Re: Busrequest always asserted

Started by Kenneth Porter January 14, 2000
One SHARC is always the bus master, and will hold BR until another
asserts its BR. If the original bus master does not need the bus at
that point, it will deassert BR, granting the transfer of ownership.
The new bus master will then hold the bus until another SHARC asks for

On Fri, 14 Jan 2000 11:38:52 +0100, peter.buschhorn wrote:

> recently I watched the Bus Request Lines of four SHARCs 21062 on a
> Transtech ASP P15. What I finally found on my Logic Analyser is quite
> confusing to me: SHARC No. 1,2 and 3 behave as described in the manual.
> I see the arbitration and every "lower" priorized SHARC deasserts it's
> busrequest signal. The "master" sharc with No. 0 always has a zero on it's
> BR0 line. As I far as I understand the manual and timing diagrams, the highest
> priorized DSP (i.e. Sharc 0) wins therefore the busmastership ?!
> Just to make sure, that no other DSP is granted the bus I wrote a little
> which lets communicate two lower priorized SHARCs via broadcast and direct-
> slave write. Amazingly the communication takes place, so the lower priorized
> DSP access the external system bus.
> I am very happy, the bus access takes place, but I really would like to know
> what is going on. Did I miss something ? Is it normal in a four-processor
> environment that Sharc 0 always asserts BR0 ?

Kenneth Porter
Kensington Laboratories, Inc.