VTref

The target reference voltage (VTRef) is the voltage reference used by SEGGER debug probes and programmers (such as J-Link, J-Trace, and Flasher) to correctly communicate with a target system operating at a specific I/O voltage level.

The target system provides its operating I/O voltage to the probe through the VTref pin (pin 1) of the target interface. This allows the probe to automatically adapt its signal levels to the target's logic voltage.

Overview

VTref stands for:
V = Voltage
T = Target
ref = Reference

VTref is therefore a reference voltage, not a power supply.

The voltage present on the VTref pin defines the logic level used by the debug probe for all target interface signals. Internally, the probe uses VTref as the reference for its input comparators and output drivers. Different target systems may operate at different I/O voltages, for example:

• 1.8 V
• 2.5 V
• 2.7 V
• 3.3 V

Other voltage levels depending on the target device.

VTref pin

On the standard 20-pin target connector of J-Link, J-Trace and Flasher, pin 1 is assigned to VTref.
For details, refer to the connector documentation:

https://kb.segger.com/20-pin_J-Link_Connector

In a typical setup, VTref is connected directly to the target board's I/O supply voltage (usually VDD or VCC).
Example:
Target VCC/VDD → VTref (pin 1)

This allows the probe to automatically detect the target voltage.

Note:
A series resistor between the target VTref signal and the probe must not be used. Adding resistance to the VTref path may affect voltage detection and can lead to communication issues.

Fixed VTref option

In some applications, connecting VTref is either impossible or undesirable.
Examples include:

• Production fixtures where connector pin count should be minimized
• Space-constrained designs
• Applications requiring a voltage different from the externally supplied VTref

For these cases, modern SEGGER J-Link, J-Trace and Flasher models support a Fixed VTref feature.
When enabled, the probe ignores the voltage measured on the VTref pin and instead uses a user-configured voltage internally.

A fixed VTref can be configured in several ways, including J-Flash and U-Flash project setup.
Refer to the corresponding documentation for more information.

FAQ

Does VTref power the target?

• No. VTref is only a voltage reference used by the probe to determine the target's logic level. It is not intended to power the target system.

Should VTref be connected even if the target is already powered?

• Yes. In normal operation, VTref should be connected to the target's I/O supply voltage so that the probe can automatically detect the correct logic level.

Can VTref be omitted?

• Yes. If the probe supports Fixed VTref and a suitable voltage is configured manually, the VTref signal may be omitted.

What happens if the wrong VTref is used?

• Communication may fail because the probe will use incorrect logic thresholds and output levels. In some cases, signal integrity and reliability may be affected.