Ethernet (ETH): media access control (MAC) with DMA controller RM0033
852/1381 RM0033 Rev 9
added after the data field but prior to the FCS (frame check sequence) field. The
length/type field is transmitted and received with the higher-order byte first.
For length/type field values in the range between maxValidLength and minTypeValue
(boundaries excluded), the behavior of the MAC sublayer is not specified: they may or
may not be passed by the MAC sublayer.
• Data and PAD fields: n-byte data field. Full data transparency is provided, it means that
any arbitrary sequence of byte values may appear in the data field. The size of the
PAD, if any, is determined by the size of the data field. Max and min length of the data
and PAD field are:
– Maximum length = 1500 bytes
– Minimum length for untagged MAC frames = 46 bytes
– Minimum length for tagged MAC frames = 42 bytes
When the data field length is less than the minimum required, the PAD field is added to
match the minimum length (42 bytes for tagged frames, 46 bytes for untagged frames).
• Frame check sequence: 4-byte field that contains the cyclic redundancy check (CRC)
value. The CRC computation is based on the following fields: source address,
destination address, QTag prefix, length/type, LLC data and PAD (that is, all fields
except the preamble, SFD). The generating polynomial is the following:
The CRC value of a frame is computed as follows:
• The first 2 bits of the frame are complemented
• The n-bits of the frame are the coefficients of a polynomial M(x) of degree (n – 1). The
first bit of the destination address corresponds to the x
n – 1
term and the last bit of the
data field corresponds to the x
0
term
• M(x) is multiplied by x
32
and divided by G(x), producing a remainder R(x) of degree
≤ 31
• The coefficients of R(x) are considered as a 32-bit sequence
• The bit sequence is complemented and the result is the CRC
• The 32-bits of the CRC value are placed in the frame check sequence. The x
32
term is
the first transmitted, the x
0
term is the last one
Gx() x
32
x
26
x
23
x
22
x
16
x
12
x
11
x
10
x
8
x
7
x
5
x
4
x
2
x1+ + + + + + + +++++++=
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