Figure 19: Gmii Block - Broadcom BCM5722 Programmer's Manual

Programmer's Guide
10/15/07
Pulse Amplitude Modulated Symbol (PAM5) encoding is leveraged for Gigabit Ethernet wire transmissions. PAM5 uses five
encoding levels: -2, -1, 0, 1, and 2. Four symbols are transmitted in parallel on the four twisted-wire pairs. The four symbols
create a code group (an eight-bit octet). The process of creating the code-group is called 4D-PAM5. Essentially, eight data
bits are represented by four symbols. Table 40-1 in the IEEE 802.3ab specification shows the data bit to symbol mapping.
The code group representation is also referred to as a quartet of quinary symbols {TA, TB, TC, TD}. The modulation rate on
the wire is measured at 125 Mbaud. The resultant bandwidth is calculated by multiplying 125 MHz by eight bits, for 1000-
Mbps wire speed.
125-MHz Ref Clock
Physical Layer
Symbol
Decoder
LED
Control
Symbol
Encoder
125-MHz Ref Clock
Document 5722-PG101-R
GMII
RXD /8
RX_CLK1
RX_ER
RX
RX_DV
I/O
COL
CRS
LED
LNKRDY
I/O
TXD /8
TX_CLK0
TX
I/O TX_ER
TX_EN
8-bit Data Path

Figure 19: GMII Block

Bro adco m Co rp or atio n
8-bit Data Path
RX Media
RX
Access
I/O
Mgmnt
Media
Status
I/O
TX Media
TX
Access
I/O
Mgmnt
MAC Sublayer
Rx Data
Decapsulation
Tx Data
Encapsulation
PHY Control
BCM5722
RX
MAC
TX
MAC
Page 38