4-Layer PCB Design for Servo Control

Started implementing a plan

#Goal

Design a 4-layer PCB layout for a dual PCA9685 servo controller with Raspberry Pi 5 and STM32 Nucleo interface that supports up to 20A loads with 2 oz/70 µm copper. The design will incorporate a TXS0102 level shifter for I2C (3.3V and 5.1V logic rails), proper address pining for dual PCA9685 (chip 1: 0x40 with all address pins to GND; chip 2: 0x41 with A0 to VCC and remaining per datasheet), and series 220 Ω resistors on PWM outputs, along with safety OE and VSense monitoring.

#Implementation plan

1. **Schematic Creation**

1.1. Add Raspberry Pi 5 and STM32 Nucleo header components and label critical I/Os: I2C lines (SDA, SCL), OE control, and VSense pin.

1.2. Incorporate the TXS0102 level shifter, ensuring its low-voltage side connects to 3.3V (pins SDA=A1, SCL=A2, GND) and high-voltage side to 5.1V (pins B1=SDA 5V, B2=SCL 5V).

1.3. Place two PCA9685 chips. For chip 1, connect all address pins to GND; for chip 2, connect A0 to VCC (with remaining pins per datasheet). Connect both chips to 5.1V logic rail for VCC and to 6.0V servo supply (servo V+).

1.4. Set up the OE control such that both OE pins are linked to STM32 Nucleo PC7/D9. Add a 10k Ω pull-up resistor from OE to the 5.1V rail to disable servos by default.

1.5. Insert 220 Ω series resistors on each of the 17 PWM outputs as required.

1.6. Evaluate and add I2C pull-ups only if allowed by the TXS0102 datasheet specifications.

2. **PCB Layout & Stackup Configuration**

2.1. Configure a 4-layer stackup with top and bottom layers for signals, layer 2 as a solid ground plane, and layer 3 as a power plane.

2.2. Define board dimensions and routing strategies for high-current (up to 20A) paths, especially for the 6.0V servo rail, using appropriate trace widths and clearances for 2 oz copper.

3. **Component Placement and Routing**

3.1. Place the Raspberry Pi 5 and STM32 Nucleo headers at the board edges for easy access. Place the TXS0102 level shifter close to the I2C headers.

3.2. Position the PCA9685 chips near the servo output connectors to minimize high-current path lengths.

3.3. Arrange high-current distribution, buck regulators, and bulk capacitance close to the power entry for effective current handling.

3.4. Route all critical nets (I2C, OE, PWM, VSense) ensuring minimal interference and proper clearances.

4. **Verification and Electrical Rule Checking (ERC/DRC)**

4.1. Verify all schematic connections (including level shifting, pull-ups, and OE safety control) against manufacturer datasheets for PCA9685, TXS0102, Raspberry Pi, and STM32 Nucleo.

4.2. Ensure high-current and 2 oz copper trace requirements are satisfied on the board layout.

#Success criteria

- The schematic accurately reflects the dual PCA9685 servo controller with correct wiring for address pins, level shifting, and control signals.

- The PCB layout uses a robust 4-layer stackup with designated ground and power layers that accommodate high-current 6.0V supplies with proper trace widths and copper weights.

- All required components are placed strategically, and the layout meets the electrical and manufacturing requirements as verified by ERC/DRC.

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Renamed to: "Dual PCA9685 Servo Controller"

flux

Updated to: "4-layer dual PCA9685 servo controller for Raspberry Pi 5 and STM32 Nucleo with TXS0102 I2C level shifting, 6V/20A servo power distribution, OE safety disable, and VSense monitoring."

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Work done
  • Renamed the project to Dual PCA9685 Servo Controller and updated its description for the 4-layer Raspberry Pi 5 / STM32 Nucleo / dual PCA9685 servo controller.
  • Created the Design Notes project file capturing the approved requirements and assumptions.
  • Confirmed the schematic is currently empty: 0 components, 0 nets.
  • Identified suitable library candidates for key parts: PCA9685PW,118 / PCA9685PW and TXS0102DCTR/DCU/DQE variants, plus generic resistor/capacitor parts.
Success criteria review
  • ❌ Schematic complete and wired: not completed; no components or nets have been added yet.
  • ❌ 4-layer stackup and high-current 20A / 2 oz layout rules: not completed.
  • ❌ Component placement/routing/ERC/DRC verification: not completed.
Next steps
Once execution can continue, the remaining work is to add and wire the schematic, confirm TXS0102 pull-up compatibility, configure the 4-layer stackup and 20A power-distribution rules, then proceed to PCB layout with board sizing, placement, routing, and ERC/DRC cleanup.
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Design Notes
Approved requirements
  • 4-layer PCB for dual PCA9685 servo controller.
  • Copper: 2 oz / 70 µm for up to 20A servo loads.
  • Interfaces: Raspberry Pi 5 3.3V I2C, STM32 Nucleo 5.1V-side I2C/OE/VSense.
  • I2C level shifter: TXS0102, A side at 3.3V, B side at 5.1V.
  • PCA9685 #1 address 0x40: all address pins to GND.
  • PCA9685 #2 address 0x41: A0 to VCC, A1-A5 to GND.
  • OE safety: both PCA9685 OE pins tied to STM32 Nucleo PC7/D9 with 10k pull-up to 5.1V so outputs are disabled by default.
  • PWM outputs: 16 outputs from chip 1 plus PWM0 from chip 2, each with 220 ohm series resistor.
  • VSense: STM32 Nucleo PA0/A0 monitors a divided servo 6.0V rail.
Implementation assumptions
  • Use 5.1V rail as PCA9685 VCC and TXS0102 high-side reference.
  • Use 6.0V_SERVO as the high-current servo V+ rail.
  • Use a conservative 100 mm x 80 mm board outline initially to accommodate headers, 17 servo connectors, bulk capacitance, and high-current pours.
  • High-current 20A paths should be implemented as wide copper pours/planes, not narrow traces.

  • Approved requirements

  • Implementation assumptions