Hi Chris,

why should anybody develop a micro, which nobody need ?

My procedure was completely different. I search not the device with the highest capabilities, I search always this one, which meet my requirements.

So on my experience I needed on my projects only 4kB up to 40kB code space. And in one case I needed also 8kB RAM and selected the P89C668.

The main reason, why the AVR with 128kB was developed was, that you can not connect external code memory to the AVR. But e.g. on the DS80C390 up to 4MB code memory can be added.

Also C programs on the AVR need most times more code space as on the 8051.
The reason was, that the C51 compilers are very efficient adapted to the 8051 architecture. The AVR compilers work like C compilers on the bigger 16 and 32 bit world with tons of push and pop on every subroutines.
But the compilers on the 8051 use data overlaying to use efficient the 128 byte internal RAM. Thus especially small subroutines and interrupts are extremely short and fast.
The only drawback of this method was, that subroutines are nonreentrant. But this was not real a drawback, since on the main loop approach this was mostly not needed. Nevertheless if needed to be reentrant, this can be declared and then some overhead was generated for these routines only.


The main drawback of the AVR architecture was, that no different interrupt priorities can be selected.
E.g. on most of my projects there are also interrupts, which must be served ASAP.
That this was an important issue can be seen on this fact, that most new 8051 derivates support not only two interrupt priorities, they can select four different interrupt priorities.
Theoretical it was possible to simulate different interrupt priorities in software, but this was gulping many of the CPU speed.
So if different interrupt priorities needed, the interrupt response was always shorter on the 8051 in comparison to the AVR.

Additional the possibility to select a different register bank on interrupts and many instructions, which not affect the status register can cause, that not only the interrupt response was shorter, also the execution of the whole interrupt handler can be faster.


Thus, why I use the AVR only on very small applications, where different interrupt priorities not needed. E.g. I use the Attiny12 and Attiny15L devices only.


And as Rob pointed right, the 8051 improvement will never be finished. So Cygnal newest device contain 128kB Flash and will run with up to 100MIPS.
Also on the Atmel web you can found always new 8051 derivates every year. The newest was the 8051 with MP3 player, the T89C51SND1.


Peter