onur
3.2 KiB
MAX2771 raw Ethernet protocol
This protocol is Ethernet Layer 2 only. It has no IP, ARP, DHCP, ICMP, UDP, or TCP component.
Ethernet envelope
| Item | Value |
|---|---|
| Preamble | seven bytes 55 |
| SFD | D5 |
| Destination | FF:FF:FF:FF:FF:FF |
| Source | 02:27:71:00:00:01 |
| EtherType | 0x88B5, most-significant byte first |
| Maximum Ethernet payload | 1500 bytes |
| VLAN / jumbo frames | not used |
| FCS | reflected Ethernet CRC-32, inverted, least-significant byte first |
| IFG | at least 12 byte times |
The CRC is initialized to 0xFFFFFFFF, updated LSB-first over destination MAC through the final payload or pad byte with reflected polynomial 0xEDB88320, then inverted. Preamble and SFD are not part of the CRC. Linux NICs normally remove preamble, SFD, and FCS before an AF_PACKET application receives the frame.
Custom payload header
All multibyte header fields use network byte order. The fixed header is exactly 32 bytes.
| Offset | Size | Field | Encoding |
|---|---|---|---|
| 0 | 4 | magic | ASCII MX27 (4D 58 32 37) |
| 4 | 1 | version | 1 |
| 5 | 1 | flags | described below |
| 6 | 2 | header length | 0x0020 |
| 8 | 4 | data-frame sequence | unsigned, wraps modulo 2^32 |
| 12 | 8 | first sample index | unsigned absolute RF sample index |
| 20 | 2 | sample count | number of real 4-bit samples in this frame |
| 22 | 1 | bits per sample | 4 |
| 23 | 1 | packing format | 1 for earlier sample in high nibble |
| 24 | 4 | dropped sample count | cumulative FPGA count, low 32 bits |
| 28 | 4 | reserved | zero |
Flags are:
| Bit | Meaning |
|---|---|
| 0 | FIFO overflow/input loss occurred since the previous reported packet |
| 1 | first data packet after FPGA reset |
| 2 | first data packet after the PHY reports a resolved 1000BASE-T full-duplex link |
| 7:3 | zero |
Normal frames contain a 32-byte custom header and 1468 data bytes: exactly 2936 samples. Sequence increments once per transmitted data frame. A sample with index N followed by N+1 is contiguous regardless of nibble values.
Packing
For samples S0, S1, S2, S3, each limited to four bits, data bytes are:
byte 0 = {S0[3:0], S1[3:0]}
byte 1 = {S2[3:0], S3[3:0]}
There is no sign extension, I/Q interpretation, bit reversal, or numeric conversion. An exceptional overflow boundary can close a short frame. If its sample count is odd, the unused low nibble of the final frame byte is zero and is not a sample. The host parser uses sample_count, so it never writes that pad nibble.
Loss semantics
Each FIFO entry carries its absolute 64-bit input sample index. The packetizer compares adjacent entries. If a full-FIFO interval creates an index discontinuity, it closes the pre-gap packet as an exceptional partial frame and begins the next packet at the first post-gap index. This makes the missing interval visible to the host. It never silently crosses a gap inside a packet.
Packet-sequence gaps indicate frames lost after the FPGA MAC. Sample-index gaps indicate missing sample time, whether due to an FPGA overflow or a lost Ethernet frame. The cumulative drop field distinguishes FPGA-side input loss. The receiver extends the low-32-bit drop count across wraps while it is running.