ST-link v2 at 1.8V?

Paul Mellor · ‎2017-06-26

Posted on June 26, 2017 at 17:37

I am using the ST-Link v2 with a target STM32 board running at 1.8V. I connect the 'target Vcc' to my boards Vcc ok, but the SWDIO and SWDCLK outputs from the ST-link are at 3.3V. Yet it looks like it is supposed to work down to 1.65V. How can I use it with my 1.8V board?

misagh · ‎2017-06-26

Posted on June 26, 2017 at 21:03

Yes, you are right. I couldn't find the ST-LINK schematic but if we assume that is same as NUCLEO ST-LINK, the SW pins are connected to 5 V tolerant pins which the Vih is 1.615 V.

Tesla DeLorean · ‎2017-06-26

Posted on June 26, 2017 at 21:32

The DISCO/NUCLEO schematic really doesn't shed much light on the stand-alone implementation.

PA4, 5, 6 and 7, and PB0, 1 are not 5V tolerant.

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misagh · ‎2017-06-26

Posted on June 27, 2017 at 06:51

You are right. JTAG pins are not FT but SW pins (SWDIO & SWCLK) are FT pins and it seems that there is no big problem to use them. However, as you said, we cannot assume with certainty that the DISCO/NUCLEO schematics are same as stand-alone implementation.

Zt Liu · ‎2017-06-27

Posted on June 27, 2017 at 09:10

Below is a cloned version from China (hey, it just somehow appeared on my desk!)

TI's voltage translator is sitting there.

However....(using above designators )

I made some short test and discover that

SWCLK and SWDIO pins(pin7 & pin9 respectively) are directly connected to

the STM32F103's pin 26 27 throuth 100 ohm resistors(RS1) and with a protection diodes (U2).

But SWDIO also connects to Voltage translator's B3 port, and A3 port connects to STM32F103's Pin25(PB12)

As for the TI's voltage translator, OE and DIR pins are both grounded!

Checked the datasheet, it says.

(A should be the 3.3V side and B for target Vcc)

I think, as least for this cloned version,

the level shifter just works for SWDIO from Vcc target side to Vcc st-link side.

Paul Mellor · ‎2017-06-27

Posted on June 27, 2017 at 10:14

Ok I tested my genuine version too. SWDIO is connected to B3 on the level shifter (SN74LVC8T245) as you say, and also OE and DIR are both grounded. So yes, the output looks like it will always be 3.3v, and the level shifter just used on input for SWDIO.