Intel Gigabit Ethernet Controllers ユーザーズマニュアル

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This loopback mode internally loops back the transmit to receive path in the PHY, exercising the 

internal GMII/MII bus. Programming both MAC and PHY is required. Following is the flow:

/* Auto-MDI/MDIX Off */

e1000_write_phy_reg(16, 0x0808);

/* reset to update Auto-MDI/MDIX */

e1000_write_phy_reg(0, 0x9140);

/* autoneg off */

e1000_write_phy_reg(0, 0x8140);

/* force 1000, set loopback */

e1000_write_phy_reg(0, 0x4140);

/* Now set up the MAC to the same speed/duplex as the PHY. */

ctrl_reg = E1000_READ_REG(CTRL);

ctrl_reg &= ~E1000_CTRL_SPD_SEL;                /* Clear the speed sel bits */

ctrl_reg |= (E1000_CTRL_FRCSPD |                   /* Set the Force Speed Bit */

                   E1000_CTRL_FRCDPLX |                /* Set the Force Duplex Bit */

                   E1000_CTRL_SPD_1000 |               /* Force Speed to 1000 */

                   E1000_CTRL_FD);                           /* Force Duplex to FULL */

/* Set the ILOS bit on the fiber Nic is half duplex link is detected. */

stat_reg = E1000_READ_REG(STATUS);

if((stat_reg & E1000_STATUS_FD) = 0)

         ctrl_reg |= (E1000_CTRL_ILOS | E1000_CTRL_SLU);

E1000_WRITE_REG(CTRL, ctrl_reg);

The Ethernet controller uses full Boundary Scan/IEEE 1149.1 JTAG standard test methods. The 
TAP controller supports EXTEST, SAMPLE/PRELOAD, IDCODE, USERCODE, and BYPASS 

instructions.