CM5 Ethernet Switch

flux

I’m blocked now because the account has no available ACUs, so I can’t continue executing tool-based schematic edits or verification.
Accomplished so far:
  • Updated Project Specification for the smart-router architecture.
  • Added the main schematic parts: CM5, LAN7431, KSZ9477, six RJ45 MagJacks total, 12 V input connector, and critical PCIe/bias/reset passives.
  • Wired the primary packet path:
    • CM5 PCIe → LAN7431
    • LAN7431 RGMII → KSZ9477 port 6
    • CM5 SPI management → KSZ9477
    • CM5 native Ethernet → WAN/management RJ45
    • KSZ9477 ports 1–5 → five switched LAN RJ45 ports
Open items:
  • Power tree is not complete: regulators, rail sequencing, decoupling, and switch/NIC rail wiring remain.
  • RJ45 center-tap/Bob Smith termination, ESD protection, and LEDs remain.
  • LAN7431 and KSZ9477 clocks/straps need to be completed.
  • DC input connector pinout/current rating should be re-verified; the provisional connector may need replacement.
  • ERC still has expected floating-pin/layout-readability warnings because support circuitry and no-connect markings are incomplete.
Suggested next steps once ACUs are available:
  1. Finish the power tree and regulator selection.
  2. Add decoupling, clocks, reset timing, and strap resistors.
  3. Add Ethernet ESD/termination networks and verify RJ45 pin mapping.
  4. Run ERC and fix/justify remaining schematic issues.
Finish and verify
Finish power tree
Add Ethernet protection
Complete support circuits
Run ERC review

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Project Specification
Project Overview
Status: Draft
Design a Raspberry Pi Compute Module 5 carrier board that acts as a smart-router / managed-switch appliance. First revision uses a two-IC Ethernet architecture: a PCIe-to-RGMII Ethernet controller bridge from the CM5 into a managed switch fabric, plus five external 1G switched LAN ports. Input power is 12 V DC with no PoE in the first revision.
Intended Use
Prototype/validation carrier board for experimenting with CM5-based packet processing, firewall/NAT/router behavior, VLANs, and managed Ethernet switching.
What the Device Should Do
  • Host a Raspberry Pi Compute Module 5 as the Linux/OpenWrt packet-processing engine.
  • Give the CM5 a real packet data path into the switch fabric through PCIe, not only SPI/I2C management.
  • Provide five external 10/100/1000BASE-T switched RJ45 LAN ports.
  • Provide a separate CM5 native Ethernet RJ45 suitable for WAN or management.
  • Accept 12 V DC input and generate all required board rails.
Main Features
  • Raspberry Pi Compute Module 5 carrier interface.
  • Microchip LAN7431 PCIe-to-RGMII Ethernet controller as the CM5 data-plane bridge.
  • Microchip KSZ9477 7-port managed gigabit switch, using ports 1-5 for RJ45 LAN and port 6 as the RGMII CPU/data port.
  • Five external RJ45 connectors with integrated magnetics for switched LAN.
  • Optional CM5 native Ethernet RJ45 for WAN/management.
  • Status LEDs, reset/boot controls, debug UART, test points.
  • 12 V input protection and regulated rails.
System Architecture
Diagram



  
    
  
  
    
  



























12 V DC Input
5 V CM5 Rail
Switch and NIC Rails
Raspberry Pi CM5
LAN7431 PCIe-to-RGMII
KSZ9477 Managed Switch
PCIe Gen2 x1
RGMII CPU/Data Port
SPI Management
5x RJ45 LAN Ports
CM5 Native GbE PHY
WAN/Management RJ45
Hardware Subsystems
  • Power: 12 V protected input, 5 V high-current rail for CM5, 3.3 V / 2.5 V / 1.2 V rails as required by LAN7431 and KSZ9477.
  • Compute: Raspberry Pi Compute Module 5 using its 200-pin carrier interface.
  • PCIe data plane: CM5 PCIe Gen2 x1 connected to LAN7431.
  • Switch fabric: KSZ9477 managed switch with RGMII port 6 connected to LAN7431 and copper PHY ports 1-5 connected to RJ45 LAN ports.
  • Ethernet WAN/management: CM5 native Ethernet MDI pairs to separate RJ45 magnetics.
  • Debug: CM5 debug UART, nRPIBOOT access, switch reset/interrupt, SPI management, test points.
Interfaces and Connections
  • External power: 12 V DC input.
  • External network: 5x switched 1G RJ45 LAN ports from KSZ9477.
  • External WAN/management: 1x native CM5 1G RJ45 port.
  • Internal data path: CM5 PCIe Gen2 x1 to LAN7431, then LAN7431 RGMII to KSZ9477 port 6.
  • Internal management: CM5 SPI to KSZ9477 register management; reset and interrupt GPIOs.
Power and Runtime Expectations
Externally powered; no battery runtime target.
Power Tree and Power Budget
Preliminary rail sizing assumptions pending final regulator selection:
  • 12 V input -> 5 V rail for CM5, target at least 2.5 A for CM5 with margin; design target around 5 A class for system headroom.
  • KSZ9477 full 1G operation typical currents: 2.5 V AVDDH about 330 mA, 3.3 V VDDIO about 45 mA, 1.2 V AVDDL about 460 mA, 1.2 V DVDDL about 750 mA, plus SGMII rails if used.
  • LAN7431 rail currents and support components must be finalized from the Microchip datasheet before regulator selection is locked.
  • Avoid deriving high-current low-voltage rails from 12 V with LDOs; use switching regulators where practical.
Manufacturing and Assembly Expectations
SMD assembly, prototype-first. High-speed Ethernet, PCIe, and RGMII routing require at least a 4-layer PCB with continuous ground reference and controlled-impedance rules.
Firmware-Relevant Hardware Requirements
CM5 Linux/OpenWrt should see a real PCIe Ethernet interface through LAN7431 and use that as the CPU/data port toward the KSZ9477 switch. KSZ9477 is managed over SPI for switch configuration, VLANs, QoS, mirroring, and related features.
Physical Design Expectations
RJ45 connectors should align along one board edge. CM5 connector placement must follow Raspberry Pi mechanical guidance. PCIe and RGMII traces must be short, impedance-controlled, and referenced to solid ground.
Important Design Decisions
  • Use two Ethernet ICs for the smart-router architecture: LAN7431 for CM5 packet data path, KSZ9477 for managed switch fabric.
  • Use KSZ9477 ports 1-5 as external switched LAN ports and port 6 as the RGMII CPU/data port.
  • Use CM5 native Ethernet as a separate WAN/management port.
  • No PoE in/out in first revision.
  • Use 4+ layer controlled-impedance PCB for Ethernet, PCIe, and RGMII.
Assumptions
  • User accepts a 5-port LAN + separate WAN/management first revision instead of 8 switched LAN ports.
  • User accepts SMD/prototype assembly.
  • LAN7431 and KSZ9477 driver/device-tree support will be validated during firmware bring-up.
  • Exact regulator choices may change after detailed power budget and datasheet extraction.
Change Notes
  • Updated architecture from 8-port switch-only concept to CM5 smart-router design using LAN7431 + KSZ9477.

  • Project Overview

  • Intended Use

  • What the Device Should Do

  • Main Features

  • System Architecture

  • Hardware Subsystems

  • Interfaces and Connections

  • Power and Runtime Expectations

  • Power Tree and Power Budget

  • Manufacturing and Assembly Expectations

  • Firmware-Relevant Hardware Requirements

  • Physical Design Expectations

  • Important Design Decisions

  • Assumptions

  • Change Notes

Documents

  • Project Specification

Assets

No assets added.

CM5 Managed Ethernet Switch

CM5 Managed Ethernet Switch thumbnail
Raspberry Pi Compute Module 5 carrier board for an 8-port 1G managed Ethernet switch with 12 V input and no PoE in the first revision.

Properties

Properties describe core aspects of the project.

Pricing & Availability

Distributor

Qty 1

Arrow

$25.49–$27.02

Digi-Key

$0.00

LCSC

$74.24

Mouser

$29.79

Verical

$51.15–$100.89

Controls