Powering STM32 Embedded Displays via expansion connector

Expansion connector on STM32U5 board (top right corner)

STM32 Embedded Displays from Riverdi provide multiple power interfaces. The recommended method is using the dedicated POWER connector (P2).

It is also possible to power the board via the expansion connector (P8), but this method is not officially intended as a primary power input.

Expansion connector – power rails

The expansion connector exposes internal power rails:

• VCC_+5V
• VCC_+3.3V
• GND

These pins are designed as outputs for external peripherals, with defined current limits.

Powering via expansion connector

Why would you do that? Main reason would be using only one connector in your setup instead of expansion connector and power connector.

The board can be powered through the expansion connector because:

• the power rails are directly connected to the internal supply
• current can flow in both directions

However, in practice:

• the board should be powered via 5V rail (VCC_+5V)
• the 3.3V rail is generated internally from 5V

Supplying only 3.3V:

• will not power the backlight (requires 5V)
• will result in limited or non-functional system behavior

Important limitation

The expansion connector:

• is not protected as a power input
• has no reverse polarity protection
• has no dedicated input filtering or protection stage

This is different from the main power connector (P2), which is designed specifically for safe power input.

Important note

Powering the board via expansion connector is possible, but:

• it is not the intended usage
• it is not protected
• it is done entirely at your own responsibility

Any instability on the supply line may directly affect the board.

Risks

Using the expansion connector as a power input may lead to:

• damage from voltage spikes
• back-powering conflicts (multiple supplies connected)
• overcurrent through unintended paths
• permanent damage to MCU or power circuitry

Practical guidelines

If this method is used:

• use only one power source at a time
• ensure correct voltage level (5V)
• avoid hot-plugging
• use stable, low-noise power supply
• avoid long or poorly shielded cables

Summary

Powering via expansion connector:

• works electrically
• is sometimes used in lab setups
• is not a safe or recommended method

Use the POWER connector (P2) whenever reliability and protection are required.