Datasheet

Micrel, Inc.
KSZ8081MNX/KSZ8081RNB
August
19, 2015 32
Revision 1.4
Loopback Mode
The KSZ8081MNX/RNB supports the following loopback operations to verify analog and/or digital data paths.
• Local (digital) loopback
• Remote (analog) loopback
Local (Digital) Loopback
This loopback mode checks the MII/RMII transmit and receive data paths between the KSZ8081MNX/RNB and the
external MAC, and is supported for both speeds (10/100Mbps) at full-duplex.
The loopback data path is shown in Figure 8.
1. The MII/RMII MAC transmits frames to the KSZ8081MNX/RNB.
2. Frames are wrapped around inside the KSZ8081MNX/RNB.
3. The KSZ8081MNX/RNB transmits frames back to the MII/RMII MAC.
4. Except the frames back to the RMII MAC, the transmit frames also go out from the copper port.
Figure 8. Local (Digital) Loopback
The following programming action and register settings are used for local loopback mode.
For 10/100Mbps loopback,
• Set Register 0h,
Bit [14] = 1 // Enable local loopback mode
Bit [13] = 0/1 // Select 10Mbps/100Mbps speed
Bit [12] = 0 // Disable auto-negotiation
Bit [8] = 1 // Select full-duplex mode
If don’t want the frames go out from the copper port in the local loopback, please follow the steps as below.
1. Set register 1Fh bit [3] to ‘1’ to disable the transmitter.
2. Run local loopback test as above.
3. Set register 1Fh bit [3] to ‘0’ to enable the transmitter.