As stated before, 8051 is here to say. Very much like the 8086 is still alive with the newest x86-64 architecture. Since the Arduino Uno is the de-facto form factor for many things, let’s try to emulate 8051 on AVR.

Planning & Choices

Some choices needs to happen, in order to be able to scope the project.


For simplicity, I’ll focus on emulating the AT89C2051 with an ATmega328P. Those 2 chips are very good examples of each architecture, as they are now ubiquitous, thanks to 4 digit clocks kits for the AT89C2051 and the Arduino UNO for the ATmega328P.

Pin Mapping

We need to assign pins. I’m trying to keep the functionality the same, even if it means to move bits around when programming.

A major difference is to assign a specific PIN to RST for the 8051, so we can reset the emulated 8051 without resetting the AVR.

8051 AVR 51 Function
P3.0 PD0 RX
P3.1 PD1 TX
P3.2 PD2 INT0
P3.3 PD3 INT1
P3.4 PD4 T0
P3.5 PD5 T1
P3.7 PD3  
P1.0 PD6 AIN0
P1.1 PD7 AIN1
P1.2 PB0  
P1.3 PB1  
P1.4 PC0  
P1.5 PC1  
P1.6 PC2  
P1.7 PC3  

Some emulator-related PINS are also used.

I’m thinking about reusing a GPIO (PC4/SLOW) to signal that the emulation is lagging from realtime. The idea being that emulator should be running at a precise clock divider from the main clock. Prescaling as much as needed to cope with the emulation tax, and also adding as much delay as needed on the fast instructions.

If there is an instruction that is expensive to emulate, I’m planning to “borrow” delay cycles from the following fast instructions in order to be accurate on average. And have the SLOW pin to signal that ‘borrowing’ state while active.

AVR EMU Comment
PC4 SDA    

Emulation Software

The chosen software is emu8051 that we covered in a previous post. It is hackable enough to be useful, and not using any library outside standard C.

Some enhancements need to happen, as it needs to be optimized for running on the limited AVR platform.

I’ll also leverage every hardware accelerated feature, such as the AVR USART, in order to decrease the emulation tax.


Let’s see that in another post. 😉