ST STM32F405 Reference Manual page 1231

RM0090
Bits 12:8 Reserved, must be kept at reset value.
Bit 7 FEF: Forward error frames
Bit 6 FUGF: Forward undersized good frames
Bit 5 Reserved, must be kept at reset value.
Bits 4:3 RTC: Receive threshold control
Note: Note that value of 11 is not applicable if the configured Receive FIFO size is 128 bytes.
Note: These bits are valid only when the RSF bit is zero, and are ignored when the RSF bit is
Bit 2 OSF: Operate on second frame
Bit 1 SR: Start/stop receive
Bit 0 Reserved, must be kept at reset value.
Ethernet (ETH): media access control (MAC) with DMA controller
When this bit is set, all frames except runt error frames are forwarded to the DMA.
When this bit is cleared, the Rx FIFO drops frames with error status (CRC error, collision
error, giant frame, watchdog timeout, overflow). However, if the frame's start byte (write)
pointer is already transferred to the read controller side (in Threshold mode), then the
frames are not dropped. The Rx FIFO drops the error frames if that frame's start byte is not
transferred (output) on the ARI bus.
When this bit is set, the Rx FIFO forwards undersized frames (frames with no error and
length less than 64 bytes) including pad-bytes and CRC).
When this bit is cleared, the Rx FIFO drops all frames of less than 64 bytes, unless such a
frame has already been transferred due to lower value of receive threshold (e.g., RTC = 01).
These two bits control the threshold level of the Receive FIFO. Transfer (request) to DMA
starts when the frame size within the Receive FIFO is larger than the threshold. In addition,
full frames with a length less than the threshold are transferred automatically.
set to 1.
00: 64
01: 32
10: 96
11: 128
When this bit is set, this bit instructs the DMA to process a second frame of Transmit data
even before status for first frame is obtained.
When this bit is set, the receive process is placed in the Running state. The DMA attempts to
acquire the descriptor from the receive list and processes incoming frames. Descriptor
acquisition is attempted from the current position in the list, which is the address set by the
DMA ETH_DMARDLAR register or the position retained when the receive process was
previously stopped. If no descriptor is owned by the DMA, reception is suspended and the
receive buffer unavailable bit (ETH_DMASR [7]) is set. The Start Receive command is
effective only when reception has stopped. If the command was issued before setting the
DMA ETH_DMARDLAR register, the DMA behavior is unpredictable.
When this bit is cleared, RxDMA operation is stopped after the transfer of the current frame.
The next descriptor position in the receive list is saved and becomes the current position
when the receive process is restarted. The Stop Receive command is effective only when
the Receive process is in either the Running (waiting for receive packet) or the Suspended
state.
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