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I have built a board for driving some external peripherals which need up to 9V. The CPU (Cortex M0) is powered with 3.3V.

My main power supply is a 9V battery. A step-down converter provides the 3.3V for the CPU (conntected to VBAT and VCC pins).

Unfortunately, I am having some troubles to connect a JTAG interface (ULINK2 with ARM20-pin interface) to the CPU which also provides VCC lines with 3.3V to power the CPU. I'm not sure how I should connect the VCC lines of the JTAG interface.

Do I have to combine these two power supplies (also connect JTAG's VCC to VBAT and VCC)? Will the JTAG interface work if I do not connect VCC at all?

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    \$\begingroup\$ What does the data sheet tell you? \$\endgroup\$ – Andy aka Sep 14 '15 at 18:25
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The ULINK2 page you linked in the question says this about the Vcc pin:

VCC Positive Supply Voltage — Power supply for JTAG interface drivers.

This means that the VCC pin on the ULINK does not supply power to your board, rather, it uses power supplied by your board to power the driver circuits on the ULINK pod. This allows the ULINK pod to be compatible with circuits running at different power supply / logic levels.

In your case, tie the ULINK VCC pin to the 3.3V supply on your board.

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