[En-Nut-Discussion] R34 on Ethernut 2.1

[Harald Kipp]

Petri Hyötylä schrieb:
> Just out of curiosity and maybe to learn something about circuit design...
> What is the reason behind R34?
>   
egnite maintains maximum backward compatibility when adding new 
features. This is specifically true for the expansion port.

In early board versions pin 64 wasn't allocated and some people used 
this pin for their own modifications. For example, you may remove the 
32kHz crystal and add two additional wires to allocate PG3 and PG4 to 
expansion pins 63 and 64. Then we were asked to provide ALE at the 
expansion board. By not directly attaching ALE,  we allow to keep 
existing modifications.

A more advanced example is the Ethernut 1.3 EEPROM emulation. In order 
not to steal port bits for this function, we managed to use the upper 
address bits as port pins during initialization, which had been really 
tricky. Nevertheless, even this can be disabled by removing R7 and 
adding R37. See
http://www.ethernut.de/en/hardware/enut1/index.html
"How to Make Rev-G and Rev-H Compatible to Rev-F"

This way an essential feature had been added without spoiling backward 
compatibility.

Regards,

Harald