AVR port of emu8051

[steveschnepp]

I want to be able to have an ATmega328P emulating an AT89C2051.

So here is the experimental branch that makes it possible. And that is the why I started #20 (fixed sized integers)

It is obviously not for merging. yet.. 😉

For the AVR port, I plan to make codeMem pointing to FLASH, which could be uploaded via TX/RX thanks to a bootloader.

Enabling 8051 programming with the Arduino IDE and bring back that glory from its past 🤣

[steveschnepp]

Btw, I do know it is totally pointless. But where would be the fun if not...

[jarikomppa]

It's not entirely pointless =) people are using microcontrollers to masquerade as other parts all the time..

[steveschnepp]

I just posted the pin mapping I'd like to reuse.

Copying over the relevant parts:

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

8051	AVR	Function
RST	PB2	RST 51
XTAL1	PB6	OSC1
XTAL2	PB7	OSC2
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
none	PC4	SLOW

Finally, 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.