Before the STM8S001J3 MCUs hit the streets, I received a few samples directly from ST Microeletronics to evaluate. ST finally jumped on a train ran by companies like Microchip or NXP for something like 20 years. It is coming from STM8S family, now being almost classic and well proven chip family.
STM8S001J3 is very similar to STM8S003F3 - so much similar it's probably the same silicon chip, actually. There is nothing inherently wrong about it, as it makes development cheaper - that is easier for manufacturer and cheaper for end customer - and allows to jump on existing development tools, again enabling customers to work with the MCUs easier and faster.
On the other hand, dark side if this decision is easily recognizable. Fitting 20-pin chip into 8-pin frame forces designers into some compromises - which pins to expose, which ones to leave unconnected? Another option is to merge multiple bonding pads to single package pin - and that is route which was chosen in ST.
It allows multiple peripherals to be shared on single pin, which could be advantageous at times, also uncomfortable, as seen later.
From 8 pins of SOIC package, two are for power supply pins - there is not way doing it otherwise. One is sacrificed for VCap pin. As this one is based on STM8S, with supply voltage range up to 5V and relatively modern core manufactured on "dense" manufacturing process, innards of MCU are supplied by lower voltage, requiring internal LDO to do this job. VCap is pin to connect capacitor to keep this LDO stable - and this pin can't be used for anything else.
I wish ST would design also STM8L device in SOIC package. Apart from Vdd being maximum 3,6V (nothing unusual in last 10-15 years), this would free up one pin (now sacrificed to VCap) and sleep current consumption would be as low as usual for STM8L devices. STM8S (including STM8S001J3) do have sleep consumption of about 5-10uA, what is one order of magnitude higher than sleep current of MCU devices designed 15 years ago!
Now, with 5 pins, guys at ST had to decide what to do with the other ones. Higher pin count STM micros do have dedicated NRST (Not ReSeT - low active reset) pin, but from obvious reasons they decided to omit it on 8-pin STM8. And SWIM pin is shared with three other pins; being it not exactly ideal solution, as confirmed by datasheet too:
So, setup your data direction register into wrong state and there you have it - OTP device. We have seen really bad programming interfaces in the past - just like AVR. With more than one way of programming of FLASH, you can easily lock one or another access, having fall-back in 1980's parallel programming. That being said, once the fuses were setup correctly, one didn't have to touch it anymore and any further access was safe. On this device it's different. DDR registers have zero protection agains fuck-ups, from programmer side or even from runaway program - that is particularly dangerous, IMO. With STM8S001J3 you should always have a few spares and hot-air soldering gun for the case you do something goofy in your program. One solution of how to escape this problem is to set-up and use some kind of bootloader to load the FLASH.
While absolutely vital SWIM pin is shared with three other pins, PB4 is just alone. I wish it would be other way around. And alternative pin functions list don't mention MISO signal of SPI interface. Either MISO is forgotten in documentation, or it's forgotten to be bonded out. On STM8S003F3, MISO is on PC7, which is explicitly being listed as NC
I hope it's alternative function to some other pin, otherwise the SPI would be seriously crippled without MISO.
All in all, while STM8 family seems to be one of the better 8-bit MCU design, this 8-pin variant does seem to suffer a bit from compromises during design stage. I really hope ST will release new 8-pin device, as there seems to be space for further optimizations.