STLink-V2 - State of SWCLK and SWDIO Without VTref Voltage

[hari_kumar]

Hi everyone,
I’m using an STM32F207ZGT6 with an STLink-V2 connected to a Raspberry Pi 5 for programming. The VTref, SWCLK, SWDIO, NRST pins of STLink-V2 are connected to the controller board. VTref is coming from the controller board.
Once the programming is completed, the power supply to the controller board is turned off, so the VTref (target voltage) is removed, while the STLink remains powered and connected to the Raspberry Pi 5.

In this condition, what happens to the SWCLK, SWDIO, and NRST pins of the STLink-V2?

Do they go to a high-impedance state, or
Are they held at some defined logic level (high/low)?

I want to understand if there will be any unwanted signals or voltages present on these lines when the target is unpowered.

Thanks in advance! :)

[TDK]

It depends on your programmer. Generally they are not driven without VCC_TARGET. The STLink-V2 isn't an ST product.

Even if these pins are driven, the chip will be fine. It's within the allowable voltages.

It would be absurd for a manufacturer to design a chip that self-destructed if a programmer is attached without power.

[hari_kumar]

Hi @TDK 
Thank you for your response.

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@TDK wrote:

The STLink-V2 isn't an ST product.

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Could you please clarify this? As far as I know, ST provides official documentation for the STLink-V2 in their website (ST-LINK/V2 | Tool - STMicroelectronics”), so I assumed it is an ST product.

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@TDK wrote:

Even if these pins are driven, the chip will be fine. It's within the allowable voltages.

I understand your point. However, I’m still not clear on one aspect:
If VTref is not present, how does the STLink determine the appropriate I/O voltage levels for SWCLK, SWDIO, and NRST? Wouldn’t the absence of VTref make it unclear what logic levels should be driven?

I’d appreciate any further clarification on this.
Thanks again!

[Andrew Neil]

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@hari_kumar wrote:

I assumed it is an ST product.

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There are clones on the market.

To be sure, see: How to recognize a genuine ST-LINK/V2 versus a cloned one

In particular, note that anything like this is certainly a fake.

Please post good, clear photos of both sides of your unit.

[Andrew Neil]

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@hari_kumar wrote:

ST provides official documentation for the STLink-V2 in their website (ST-LINK/V2 | Tool - STMicroelectronics”)

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Possibly @TDK is being pedantic about "ST-LINK/V2" vs "STLink-V2" ?

But, as previously noted, it is certainly true that many things sold as "STLink-V2" are not genuine ST products - so you do need to verify!

Example of a clone here.

[AM]

Hello Harikumar,

ST-LINK/V2 probe drives SWCLK, SWDIO, while there is a debug session on going: while a programmer has asked to start the JTAG/SWD communication, until the debug session is closed by the programmer. When there is no debug session on going these pins are configured as input floating.

Target VCC (VTref in your case) voltage has no impact on this behaviour (there is no level shifters on ST-LINK/V2): target voltage is displayed in CubeProgrammer as an information.

TNRST is configured as output open drain, (high state by default) while not managed by a tool.

Best regards

[TDK]

The ST-LINK/V2 is an official tool. The STLink-V2 is not. Which one do you have?

[S C]

Hello,

regarding the voltage, I'd like to point out the note in the first chapter of the user manual: "The ST-LINK/V2 can communicate with targets operating below 3.3 V but generates output signals at this voltage level. STM32 targets are tolerant to this overvoltage. If some other components of the target board are sensible, use ST-LINK/V2-ISOL, STLINK-V3MINIE, or STLINK-V3SET with a B-STLINK-VOLT adapter to avoid the impact of overvoltage injection on the board.".

This applies to genuine ST-Link/V2 with genuine STM32 target.

[hari_kumar]

Hi @Andrew Neil 
I'm using ST-LINK/V2. Sorry for the misunderstanding

-Thanks

[hari_kumar]

Hi @TDK 

I'm using ST-LINK/V2. Sorry for the misunderstanding

-Thanks