Embedded Peripherals IP User Guide

ID 683130
Date 12/13/2021
Public

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Document Table of Contents
1. Introduction 2. Avalon® -ST Multi-Channel Shared Memory FIFO Core 3. Avalon® -ST Single-Clock and Dual-Clock FIFO Cores 4. Avalon® -ST Serial Peripheral Interface Core 5. SPI Core 6. SPI Agent/JTAG to Avalon® Host Bridge Cores 7. Intel eSPI Agent Core 8. eSPI to LPC Bridge Core 9. Ethernet MDIO Core 10. Intel FPGA 16550 Compatible UART Core 11. UART Core 12. JTAG UART Core 13. Intel FPGA Avalon® Mailbox Core 14. Intel FPGA Avalon® Mutex Core 15. Intel FPGA Avalon® I2C (Host) Core 16. Intel FPGA I2C Agent to Avalon® -MM Host Bridge Core 17. Intel FPGA Avalon® Compact Flash Core 18. EPCS/EPCQA Serial Flash Controller Core 19. Intel FPGA Serial Flash Controller Core 20. Intel FPGA Serial Flash Controller II Core 21. Intel FPGA Generic QUAD SPI Controller Core 22. Intel FPGA Generic QUAD SPI Controller II Core 23. Interval Timer Core 24. Intel FPGA Avalon FIFO Memory Core 25. On-Chip Memory (RAM and ROM) Intel FPGA IP 26. On-Chip Memory II (RAM or ROM) Intel FPGA IP 27. Optrex 16207 LCD Controller Core 28. PIO Core 29. PLL Cores 30. DMA Controller Core 31. Modular Scatter-Gather DMA Core 32. Scatter-Gather DMA Controller Core 33. SDRAM Controller Core 34. Tri-State SDRAM Core 35. Video Sync Generator and Pixel Converter Cores 36. Intel FPGA Interrupt Latency Counter Core 37. Performance Counter Unit Core 38. Vectored Interrupt Controller Core 39. Avalon® -ST Data Pattern Generator and Checker Cores 40. Avalon® -ST Test Pattern Generator and Checker Cores 41. System ID Peripheral Core 42. Avalon® Packets to Transactions Converter Core 43. Avalon® -ST Multiplexer and Demultiplexer Cores 44. Avalon® -ST Bytes to Packets and Packets to Bytes Converter Cores 45. Avalon® -ST Delay Core 46. Avalon® -ST Round Robin Scheduler Core 47. Avalon® -ST Splitter Core 48. Avalon® -MM DDR Memory Half Rate Bridge Core 49. Intel FPGA GMII to RGMII Converter Core 50. Intel FPGA MII to RMII Converter Core 51. Intel FPGA HPS GMII to TSE 1000BASE-X/SGMII PCS Bridge Core 52. Intel FPGA HPS EMAC to Multi-rate PHY GMII Adapter Core 53. Intel FPGA MSI to GIC Generator Core

32.3.2. DMA Descriptors

DMA descriptors specify data transfers to be performed. The SG-DMA core uses a dedicated interface to read and write the descriptors. These descriptors, which are stored as a linked list, can be stored on an on-chip or off-chip memory and can be arbitrarily long.

Storing the descriptor list in an external memory frees up resources in the FPGA; however, an external descriptor list increases the overhead involved when the descriptor processor reads and updates the list. The SG-DMA core has an internal FIFO to store descriptors read from memory, which allows the core to perform descriptor read, execute, and write back operations in parallel, hiding the descriptor access and processing overhead.

The descriptors must be initialized and aligned on a 32-bit boundary. The last descriptor in the list must have its OWNED_BY_HW bit set to 0 because the core relies on a cleared OWNED_BY_HW bit to stop processing.

See the DMA Descriptors section for the structure of the DMA descriptor.

Descriptor Processing

The following steps describe how the DMA descriptors are processed:

  1. Software builds the descriptor linked list. See the Building and Updating Descriptors List section for more information on how to build and update the descriptor linked list.
  2. Software writes the address of the first descriptor to the next_descriptor_pointer register and initiates the transfer by setting the RUN bit in the control register to 1. See the Software Programming Model section for more information on the registers.

    On the next clock cycle following the assertion of the RUN bit, the core sets the BUSY bit in the status register to 1 to indicate that descriptor processing is executing.

  3. The descriptor processor block reads the address of the first descriptor from the next_descriptor_pointer register and pushes the retrieved descriptor into the command FIFO, which feeds commands to both the DMA read and write blocks. As soon as the first descriptor is read, the block reads the next descriptor and pushes it into the command FIFO. One descriptor is always read in advance thus maximizing throughput.
  4. The core performs the data transfer.
    • In memory-to-memory configurations, the DMA read block receives the source address from its command FIFO and starts reading data to fill the FIFO on its stream port until the specified number of bytes are transferred. The DMA read block pauses when the FIFO is full until the FIFO has enough space to accept more data. 

The DMA write block gets the destination address from its command FIFO and starts writing until the specified number of bytes are transferred. If the data FIFO ever empties, the write block pauses until the FIFO has more data to write.
    • In memory-to-stream configurations, the DMA read block reads from the source address and transfers the data to the core’s streaming port until the specified number of bytes are transferred or the end of packet is reached. The block uses the end-of-packet indicator for transfers with an unknown transfer size. For data transfers without using the end-of-packet indicator, the transfer size must be a multiple of the data width. Otherwise, the block requires extra logic and may impact the system performance.
    • In stream-to-memory configurations, the DMA write block reads from the core’s streaming port and writes to the destination address. The block continues reading until the specified number of bytes are transferred.
  5. The descriptor processor block receives a status from the DMA read or write block and updates the DESC_CONTROL, DESC_STATUS, and ACTUAL_BYTES_TRANSFERRED fields in the descriptor. The OWNED_BY_HW bit in the DESC_CONTROL field is cleared unless the PARK bit is set to 1.

    Once the core starts processing the descriptors, software must not update descriptors with OWNED_BY_HW bit set to 1. It is only safe for software to update a descriptor when its OWNED_BY_HW bit is cleared.

    The SG-DMA core continues processing the descriptors until an error condition occurs and the STOP_DMA_ER bit is set to 1, or a descriptor with a cleared OWNED_BY_HW bit is encountered.

Building and Updating Descriptor List

Intel recommends the following method of building and updating the descriptor list:

  1. Build the descriptor list and terminate the list with a non-hardware owned descriptor (OWNED_BY_HW = 0). The list can be arbitrarily long.
  2. Set the interrupt IE_CHAIN_COMPLETED.
  3. Write the address of the first descriptor in the first list to the next_descriptor_pointer register and set the RUN bit to 1 to initiate transfers.
  4. While the core is processing the first list, build a second list of descriptors.
  5. When the SGDMS controller core finishes processing the first list, an interrupt is generated. Update the next_descriptor_pointer register with the address of the first descriptor in the second list. Clear the RUN bit and the status register. Set the RUN bit back to 1 to resume transfers.
  6. If there are new descriptors to add, always add them to the list which the core is not processing. For example, if the core is processing the first list, add new descriptors to the second list and so forth.

    This method ensures that the descriptors are not updated when the core is processing them. Because the method requires a response to the interrupt, a high-latency interrupt may cause a problem in systems where stalling data movement is not possible.