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RE: [Discuss-gnuradio] Embedded control of USRP

From: Jim Hanlon
Subject: RE: [Discuss-gnuradio] Embedded control of USRP
Date: Thu, 20 Apr 2006 16:06:44 -0500


> -----Original Message-----
> From: address@hidden 
> As always, there are lot of differences between processors.  
> Some of the VIA's are dogs, other's aren't so bad.  We've run 
> the USRP and
> gmsk2 code on one particular compact VIA board, and it wasn't 
> horrible.
> We were able to run GMSK at a few 100 kb/sec with it.
> The USB expansion card could have been the problem.  How do 
> you know it was lack of CPU?
To be honest, we did not try a controlled experiment of varying CPU speed, nor 
engage in application profiling, or a lot of other
things we might have done. We were in cut-and-try mode. And our trial of the 
"fanless industrial control computer" indicated that we
were on the margins of acceptable behavior; not promising for a potential 
product. We saw to it that the USB card was not sharing
interrupts with anything demanding (as I recall, we did not have the USB card 
isolated on its own IRQ.) And all the other reports of
success from the group involved multi-GHz machines. There was even sage advice 
to move to 64 bit CPUs. The prudent call seemed to
move away from the "slow and cool" mode, as attractive as it is for a lot of 
good reasons.

That said, CPU clock speed and USB signal timing are two different things. 
Probably the card's interconnection bus protocol
(careful, some single board computers have oddball expansion connectors), and 
the IRQ service logic, are more important factors. And
as Eric points out, the answer to these "is the computer fast enough" questions 
all depends on the intended application's sample
rates. So if your app allows it, and you have the time and the hardware, by all 
means dig deeper.
Jim Hanlon 

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