Functional Description
901
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
Table 15-15. Enhanced Receive Descriptor 7 (RDES7)
Bit
Description
31:0
RTSH: Receive Frame Timestamp High
This field is updated by DMA with the most significant 32 bits of the timestamp captured for the corresponding receive
frame. This field is updated by DMA only for the last descriptor of the receive frame which is indicated by Last
Descriptor status bit (RDES0[8]).
15.3.3.6 DMA Transmission Operation
The following sections describe the modes of the transmit operation.
15.3.3.6.1 TX DMA Default Operation
The TX DMA engine in default mode operates as follows:
1. The CPU configures the transmit descriptor (TDES0-TDES3) and sets the OWN bit (TDES0[31]) after
setting up the corresponding data buffers with the Ethernet frame.
2. When the ST bit of the Ethernet MAC DMA Operation Mode (EMACDMAOPMODE) register, offset
0xC18, is set, the DMA enters the RUN state.
3. While in RUN state, the DMA polls the Transmit Descriptor list for frames requiring transmission. After
polling starts, it continues in either sequential descriptor ring order or chained order. If the DMA detects
a descriptor flagged as owned by the CPU (TDES0[31] = 0), or if an error condition occurs,
transmission is suspended and both the Transmit Buffer Unavailable (TU) bit and the Normal Interrupt
Summary (NIS) bit are set in the Ethernet MAC DMA Interrupt Status (EMACDMARIS) register, offset
0xC14. The transmit engine the proceeds to Step 9.
4. If the acquired descriptor is flagged as owned by the DMA (TDES0[31] = 1), the DMA decodes the
transmit data buffer address from the acquired descriptor.
5. If the acquired descriptor is flagged as owned by DMA (TDES0[31] = 1), the DMA decodes the
Transmit Data Buffer address from the acquired descriptor.
6. The DMA fetches the transmit data from system memory and transfers the data to the TX/RX
Controller for transmission.
7. If the Ethernet frame is stored over data buffers in multiple descriptors, the DMA closes the
intermediate descriptor and fetches the next descriptor. Step 3, Step 4, and Step 5 are repeated until
the end of the Ethernet frame data is transferred to the TX/RX Controller.
8. When frame transmission is complete, if IEEE 1588 timestamping was enabled for the frame (as
indicated in the transmit status) the timestamp value is written to the transmit descriptor (TDES6 and
TDES7) that contains the end-of-frame buffer. The status information is then written to transmit
descriptor TDES0. Because the OWN bit is cleared during this step, the CPU now owns this descriptor.
If timestamping was not enabled for this frame, the DMA does not alter the contents of TDES6 and
TDES7.
9. The Transmit Interrupt (TI) bit is set in the EMACDMARIS register after transmission completion of a
frame that has Interrupt on Completion set in its last descriptor. The Interrupt on Completion bit resides
in TDES0[30]. The DMA engine then returns to Step 3.
10. In the suspend state, the DMA tries to reacquire the descriptor (and thereby return to Step 3) when it
receives a transmit poll demand in the Ethernet MAC Transmit Poll Demand (EMACTXPOLLD)
register, and the Underflow Interrupt Status (UNF) bit is cleared in the EMACDMARIS register. If the
CPU stopped the DMA by clearing the ST bit of the EMACDMAOPMODE register, the DMA enters the
STOP state.
shows the flow for the TX DMA default operation.