RM0090: Reference manual - STMicroelectronics

RM0090 Ethernet (ETH): media access control (MAC) with DMA controller
Unavailable (ETH_DMASR register[2]) and Normal Interrupt Summary (ETH_DMASR
register[16]) bits are set. The transmit engine proceeds to Step 9.
4. If the acquired descriptor is flagged as owned by DMA (TDES0[31] is set), the DMA
decodes the transmit data buffer address from the acquired descriptor.
5. The DMA fetches the transmit data from the STM32F4xx memory and transfers the
data.
6. If an Ethernet frame is stored over data buffers in multiple descriptors, the DMA closes
the intermediate descriptor and fetches the next descriptor. Steps 3, 4, and 5 are
repeated until the end of Ethernet frame data is transferred.
7. When frame transmission is complete, if IEEE 1588 time stamping was enabled for the
frame (as indicated in the transmit status) the time stamp value is written to the transmit
descriptor (TDES2 and TDES3) that contains the end-of-frame buffer. The status
information is then written to this transmit descriptor (TDES0). Because the OWN bit is
cleared during this step, the CPU now owns this descriptor. If time stamping was not
enabled for this frame, the DMA does not alter the contents of TDES2 and TDES3.
8. Transmit Interrupt (ETH_DMASR register [0]) is set after completing the transmission of
a frame that has Interrupt on Completion (TDES1[31]) set in its last descriptor. The
DMA engine then returns to Step 3.
9. In the Suspend state, the DMA tries to re-acquire the descriptor (and thereby returns to
Step 3) when it receives a transmit poll demand, and the Underflow Interrupt Status bit
is cleared.
Figure 347 shows the TxDMA transmission flow in default mode.
Doc ID 018909 Rev 3 944/1416