Reference Voltage (Vtref) - Segger J-Link User Manual

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15.4

Reference voltage (VTref)

VTref is the target reference voltage. It is used by the J-Link to check if the target has
power, to create the logic-level reference for the input comparators and to control the
output logic levels to the target. It is normally fed from Vdd of the target board and must
not have a series resistor.
In cases where the VTref signal should not be wired to save one more pin / place on the
target hardware interface connector (e.g. in production environments), SEGGER offers a
special adapter called J-Link Supply Adapter which can be used for such purposes. Further
information regarding this, can be found on the SEGGER website (
To guarantee proper debug functionality, please make sure to connect at least on of the
GND pins to GND (Pin 4, 6, 8, 10, 12, 14*, 16*, 18*, 20*).
Note
*On later J-Link products like the J-Link ULTRA+, these pins are reserved for firmware
extension purposes. They can be left open or connected to GND in normal debug
environment. They are not essential for JTAG/SWD in general.
J-Link / J-Trace (UM08001)
CHAPTER 15
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Reference voltage (VTref)
J-Link supply adapter )