4-Layer PCB Design for Servo Controller

flux

1:03

Thought for 2 sec

Created 4 tasks.

1/4 Inspect U3 and R19 current parts, values, pins, and connectivity

Thought for 3 sec

Completed design checks — found 170 issues.

Read BOM.

Read schematic.

Thought for 5 sec

2/4 Determine valid replacement action for U3 and R19

Thought for a moment

I inspected U3 and R19. U3 is currently TXS0102DCTR wired as the 3.3 V ↔ 5.1 V I2C level shifter, and R19 is the 4.7 kΩ SDA_5V pull-up to 5V1_LOGIC.
I’m blocked because replacing U3 requires the exact new level-shifter part/MPN, and replacing R19 requires the intended resistor value/package or whether it should be removed. Send the replacement part for U3 and the intended change for R19 — for example: “replace U3 with PCA9306” and “change R19 to 10 kΩ / remove R19.”
robocraz

robocraz

remove both and place new one

flux

0:22

Thought for a moment

I’ll remove the existing level shifter and pull-up resistor, then add a fresh level-shifter component back in place and restore its I2C/power connections without the R19 pull-up.

flux

11:31

Thought for 4 sec

3/5 Remove U3 and R19, add a fresh TXS0102DCTR level shifter, and restore connections without R19

Thought for 8 sec

Thought for 3 sec

Thought for 8 sec

4/5 Validate schematic/review for new U3 after change

Thought for 8 sec

Completed design checks — found 167 issues.

Read layout.

Read schematic.

Thought for 2 sec

5/5 Place the new U3 in the interface cluster on the PCB

Thought for a moment

Thought for a moment

Thought for a moment

Thought for 4 sec

Thought for a moment

Thought for 8 sec

Thought for a moment

Thought for 2 sec

Thought for 36 sec

Thought for 15 sec

Thought for 7 sec

6/6 Run final focused review after U3 placement

Thought for 3 sec

Completed design checks — found 242 issues.

Thought for 12 sec

6/6 tasks completed.

Thought for a moment

Done. Removed the old U3 and R19, added a fresh U3 TXS0102DCTR, restored its SDA/SCL, power, OE, and GND connections, and placed it back in the interface cluster near J1/J2 and the related resistors.
Note: R19 is now removed, so the SDA_5V pull-up is gone; SCL_5V still has R20. The focused check did not report U3/R19-related issues, but the broader project may still have the previous layout/routing issues.
Add SDA pull-up
Check level shifter
Check pull-up values
Move R20 closer

Finished working

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Project Specification
Goal
Design a compact 4-layer PCB for a dual PCA9685 servo controller interfaced with a Raspberry Pi 5 on the 3.3 V I2C side and an STM32 Nucleo on the 5.1 V logic side.
Required rails
  • 3.3 V reference from Raspberry Pi 5 header for TXS0102 LV side.
  • 5.1 V logic rail from buck converter for TXS0102 HV side, PCA9685 VCC, STM32 Nucleo 5V pin, OE pull-up, and HV I2C pull-ups if used.
  • 6.0 V dedicated servo rail sized for 20 A load with 2 oz / 70 um copper.
  • Common GND across Pi, Nucleo, logic, and servo rails.
Required ICs and interfaces
  • TXS0102 level translator: A1=SDA_LV, A2=SCL_LV, B1=SDA_5V, B2=SCL_5V.
  • PCA9685 chip 1: address 0x40, all address pins to GND.
  • PCA9685 chip 2: address 0x41, A0 to 5.1 V, remaining address pins to GND.
  • STM32 Nucleo header: PB8/D15=SCL_5V, PB9/D14=SDA_5V, PC7/D9=OE, PA0/A0=VSense divider midpoint, 5V fed by 5.1 V logic rail.
Required passives
  • 17x 220 ohm PWM series resistors, one for each used PCA9685 output.
  • 1x 10k OE pull-up to 5.1 V.
  • VSense divider to PA0/A0.
  • Decoupling capacitors for IC supplies and bulk capacitance on logic/servo rails.
  • 4.7k I2C pull-ups on HV side only if allowed/needed after TXS0102 datasheet check.
Layout requirements
  • 4-layer PCB.
  • Inner GND plane and inner power plane.
  • 2 oz (70 um) copper for 20 A servo load paths.
  • Compact functional placement: Pi/TXS0102, Nucleo/VSense, PCA9685/series resistors/servo outputs.

    Goal

    Required rails

    Required ICs and interfaces

    Required passives

    Layout requirements

Documents

    Project Specification

    Implementation Status

Assets

Assets are files uploaded to this project which can be used in various ways.

Dual PCA9685 Servo Controller thumbnail
Compact 4-layer dual PCA9685 servo controller with Raspberry Pi 5 and STM32 Nucleo interfaces

Properties

Properties describe core aspects of the project.

Pricing & Availability

Distributor

Qty 1

Arrow

$7.40–$8.10

Digi-Key

$3.49–$7.03

HQonline

$5.32

LCSC

$13.89

Mouser

$33.71

TME

$0.78

Verical

$26.41–$32.80

Controls