OPB versus PLB

Could somebody explain me why the OPB bus is used for "slow" peripheral and 
the PLB bus for "fast" peripherals? I have a small design with both an OPB 
and PLB bus and both are running at 100 MHz. Of course there is a delay when 
access devices at the OPB bus from a PowerPC (OPB2PLB bridge). But is this 
the only delay or are there more (like burst supports etc.).

TIA,
Frank 

Frank van Eijkelenburg wrote on 28.10.2004 09:55:
> Could somebody explain me why the OPB bus is used for "slow" peripheral and 
> the PLB bus for "fast" peripherals? I have a small design with both an OPB 
> and PLB bus and both are running at 100 MHz. Of course there is a delay when 
> access devices at the OPB bus from a PowerPC (OPB2PLB bridge). But is this 
> the only delay or are there more (like burst supports etc.).
The PLB just has some features that make it better suited for high-speed 
accesses. You can do 64bit-transfers, simultaneous read and write, it's 
hooked up directly to the PPC (so there's no need for a bridge in 
between, which could add more delay), has less latency and so on. The 
PLB is specifically designed to connect your PPC to memories and other 
stuff that needs really high data rates.

The OPB on the other hand is designed to be as simple as possible (i.e. 
use little logic ressources) and interface as easily to your logic as 
possible. It just lacks some of the "advanced features" the PLB has. 
Doesn't necessarily mean it's a lot slower, just that the theoretical 
maximum throughput is a lot higher for the PLB.

cu,
Sean