponds to the minimum servo position.

• Default: 650
• Typical range: 500-1000

Lower values = more travel, but too low can damage the servo.

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Max s

The pulse width that corresponds to the maximum servo position.

• Default: 2250
• Typical range: 2000-2500

Higher values = more travel, but too high can cause buzzing or strain.

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Port Range

Selects which 16-port window of ARC virtual servo ports this plugin instance binds to. The chosen window is mapped onto the PCA9685's physical channels 0..15 in order.

• Default: V0 - V15
• Options: V0 - V15, V16 - V31, V32 - V47, ... (the list is populated from the Vx ports available in your ARC build, grouped into 16-port windows)

When to change this: only when you have more than one PCA9685 board in the same project. Give each board a unique I2C address and a unique port range so their ARC virtual ports don't overlap. For a single-board setup, leave this on the default V0 - V15.

Example - three PCA9685 boards driving 48 servos:

Plugin instance	I2C Address	Port Range
Board 1	0x80	V0 - V15
Board 2	0x82	V16 - V31
Board 3	0x84	V32 - V47

Any ARC skill (Auto Position, Inverse Kinematics, scripts, etc.) can then command V0..V47 and the correct board will respond.

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Beginner Tip

If you’re unsure:

• Start with defaults
• Slowly adjust while watching the servo
• Stop if you hear buzzing or see resistance

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Troubleshooting

• PCA9685 init failed @ 0x80 Check wiring and address

• EZ-Robot EZB V4.x locks up (solid red LED) Likely I2C wiring issue Fix wiring, then power cycle EZB

• Servos jitter Almost always a power issue Use a stronger external power supply

• Servo hits hard stop Adjust Min/Max s range

• Nothing happens on V0-V15 Confirm initialization message appears Confirm the Port Range in the configuration matches the ports you are commanding (e.g. if the range is V16 - V31, commanding V0 will do nothing on this board)

• Random movement on startup Normal behavior during initialization

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Limits and Notes

• Fixed at 50 Hz (standard for servos)
• Only supports 16 channels per board
• Each plugin instance binds to one configurable 16-port Vx window (default V0..V15); add more instances with different port ranges to drive additional boards
• Does not "release" servos - it continuously sends position signals

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Final Notes for New Users

If this is your first time using I2C or external servo drivers:

• Take your time wiring - most issues are wiring-related
• Always use a proper power supply for servos
• Start with one servo before connecting many
• Use ARC’s simple servo commands to test before building complex behaviors