TWIST routing

This page explains the choices made in its routing, the problems we faced and how we solved them.

Routing Constraints Overview

The SPIN is a four-layered PCB with a particular set of challenges when routing:

• Signal integrity - routing power away from signals
• Pin availability - routing most pins to the castellated holes in the outside of the board
• Appropriate grouding - route the castellated holes according to the state of the art in digital grounding
• Oscillator routing - oscillators require a special technique for grounding which shield their signal from interference
• USB routing - USB require a controlled impedance routing to guarantee signal integrity
• Using the inner layers of the PCB as noise shields
• Routing signals on the bottom layer
• Using the top layer as a component layer

Component placement and Board overview

The board uses a feather footprint of 21mmx51mm with castellated and through holes on the edges. A 3D view of the board shows its component placement.

3D view of the SPIN board showing its components placement

The image above shows the placement of the main blocks of components of the board:

• The USB-C is placed on the right, followed closely by its protections.
• The LEDs are places on either side of the USB connector.
• The LDO and the decoupling capacitors are placed very closely to the USB.
• The STM32G4 is in the middle of the board and tilted to allow easy access to the pins during routing
• The buttons are on the other side of the board, near the JTAG connector.
• The HSE is placed near the OwnTech logo, with plenty of space for their special routing needs.

Overall routing strategy

In terms of routing, the overall strategy is to separate the board signals mainly into two parts: top and bottom. The middle of the board will be used by ground planes which will act as a shield between the top and bottom signals.

Top layer - Components, USB and Power

On the top layer, the

Top-layer routing - Mainly power signals and some of the pins are routed to this layer.

A main +3.3V axis can be seen from the left to the right of the board, bringing the power to the STM32 via its decoupling capacitors, the buttons and the JTAG.

Mid layers - Shielding signals

Mid-layer 1 routing - It is possible to see the special routing technique for the HSE on the bottom left

The HSE requires a special routing technique where its ground is semi-isolated from the ground plane to avoid noice interfering with the oscillators' output signal.

Mid-layer 2 - Mainly used as a GND shield

Bottom layer - Signals

On the bottom layer, signals are seen coming from the through and castellated holes to the center of the board to connect with the STM32G4.

Bottom layer routing - Signals from several pins are routed to the center of the board.