Unknown TSSOP-20 chip identification

[MPrih.1]

Hi all.

I'm struggling with certain device and I'd like to identify unknown TSSOP-20 chip on the PCB. Chip must be an MCU and based on the pinout it's very close to some STM8, eg. STM8AF6213. 

There are VCC and GND connected to Pin 9 and 7 respectively, and there is also kind of debugging header nearby and its two pins are connected to pins 4 and 18 of my unknown chip which seems like NRST/SWIM on STM8 chip.

But, problem starts here - my device has pins 5 and 6 connected to analog signals (from potentiometers) and STM8AFxxxx do not have ADin(s) on pins 5 and 6...At least i was unable to find such a chip.

I'm not very familiar with STM8 and maybe my unknown chip is some legacy/deprecated chip so that's my question. Are you aware of some STM MCU where VCC, GND, RST and Debug iface is on 9,7,4 and 18 pin respectively (for TSSOP-20 package) and pins 5,6 are Analog IN???

Many thanks!!!

[Tesla DeLorean]

Or it could be something entirely different..

Got some pictures of this board you're trying to reverse engineer?

[MPrih.1]

Thank you for your answer.

Yes It could be something completely different. The device is control unit for hobby MIG welder.

The control unit is taking care about triac drive (Power regulation for welding transformer), it's taking care about the wire feeder, etc...

The unknown chip U2 is without any text on the package. Maybe some text was there but it was under thick layer of paint and it's gone since I've scratched the paint, only dot at pin 1 is present. I do not think that this chip is ASIC, because if it would be an ASIC, why the vendor has placed J1 header on the PCB. There is no adjusting trimmer on the PCB (e.g. for final setting during manufacturing process) and nothing is connected to this J1 header. Header pins are +5V, GND and two pins are exclusively connected to the U2 to pins 4 and 18. That's why I'm convinced that U2 is an MCU.

I'm familiar with MSP430 and AVR family and Im not aware of a chip with Vcc and GND on pins 7 and 9 (There is no other pin with direct connection to the GND and Vcc) so i went through different MCUs and I found that STM8 is veeery close...Except for pins 5 and 6 which must be AD inputs in my case and there is also no voltage reference on pin 8 in my case... etc...

[waclawek.jan]

The STM8(S003F3) pinout is very popular amongst various chinese and even some non-chinese manufacturers (often, chips using it do have "003" in their marking), so it's most probably a non-ST chip.

JW

[dlkeng]

I recently reverse-engineered a USB multifunction tester device that had a similar unmarked MCU chip on it that had a similar pinout. From what I could find, this MCU appeared to be a CH32X033F8P6 Chinese device.

See: https://www.wch-ic.com/products/CH32X035.html

See: https://hackaday.io/project/194408-ch32x033-f8p6-development-board

See: https://github.com/wagiminator/Development-Boards/tree/main/CH32X033F8P6_DevBoard