This project provides an stlink "clone" that takes < 13K flash on an stm32f042. It does not support tracing (swo), but it does play nicely with openocd. We developed this so that we could include stlink support in a baseboard for programming/configuring sub-gram data loggers that we are developing for collecting actvity data of songbirds.
In our application we needed to include battery charging hardware and did not want to ask our biology partners to use a platform cobbled together from off-the-shelf stlink hardware + adapter boards + power supplies.
An example board schematic is provided to help understand the code. There are a few things to note about the board design that may not fit your needs
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This board was designed to talk to a device with a npn transistor in the reset line. Thus, the firmware reverses the sense of reset from regular stlink boards
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This board includes devices to electrically isolate the attached "tag" when powered off. This may or may not be necessary in your application
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The simplest case hardware would be: stm32f042, connectors, voltage regulator, passives, and a boot0 button. In our application we use dfu-util to flash the stm32f042.
A few notes about the firmware
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The firmware uses ChibiOS which is available under various licensing options. If you plan to use this firmware in a commercial product you will need to obtain an appropriate ChibiOS license.
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The firmware includes header information from various sources (marked) The position of FSF seems to be that a GPL header can be used without forcing the whole to be GPL'd. I don't know the answer to this
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In my ChibiOS projects, I use FMPP to generate board .h/.c files. This repo includes the generated files (board/), but if you port this to another context, you may want to use the boardcfg/ files
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The firmware uses an STMicroelectronics VID/PID. You should obtain your own VID/PID if you plan to use this code in anything other than a hobby/research environment. We do not have permission to use this VID/PID
Good Luck !