Broadcom BCM5722 - Figure 19: GMII Block

Programmer’s Guide BCM5722
10/15/07
Broadcom Corporation
Document 5722-PG101-R PHY Control Page 38
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.
Figure 19: GMII Block
RX
I/O
RXD /8
RX_CLK1
RX_ER
RX_DV
TX
I/O
TXD /8
TX_CLK0
TX_ER
TX_EN
Media
Status
I/O
COL
CRS
LNKRDY
RX Media
Access
Mgmnt
RX
MAC
Rx Data
Decapsulation
TX Media
Access
Mgmnt
TX
MAC
Tx Data
Encapsulation
MAC Sublayer
Physical Layer
RX
I/O
Symbol
Decoder
LED
Control
TX
I/O
Symbol
Encoder
125-MHz Ref Clock 8-bit Data Path
LED
I/O
8-bit Data Path125-MHz Ref Clock
GMII