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

16.5 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 con-
trol 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 pur-
poses. Further information regarding this, can be found on the SEGGER website
( https://www.segger.com/jlink-adapters-supply.html )
To gurantee 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 nor-
mal debug environment. They are not essential for JTAG/SWD in general.
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