Due : DMAC BTSIZE trimmed modulo 8192?
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The dmac.h file in asf library has a macro named DMA_MAX_LENGTH set to 0xFFF = 4095. We tried changing it to higher values but still Buffer Transfer size (BTSIZE) register was showing only values till 4095. This seemed to us to a hardware limitation of the DMAC in Atmel SAM3x8E (which we did'nt see mentioned in the datasheet). So this closed the option of single buffer transfer for us and we moved to multi-buffer transfer
It was a simple way out (even though I took some time to figure it out :P ) - Use multi-buffer transfer with contiguous source address! (see table 22.4 Row 3)