Direct Register Mode (Scatter Gather Engine is disabled) provides a configuration for doing simple DMA transfers on MM2S and S2MM channels that requires less FPGA resource utilization. Transfers are initiated by accessing the DMACR, the Source or Destination Address and the Length registers. When the transfer is completed, a DMASR.IOC_Irq asserts for the associated channel and if enabled generates an interrupt out.
A DMA operation for the MM2S channel is set up and started by the following sequence:
1. Start the MM2S channel running by setting the run/stop bit to 1 (MM2S_DMACR.RS = 1). The halted bit (DMASR.Halted) should deassert indicating the MM2S channel is running.
2. If desired, enable interrupts by writing a 1 to MM2S_DMACR.IOC_IrqEn and MM2S_DMACR.Err_IrqEn. The delay interrupt, delay count, and threshold count are not used when the AXI DMA is configured for Direct Register Mode.
3.
Write a valid source address to the MM2S_SA register. If AXI DMA is configured for an address space greater than 32, then program the MM2S_SA MSB register. If the AXI DMA is not configured for Data Re-Alignment, then a valid address must be aligned or undefined results occur. What is considered aligned or unaligned is based on the stream data width. When AXI_DMA is configured in Micro Mode, it is your responsibility to specify the correct address. Micro DMA does not take care of the 4K boundary.
For example, if Memory Map Data Width = 32, data is aligned if it is located at word offsets (32-bit offset), that is 0x0, 0x4, 0x8, 0xC, and so forth. If DRE is enabled and Streaming Data Width < 128, then the Source Addresses can be of any byte offset.
4. Write the number of bytes to transfer in the MM2S_LENGTH register. A value of zero written has no effect. A non-zero value causes the MM2S_LENGTH number of bytes to be read on the MM2S AXI4 interface and transmitted out of the MM2S AXI4-Stream interface. The MM2S_LENGTH register must be written last. All other MM2S registers can be written in any order. In the case of Micro DMA, this value cannot exceed [Burst_length * (Memory Mapped Data Width)/8].
A DMA operation for the S2MM channel is set up and started by the following sequence:
1. Start the S2MM channel running by setting the run/stop bit to 1 (S2MM_DMACR.RS = 1). The halted bit (DMASR.Halted) should deassert indicating the S2MM channel is running.
2. If desired, enable interrupts by writing a 1 to S2MM_DMACR.IOC_IrqEn and S2MM_DMACR.Err_IrqEn. The delay interrupt, delay count, and threshold count are not used when the AXI DMA is configured for Direct Register Mode.
3. Write a valid destination address to the S2MM_DA register. If AXI DMA is configured for an address space greater than 32, program the S2MM_DA MSB register.
4.
If the AXI DMA is not configured for Data Re-Alignment then a valid address must be aligned or undefined results occur. What is considered aligned or unaligned is based on the stream data width.
For example, if Memory Map Data Width= 32, data is aligned if it is located at word offsets (32-bit offset), that is, 0x0, 0x4, 0x8, 0xC, and so forth. If DRE is enabled and Streaming Data Width < 128 then the Destination Addresses can be of any byte offset.
5.
Write the length in bytes of the receive buffer in the S2MM_LENGTH register. A value of zero has no effect. A non-zero value causes a write on the S2MM AXI4 interface of the number of bytes received on the S2MM AXI4-Stream interface. A value greater than or equal to the largest received packet must be written to S2MM_LENGTH. A receive buffer length value that is less than the number of bytes received produces undefined results.
When AXI DMA is configured in Micro mode, this value should exactly match the bytes received on the S2MM AXI4-Stream interface. The S2MM_LENGTH register must be written last. All other S2MM registers can be written in any order.