Electronic – JTAG and additional external power supply

cortex-m0jtagpower supply

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?

Best Answer

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.

Related Solutions

Electronic – How to use RPI 2 to debug RPI model B via JTAG with OpenOCD

Hmm, looking at your pinout,

TDO: Pin 19 (Model B) -> Pin 21 (RPI2)

TDO is Pin 18 or Pin 29, but not pin 19.

Electrical – How to identify a VCC pin on PCB use digital multimeter

Turn the board on, turn the DMM to measure DC voltage up to 20V (or auto range), put your black probe to a ground pad an look for some capacitors - electrolytic, tantalum or even SMD ceramics which look thicker then others. Put your red probe on capacitor's (+) pin/pad and check if this is the desired voltage (3.3V). Then if you are looking for a more convenient to use pad you can turn off the device. Test with the multimeter if the voltage is fallen enough (below 0.5V), switch the DMM to measure diode, put one of the probes on the pad you already know is VCC from previous measurement an search for a pad on which if you put the other probe the DMM will say "0.000" or similar.

About measuring continuity and diodes. Actually these tests are quite similar, even on some models both test are combined. The difference is that continuity test show on the display something close to the ohmic resistance of the connection, while diode test shows mV of PN junction.

Most commercial DMMs inject curent and measure resulting voltage. That's because it's simpler and much more accurate to inject a predefined constant current and to measure low voltage than measuring very small current, resulting form a constant voltage. The current depending on DMM's model is between 0.1mA and 2mA and is very unlikely to burn anything. There are 3 main reasons you have to make a continuity measure on a turned off equipment

for a HV equipment it can burn your DMM, not the equipment itself;
for a sensitive LV equipment multimeter's current can change some input's state and lead to undesired response of the system;
when you place your probes over a certain voltage (say 1V or more) in one direction your DMM will state "connection", on other direction it will say "disconnected" and none of them would be true :)

Best Answer

Related Solutions

Electronic – How to use RPI 2 to debug RPI model B via JTAG with OpenOCD

Electrical – How to identify a VCC pin on PCB use digital multimeter

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