mdio_mdio是什么缩写_sgmii和serdes的区别(2)

Clause 22

Clause 22 defines the MDIO communication basic frame format (figure) which is composed of the following elements:

The frame format only allows a 5-bit number for both the PHY address and the register address, which limits the number of MMDs that the STA can intece. Additionally, Clause 22 MDIO only supports 5V tolerant devices and does not have a low voltage option.

Clause 45

In order to address the deficiencies of Clause 22, Clause 45 was added to the 802.3 specification. Clause 45 added support for low voltage devices down to 1.2V and extended the frame format (figure) to provide access to many more devices and registers. Some of the elements of the extended frame are similar to the basic data frame:

Figure 14: Extended MDIO Frame Format

The primary change in Clause 45 is how the registers are accessed. In Clauses 22, a single frame specified both the address and the data to read or write. Clause 45 changes this paradigm. First an address frame is sent to specify the MMD and register. A second frame is then sent to perform the read or write.

The benefits of adding this two cycle access are that Clause 45 is backwards compatible with Clause 22, allowing devices to interoperate with each other. Secondly, by creating a address frame, the register address space is increased from 5 bits to 16 bits, which allows an STA to access 65,536 different registers.

In order to accomplish this, several changes were made in the composition of the data frame. A new ST code (00) is defined to identify Clause 45 data frames. The OP codes were expanded to specify an address frame, a write frame, a read frame, or a read and post read increment address frame. Since the register address is no longer needed, this field is replaced with DEVTYPE to specify the targeted device type. The expanded device type allows the STA to access other devices in addition to PHYs.

Additional details about Clause 45 can be found on the IEEE 802.3 workgroup website.

References

IEEE 802 LAN/MAN Standards Committee

Use The MDIO Bus To Interrogate Complex Devices–Electronic Design Magazine

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