AM79C978AKCW Ver la hoja de datos (PDF) - Advanced Micro Devices
Número de pieza
componentes Descripción
fabricante
AM79C978AKCW Datasheet PDF : 256 Pages
| |||
MII Management Frames
MII management frames are automatically generated
by the Am79C978A controller and conform to the MII
clause in the IEEE 802.3u standard.
The start of the frame is a preamble of 32 ones and
guarantees that all of the external PHYs are synchro-
nized on the same interface. See Figure 2. Loss of
synchronization is possible due to the hot-plugging
capability of the exposed MII.
The IEEE 802.3 specification allows you to drop the
preamble, if after reading the MII Status Register from
the external PHY you can determine that the external
PHY will support Preamble Suppression (BCR34, bit
6). After having a valid MII Status Register read, the
Am79C978A controller will then drop the creation of the
preamble stream until a reset occurs, receives a read
error, or the external PHY is disconnected.
Preamble
1111....1111
32
Bits
ST
OP
01
10 Rd
01 Wr
2
2
Bits
Bits
PHY
Address
5
Bits
Register
Address
5
Bits
TA
Z0 Rd
10 Wr
2
Bits
Data
16
Bits
Figure 2. Frame Format at the MII Interface Connection
Idle
Z
1
Bit
22399A-5
This is followed by a start field (ST) and an operation
field (OP). The operation field (OP) indicates whether
the Am79C978A controller is initiating a read or write
operation. This is followed by the external PHY ad-
dress (PHYAD) and the register address (REGAD) pro-
grammed in BCR33. The PHY address of 1Fh is
reserved and should not be used. The external PHY
may have a larger address space starting at 10h - 1Fh.
This is the address range set aside by the IEEE as ven-
dor usable address space and will vary from vendor to
vendor. This field is followed by a bus turnaround field.
During a read operation, the bus turnaround field is
used to determine if the external PHY is responding
correctly to the read request or not. The Am79C978A
controller will tri-state the MDIO for both MDC cycles.
During the second cycle, if the external PHY is syn-
chronized to the Am79C978A controller, the external
PHY will drive a 0. If the external PHY does not drive a
0, the Am79C978A controller will signal a MREINT
(CSR7, bit 9) interrupt, if MREINTE (CSR7, bit 8) is set
to a 1, indicating the Am79C978A controller had an MII
management frame read error and that the data in
BCR34 is not valid. The data field to/from the external
PHY is read or written into the BCR34 register. The last
field is an IDLE field that is necessary to give ample
time for drivers to turn off before the next access. The
Am79C978A controller will drive the MDC to 0 and tri-
state the MDIO anytime the MII Management Port is
not active.
To help to speed up the reading and of writing the MII
management frames to the external PHY, the MDC can
be sped up to 10 MHz by setting the FMDC bits in
BCR32. The IEEE 802.3 specification requires use of
the 2.5-MHz clock rate, but 5 MHz and 10 MHz are avail-
able for the user. The intended applications are that the
10-MHz clock rate can be used for a single external PHY
on an adapter card or motherboard. The 5-MHz clock
rate can be used for an exposed MII with one external
PHY attached. The 2.5-MHz clock rate is intended to be
used when multiple external PHYs are connected to the
MII Management Port or if compliance to the IEEE
802.3u standard is required.
Auto-Poll External PHY Status Polling
As defined in the IEEE 802.3 standard, the external
PHY attached to the Am79C978A controller’s MII
has no way of communicating important timely sta-
tus information back to Am79C978A controller. The
Am79C978A controller has no way of knowing that
an external PHY has undergone a change in status
without polling the MII status register. To prevent
problems from occurring with inadequate host or
software polling, the Am79C978A controller will
Auto-Poll when APEP (BCR32, bit 11) is set to 1 to
insure that the most current information is available.
See 10BASE-T PHY Management Registers for the
bit descriptions of the MII Status Register. The con-
tents of the latest read from the external PHY will be
stored in a shadow register in the Auto-Poll block.
The first read of the MII Status Register will just be
stored, but subsequent reads will be compared to
the contents already stored in the shadow register.
If there has been a change in the contents of the MII
Status Register, a MAPINT (CSR7, bit 5) interrupt
will be generated on INTA if the MAPINTE (CSR7,
bit 4) is set to 1. The Auto-Poll features can be dis-
abled if software driver polling is required.
Am79C978A
31
Share Link: 