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Andrew Zonenberg

Ethernet nerds: Does this section from 802.3-2022 actually require that you be able to read the currently negotiated speed and duplex state back from this register, or only write to force a specific speed?

Every PHY I've ever seen except the VSC8512 lets you read back the actual operating conditions, but reading the spec it seems that there's not actually a mandate that this capability be there.

The register is defined as readable but it's not well defined whether it reads the configured value or the negotiated value.

45.2.1.1.3 Speed selection (1.0.13, 1.0.6, 1.0.5:2) For devices operating at 10 Mb/s, 100 Mb/s, or 1000 Mb/s the speed of the PMA/PMD may be sclected using bits 13 and 6. The speed abilities of the PMA/PMD are advertised in the PMA/PMD speed ability register. These two bits use the same definition as the speed selection bits defined in Clause 22. For devices not operating at 10 Mb/s, 100 Mb/s, or 1000 Mb/s, the speed of the PMA/PMD may be selected using bits 5 through 2. When bits 5 through 2 are set to 0000 the use of a 10G PMA/PMD is sclected. More specific selection is performed using the PMA/PMD control 2 register (register 1.7) (sce 45.2.1.6). The speed abilities of the PMA/PMD are advertised in the PMA/PMD speed ability register. A PMA/PMD may ignore writes to the PMA/PMD speed selection bits that select speeds it has not advertised in the PMA/PMD speed ability register. It is the responsibility of the STA entity to ensure that mutually acceptable speeds are applied consistently across all the MMDs on a particular PHY. The PMA/PMD speed selection defaults to a supported ability. When set to 0001, bits 5:2 select the use of the 10PASS-TS or 2BASE-TL PMA/PMD. More specific mode selection is performed using the 10P/2B PMA control register (45.2.1.28).
Jul 04, 2025, 12:13 AM·Public·Web
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✧✦Catherine✦✧

@azonenberg this text doesn't appear to say anything about readback

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Andrew Zonenberg

@whitequark It's mentioned as readable a bit up in the table, but the spec doesn't seem to say anything about whether the read value is what you last wrote, or the operating mode

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Andrew Zonenberg

@whitequark and this behavior seems to vary from phy to phy

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