[Gb-ccb] FPGA design

[John Ford]

Martin Shepherd writes:
 > 
 > On Tue, 30 Mar 2004, Brian Mason wrote:
 > >...
 > > taking such care in the documentation.  Our only major comment is that we
 > > think the approach of having a write-only interface to the device is a
 > > mistake-- this is a view that emerges from long and painful experience
 > > with other devices.
 > 
 > In the CBI Steve Padin implemented lots of write-only VME registers,
 > and there were no problems, since I just kept a record of what I had
 > sent. I don't understand the utility of having readback of values that
 > are never changed by the FPGA. 

I might agree with you if we had a PCI or VME interface, but, alas, we
have a parallel port, and there is a world of difference in the
reliabilty of the communications channel.  As Brian says, through
bitter experience we do not approve of write-only communications
channels.

 > All this would tell you was that the
 > EPP interface in the FPGA was keeping a record of these register
 > values. 

Indeed, this is precisely what we want to know!

John