ARM Technical Support Knowledge Articles

What's the value of HBURST in PL081 when burst transfer request size = 4 (DBSize = 0b001)?

Applies to: PL08x DMAC (DM & SM)

Answer

DBSize/SBSize is the amount of data transferred when the DMACxBREQ signal goes active. The value 
of HBURST depends on DBSize/SBSize and DWidth/SWidth. Since the FIFO is only four words deep, an 
INCR4 is the maximum HBURST possible when transferring words. When transferring half-words (16 
bit quantities), INCR8 is the maximum HBURST size possible; with BYTES it is INCR16. 

Let's say that PL081 is told to transfer 15 words. Assuming the initial conditions of DBSize = SBSize = 4,
DWidth = SWidth = WORD, it will transfer this burst as 3 x HBURST = INCR4, followed by 3 x HBURST = INCR where the burst 
length is one.


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