网络控制寄存器(00H)
Loopback Mode
Bit 2 1
0 0 Normal
0 1 MAC Internal loopback
1 0 Internal PHY 100M mode digital loopback
1 1 (Reserved)
PHY基本模式控制寄存器(offset:00)
Loopback
Loop-back control register
1 = Loop-back enabled
0 = Normal operation
When in 100Mbps operation mode, setting this bit may cause
the descrambler to lose synchronization and produce a 720ms
"dead time" before receive