Reverse Loopback receives data on the MDI and passes it through the entire receive block where it is then looped back within the PCS layer to the transmit block. The data is transmitted back out on the MDI to the attached Link Partner. To avoid contention, MAC transmit path is isolated.

Enable Loopback

Write register 0x0016 = 0x0110

Enable data generator/checker for MAC side

Use the following register settings to enable checker depending on the MAC interface mode.

• For RGMII, write register 0x0619 = 0x1004
• For SGMII, write register 0x0619 = 0x1114
• For RMII, write register 0x0619 = 0x1224
• For MII, write register 0x0619 = 0x1334

Write register 0x0624 = 0x55BF

Check incoming data from MAC side

Data can also be checked internally by reading registers 0x063C, 0x063D, 0x063E

Check data for Cable side

1. Write register 0x0620[1] = 1'b1
2. Read register 0x620
   1. Bit [7:0] = Number of errors bytes received
   2. Bit [8] = PRBS checker lock status on incoming data (1'b1 indicates lock)

Repeat steps 1 and 2 to continously check error status of incoming data stream.

Other system requirements

Data generate by the internal PRBS will be transmitted over the MDI and the MAC interface.