AVR-Chat

Dave VanHorn schrieb:
> At 09:35 PM 3/27/2005, Reza wrote:
> 
>>cpu/micro depends on application. but as usual, you
>>shoud use but tranceivers and isolate address/data bus
>>of each cpu from the other.
> 
> Not really necessary with a microcontroller, if you design carefully.

This is what I am thinking about: What needs to be considered so I can 
drop the transceivers. Board space is tight and each chip I can remove 
is worth a lot (of time to think about other solutions).

> The chips power up with the ports as inputs.
> If you implement a couple of handshaking lines and a few logic gates, you 
> can connect both chips to the SRAM directly.

Exactly what the initial idea was. Now, what would "directly" really 
mean? Is it really a good idea to directly connect the busses to each 
other? This leaves the possibility that two devices are driving the same 
lines when software fails.

If I can't do this - will series resistors be enough and what would the 
penalty be?

> You don't HAVE to use the SRAM interface after all, you can do the lines as 
> I/O pins.

That is in fact correct, but using the SRAM interface gives some very 
nice properties:
- Lowest number of cycles from data ready to data written to the shared 
memory.
- Small number of additional pins needed: There might the the need for 
an additional SRAM attached to one of the AVRs that is not shared but 
connected to the same bus. Both chips (the shared one and the other one) 
would be switched through a port pin.

Regards,
Philipp