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GD32F20x User Manual
808
Transmission and reception of frames with the PTP feature
After enabled the IEEE 1588 (PTP) timestamp function, timestamp is recorded when the
frame’s SFD field is outputting from the MAC or the MAC receives a frame’s SFD field. Each
transmitted frame can be marked in TxDMA descriptor to indicate whether a timestamp should
be captured or not.
Together with the state information of frame, the recorded timestamp value will also be stored
in the corresponding transmission/reception descriptor. The 64-bit timestamp information of
transmission frame is written back to the transmit descriptor and the 64-bit timestamp
information of reception frame is written back to the receive descriptor. See the detailed
description in “Transmit DMA descriptor” and “Receive DMA descriptor”.
PTP trigger internal connection with TIMER1
MAC can provide trigger interrupt when the system time is no less than the target time. Using
an interrupt imports a known latency and an uncertainty in the command execution time. In
order to calculate the time of this known latency part, when the system time is greater than
target time, the PTP module sets an output signal. Set bit 29 of AFIO_PCF0 register to 0 can
make this signal internally connected to the ITI0 input of TIMER1. For this feature designed,
no uncertainty is introduced because the clock of the TIMER1 and PTP reference clock
(HCLK) are synchronous.
PTP pulse-per-second (PPS) output signal
Application configures ETH_PPS_OUT pin to AF output push-pull to enable the PPS output
function. This function can output a signal with the pulse width of 125ms which can be used
to check the synchronization between all nodes in the network. To test the difference between
the slave clock and the master clock, both of the slave and master can output PPS and
connect them to one oscilloscope for clock measurement.
27.3.6.
DMA controller description
Ethernet DMA controller is designed for frame transmission between FIFO and system
memory which can reduce the occupation of CPU. Communication between the CPU and the
DMA is achieved by the following two kinds of data structures:
Descriptor table (ring or chain type) and data buffer
Control and status register
Applications need to provide the memory for storage of descriptor tables and data buffers.
Descriptors that reside in the memory act as pointers to these buffers. Transmission has
transmission descriptor and reception has reception descriptor. The base address of each
table is stored in ENET_DMA_TDTADDR and ENET_DMA_RDTADDR register. Descriptors
of transmission constituted by 4 descriptor word (TDES0-TDES3). Likewise, reception
descriptors constituted by 4 descriptor word (RDES0-RDES3). Each descriptor can point to a
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