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Functional Description
888
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
Table 15-1. MII and RMII Signals (continued)
MII Signal
RMII Signal
RMII Standard Name and Function
EN0CRS
Not used
N/A
EN0MDC
EN0MDC
MDC: Management Data Clock
EN0MDIO
EN0MDIO
MDIO: Management Data Input/Output
EN0PPS
EN0PPS
Pulse-Per-Second (PPS) Output (optional, this is not a standard RMII signal)
EN0INTRN
EN0INTRN
Interrupt to Ethernet PHY (optional, this is not a standard RMII signal)
15.3.3 DMA Controller
The integrated DMA of the Ethernet Controller is used to optimize data transfer between the MAC and
system SRAM. The DMA has independent transmit and receive engines.
The DMA transmit engine transfers data from system memory to the Ethernet TX/RX Controller, while the
receive engine transfers data from the RX FIFO to the system memory. The controller uses descriptors to
efficiently move data from source to destination with minimal CPU intervention. The DMA is designed for
packet-oriented data transfers such as frames in Ethernet. Fixed burst lengths of 1, 4, 8, or 16 words are
supported with reinitiation of bursts when retry or burst termination responses occur. For a burst retry, if
the remaining address count is greater than 1 and the RIB bit in the Ethernet MAC DMA Bus Mode
(EMACDMABUSMOD) register is clear, then the transfer resends data in one continuous burst. When one
transfer is left, it is done as a single burst and the transaction is terminated immediately afterward. If the
RIB bit in the EMACDMABUSMOD register is set, the DMA sends the remaining data in fixed burst sizes
of 1, 4, 8, or 16 words.
The application may also choose between solely fixed bursts or mixed bursts by the DMA. If the MB bit is
set and the FB bit is clear in the EMACDMABUSMOD register, then the DMA uses fixed bursts for burst
sizes less than 16 and a full, non-divided burst for lengths greater than 16. Fixed burst lengths allow for
more DMA bus arbitration with other masters. Maximum burst transfer lengths can be programmed for
both the receive and transmit channels of the DMA through the PBL, RPBL, and 8xPBL bit fields in the
EMACDMABUSMOD register.
The DMA Controller requests a read transfer only when it can accept the received burst data completely.
Data read from the bus is always pushed into the DMA without any delay or busy cycles. The DMA
requests write transfers only when it has sufficient data to transfer the burst completely. When operating in
fixed burst length mode, the DMA interface continues to burst with dummy data until the specified length is
completed. The Ethernet controller can be programmed to interrupt the CPU in situations such as Frame
Transmit and Receive transfer completion, and other normal/error conditions.
The integrated Ethernet DMA communicates through two data structures:
•
Control and Status registers
•
Descriptor lists and data buffers
The DMA writes data frames received by the MAC to the receive buffer in system memory and transfers
data frames for transmission from system memory to the MAC. Descriptors that reside in the system
memory act as pointers to these buffers.
There are two descriptor lists: one for reception and one for transmission. The base address of each list is
written into the Ethernet MAC Receive Descriptor List Address (EMACRXDLADDR) register at offset
0xC0C and the Ethernet Mac Transmit Descriptor List Address (EMACTXDLADDR) register at offset
0xC10, respectively.
The descriptor structure can contain up to 8 words (32 bytes). These are described in more detail in
. A descriptor list is forward linked (either implicitly or explicitly). The last descriptor may
point back to the first entry to create a ring structure. Explicit chaining of descriptors is accomplished by
enabling second address chaining in both the Receive and Transmit descriptors (RDES0[14] and
TDES0[20]). The descriptor lists reside in the SRAM address space. Each descriptor can point to a
maximum of two buffers. This enables two buffers to be used at different physical addresses rather than
contiguous buffers in memory.