MM2S_TAILDESC (MM2S DMA Tail Descriptor Pointer Register - Offset 10h) - 7.1 English

AXI DMA v7.1 LogiCORE IP Product Guide (PG021)

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7.1 English

This register provides the Tail Descriptor Pointer for the Memory Map to Stream DMA Scatter Gather Descriptor Management.

Figure 1. MM2S_TAILDESC Register

Table 1. MM2S_TAILDESC Register Details
Bits Field Name Default Value Access Type Description
5 to 0 Reserved 0 RO Writing to these bits has no effect, and they are always read as zeros.
31 to 6 Tail Descriptor Pointer zeros R/W Indicates the pause pointer in a descriptor chain. The AXI DMA SG Engine pauses descriptor fetching after completing operations on the descriptor whose current descriptor pointer matches the tail descriptor pointer.

When AXI DMA Channel is not halted (DMASR.Halted = 0), a write by the CPU to the TAILDESC_PTR register causes the AXI DMASG Engine to start fetching descriptors or restart if it was idle (DMASR.Idle = 1). If it was not idle, writing TAILDESC_PTR has no effect except to reposition the pause point.

Note: The software must not move the tail pointer to a location that has not been updated. The software processes and reallocates all completed descriptors (Cmplted = 1), clears the completed bits and then moves the tail pointer. The software must move the pointer to the last descriptor it updated. Descriptors must be 16-word aligned, that is, 0x00, 0x40, 0x80, and so forth. Any other alignment has undefined results.