8051 Mini Development board

Hello? Any one who can help out on this one?

Is this your project? Or did you just find it somewhere?
You wrote 8085 once in the beginning, that’s something a bit different.
Is the KiCad project available? I could have a look at it.
The posted schematic is unreadable. Both because resolution is too low, and because texts are printed over each other and over the resistors. I don’t like the boxes drawn arount schematic sections. They add nothing that a bit of whitespace on the schematic can not do, and they often need redraws during revisions. They also make the schematic look more crowded, while some whitespace also separates blocks and is more flexible for making revisions.
U1 is a DIP 4 (almost?) exclusively used for optocouplers. What is it doing there? Is it possible it has the wrong footprint assigned?
Hardening I/O with series resistors is sometimes a good Idea, but I’m not sure if it belongs on a generic development board. Personally I prefer the narrower boards that can be put on a breadboard and still have some room left to add some wires. If it’s too wide for a breadboard, I prefer IDC sockets. They can be used both with “Dupont” wires or a flatcable. Putting all I/O on one side of the PCB is then also easier to put it next to a breadboard.

This does not work:

The first thing to do for a good layout, is to have a continuous GND plane, or at least minimize the holes in the GND plane, for example by rotating the row of resistors on the PCB (or in the schematic) by 180 degrees.

I’d also say that 8051 is obsolete. It has gained an enormous amount of momentum, and therefore also takes a long time to die, but why design an 8051 development board in 2022? I never used 8051 myself, I started with uC’s when they became cheap. Used PIC16F84 a little bit, but thoroughly disliked the limited architecture and asm, so I switched to at90s2313 when that became available. It’s also one of the first flash based uC’s, and you could program it with GCC, but it’s also obsolete now.

Your posted BOM does not work out. I do not see the bunch of identical series resistors or the optocoupler like thing.

Routing is not complete, you’ve still got a bunch of ratsnest lines left.

The routing for the crystal is extremely bad. Crystals need short wires Net-(C5-Pad2) Is far too long, it also goes around some parts to then connect to the other side of the capacitor ???

Why use a THT led when (almost) al lthe rest is SMT? It also gets easily damaged at the corner of the CB.

Do you have any GND pins on those single row headers?


You don’t say what the goal of your project is. Is it to:

  • teach how to make a PCB? There are lots of flaws with your design. One is you don’t seem to have made provision for in-circuit programming, obviously you cannot remove the MCU to program externally.
  • teach 8051 programming? You cannot compete with the mass produced boards on AliExpress many of feature advanced 8051 derivatives with higher clock speed, more peripherals, etc.
  • teach MCU programming? Does it have to be the 8051 architecture? There are much better architectures out there.

You can get an ARM MCU, a RISC-V MCU, or an ESP32 MCU for just a bit more money. Assembled.

Also LM7805 is not a LDO regulator. An AMS1117 would be, and a better choice.


I can’t find a decoupling capacitor for at the VDD PIN.
As paulvdh mentioned, the routing for crystal is unacceptable.
Start the layout with placing crystal and it’s load capacitors close to the IC, as well as the decoupling capacitor close to the VDD pin, and the capacitor for RESET line. 100nf should do.

I know it’s the aesthetics only, but I can’t stand this messy out-of-grid rows of resistors. Place them on grid, and the layout will look much more professional.
And avoid putting a VIA within the SMT pads, as you’ll loose control over solder paste distribution.
I’d also recommend to include some ESD protection for the I/O pins, especially since they’re equipped with pin headers which will be ESD magnets.

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