almanach network
transceiver status
The MII status word is the most useful datum. A typical good status value 0x784D,
while a transceiver without link beat typically reports 0x7849. If you don't have
link beat, check your cable and hub. If you have link beat, the next register
to check is the Link Partner Ability. It reports what the machine at the other
end of the wire can do.
MII Transceiver Registers
MII transceivers have 32 management registers. The first 16 are reserved for standard-defined
uses, and the remaining one are available for chip-specific features. Only the
first seven registers are currently defined.
| Offset |
Register Name |
| 00 |
Basic Mode Configuration |
| 01 |
Basic Mode Status |
| 02 |
PHY ID #1 |
| 03 |
PHY ID #2 |
| 04 |
AutoNegotation Advertisement |
| 05 |
Link Partner Ability |
| 06 |
AutoNegotation Expansion |
These registers are reported by program, and sometimes by device driver status
messages.
Basic Mode Control
This register configures the transceiver operation, with enable bits for autonegotation,
test and loopback modes, and forcing speed and duplex.
MII status word
The MII status word is the most useful MII register. A typical good status value
0x78*D, while a transceiver without link beat typically reports 0x78*9.
| MII status word bits |
| 0x8000 |
Capable of 100baseT4. |
| 0x7800 |
Capable of 10/100 HD/FD (most common). |
| 0x0040 |
Preamble suppression permitted. |
| 0x0020 |
Autonegotiation complete. |
| 0x0010 |
Remote fault. |
| 0x0008 |
Capable of Autonegotiation. |
| 0x0004 |
Link established ("sticky"* on link failure) |
| 0x0002 |
Jabber detected ("sticky"* on transmit jabber) |
| 0x0001 |
Extended MII register exist. |
Notes:
* Some bits are "sticky", which means they indicate that an event has occured.
Registers with set sticky bits need to be read a second time to get the current
status.
On some transceivers bit 0x0020 means "autonegotiation failed, but autosensing
worked".
PHY Identification
The PHY ID registers uniquely identify the transceiver chip. The are a combination
of the IEEE-assigned organization (company) number
and a part number assigned by the designer.
Ability Advertisement Register
The capabilities we advertise are set in the Autonegotiation Advertisement Register.
This value is encoded on the link beat pulses and sent to the remote machine.
The meaning of most bits are identical to the link partner ability register below.
| 0x8000 |
We can send additional information. |
| 0x4000 |
Link partner got our advertised abilities. |
| 0x2000 |
Advertise that we detected a link fault. |
| 0x0400 |
Flow control supported (currently uncommon) |
| 0x0200 |
100baseT4 supported |
| 0x0100 |
100baseTx-FD supported |
| 0x0080 |
100baseTx supported |
| 0x0040 |
10baseT-FD supported |
| 0x0020 |
10baseT supported |
| 0x001F |
Protocol selection bits, always 0x0001 for Ethernet. |
This register usually initialized at power-up to the proper value. A few board
designs (e.g. the D-Link DFE500TX rev C3) are
broken and do not set up the transceiver to reset with the correct value. For
these boards the device driver must set the advertised value, which often causes
problems as the value changes.
Link Partner Ability
If autonegotation completes the transceiver will report the link types the remote
machine (the "link partner") will support.
A common link partner ability word is 0x01E1, which means the remote machine will
support 10 and 100mbps at half and full duplex. Also common are 0x0080 and 0x0020,
which mean the remote end is a 100baseTx or 10baseT repeater.
The bits of the AutoNegotation Link Partner Ability Register are
| 0x8000 |
Link partner can send more info. |
| 0x4000 |
Link partner got our advertised abilities. |
| 0x2000 |
Fault detected by link partner (uncommon). |
| 0x0400 |
Flow control supported (currently uncommon) |
| 0x0200 |
100baseT4 supported (uncommon) |
| 0x0100 |
100baseTx-FD (full duplex) supported |
| 0x0080 |
100baseTx supported |
| 0x0040 |
10baseT-FD supported |
| 0x0020 |
10baseT supported |
| 0x001F |
Protocol selection bits, always 0x0001 for Ethernet. |
While it's not universal, many transceivers set the 0x0080 or 0x0020 bits to indicate
the link speed when the link partner does not autonegotiate.
AutoNegotation Expansion
This register supports additional communication about link level information.
It uses the same mechanism as the media type advertisement. While a protocol has
not yet been defined, this functionality could be used to communicate willingness
to accept jumbo frames (oversized packets), VLAN connections, channel bonding
aka trunking, etc.