DIY STM32 Programmer (ST-Link/V2-1)
ST-Link devices are almost essential when developing software for the STM32 MCUs. They are relatively cheap and easily available.
They roughly fall into two different categories. ST themselves make a number of them and they all work quite well.
One thing that annoys me with the original ST-Link devices is that the programming header is unlabelled. This essentially mean that every time I use it I have to Google for the actual pinout.
Also, the ones I've got (the one in the photo to the right) are ST-Link/V2 devices.
Before diving into any further details, credits to Martin from EmbeddedBlog who went through the same as here. His project is linked under Miscellaneous Links.
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There are also a lot of Chinese clones available and while they have some limitations they also work quite well.
Unfortunately these devices are also ST-Link/V2 devices, and they do not have the SWO pin on the header, so any debug trace is impossible. For flashing and normal debugging they work fine though.
In short, the ST-Link/V2.1 devices offer a few extra features:
- Serial Wire Trace (SWO)
- Virtual serial port
- Mass storage
All features that, while not absolutely essential, does come quite handy.
Another reason for creating my own "programmer" is that I can experiment with using a GigaDevice GD32F103, which are much easier to source at the moment.
Original ST-Link V2.1
The schematic of the original ST-Link is readily available from the various Nucleo development modules.
A bit messy schematics but it'll do.
Schematics
Based on ST's original schematics, creating a version using only parts from JLCPCB's parts library was easy (and much nicer if I shall say so myself).
One thing I left out was the serial resistors of the programming header. This might very well come back and bite me in the ass but space is going to be at a premium.
The main difference hardware wise between ST-Link/V2 and ST-Link/V2-1 is the renumeration circuitry:
I think what is going on there is ensuring the D+ is pulled high through the 1.5 kOhm resistor unless the RENUM signal is forced low. The voltage divider R4/R5 and R6 are merely there to protect the transistor itself. While I do understand what they (ST) are trying to achieve, I would say they could have achieved this much simpler. IF the D+ was simply permanently pulled high by the 1.5 kOhm resistor, since it is connected to a GPIO pin, this GPIO pin could simply be briefly yanked low at MCU reset.
As for MCU the original ST-Link/V2-1 uses an STM32F103CB (128 kB flash).
PCB
For hobby projects like this I will usually stick with double-sided PCB. But in this particular case I wanted this to fit into the aluminium "sleeve" of the Chinese knock off ST-Link devices, so space was at a premium. Also, pricing wise there's not much difference these days, so I went for a four layer PCB and kept +3.3V and GND on the two inner layers.
For SMT assembly, JLCPCB has a minimum size of 20 mm in either direction:
For a USB stick 20 mm is too wide, so I had to design the PCB with a breakaway part connected with mouse bites.
The final rev. a layout looks like this:
The KiCAD 3D model:
Revision a
A quick order to JLCPCB and within a week I had my first "samples".
I had to solder the USB connector by hand and it turned out that I was either out of IDC connectors (or couldn't be bothered to search long enough to dig them out of a box), so I used a simple dual-row standard 100 mil header.
That is "good enough". For rev. b, I will put 1 mm more spacing between the IDC header and the components - that was a bit tight.
Firmware
To be added
Testing
To be added