51 microcontroller development board (compatible with STC and AT chips)

This project
is a creation based on my ideas for a 51 microcontroller development board
. Recently, my school offered a 51 microcontroller course, and I created this project with a learning attitude. I also hope to add my own ideas and make some improvements to the project. Here are
some things I did:
Compared to other development boards on the market, this board has an ISP interface that can be used with STC. Alternatively, you can use either an AT51 microcontroller or an STC chip (although an STC chip is sufficient
and a Type-C interface is added, reducing the need for cables; it has a built-in CH340G chip for direct programming and an automatic download circuit, eliminating the need for a switching power supply for downloading
– most importantly, and also my biggest mistake). If you want to expand with RAM or ROM, that's also possible, as the development board has DIP switches for these functions (if you really want to try it out...).
The bottom has an FPC cable to help you quickly connect an external expansion board. (Of course, you'll need to modify
the M3 screw post; the upper right corner connects to the VCC for easy stacking, and the other copper posts are grounded).
It provides 3.3V power and TXS0108 bidirectional level conversion communication,
eliminating the need for a 3.3V module with a 5V level for the 51 microcontroller.
The rest are optional:
8 LEDs, an infrared receiver, a buzzer, a common cathode digital tube driven by 74HC138 and 74HC245,
a built-in DS1302 time chip, a DS18B20 temperature chip, 4 buttons, and an AT24C02. The memory chip includes ISP and TTL auto-download circuitry
(CH340G), the USB-to-serial chip has a step-down chip that can output 3.3V, and the TXS0108 bidirectional level conversion communication chip
board was prototyped several times, but thankfully all the problems were solved (thanks to JLCPCB QAQ
for providing the schematic).