When the DMA controller accesses a memory in the PL, the flow control interface (FCI) signals are used to provide the data transfer handshake. There are eight sets of FCI signals; a set for each channel. The PS_DMA.CH_FCI register configures the FCI flow control interface.
The PL provides credits to the DMA channel. Each credit is a permission for a single AXI transaction. When the FCI is attached to the SRC (read), there is a permission to generate one AXI data read transaction (write transaction when the FCI is attached to write DST). The following table lists the FCI signals.
|PL2DMA_CLK||PL clock: signals from/to PL are synchronous to PL2DMA_CLK. The DMA handles all clock domain crossing.|
|PL2DMA_CVLD||Credit valid signal to DMA.|
Credit acknowledgment from DMA:
|PL2DMA_TACK||Transaction acknowledgment: the DMA channel indicates that one write transaction is done (AXI write command was generated and a BRESP is received) when DMA2PL_TVLD and PL2DMA_TACK are True.|
The timing diagram for the flow control interface is shown in the following figure.
Software can configure FCI to flow control either the SRC or DST based on whether the DMA channel is reading from or writing to the PL memory.
- PL memory reads: program and configured SRC flow control
- PL memory writes: program and configure DST flow control