VSC8540RT datasheet(63/87 Pages) MICROCHIP

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DS60001603E-page 63

VSC8540RT

7.0

DESIGN CONSIDERATIONS

This section provides information about design considerations for the VSC8540RT device.

7.1

Link status LED remains on while COMA_MODE pin is asserted high

When the COMA_MODE is asserted high, the link status LED may not deactivate unless the media cable is discon-

nected from the device.

While using COMA_MODE, link status should be verified using status registers rather than LED indicators.

7.2

10BASE-T signal amplitude

10BASE-T signal amplitude can be lower than the minimum specified in IEEE 802.3 paragraph 14.3.1.2.1 (2.2V) at low

supply voltages. Additionally, associated templates may be marginal or have failures.

This issue is not estimated to present any system-level impact. Performance is not impaired with cables up to 130 m

with various link partners.

7.3

Receive error counter only clears when NRESET applied in RMII mode

When operating in RMII MAC interface mode, the receive error counter of register 19 (Table 6-16) does not clear upon

read-back or software reset of the device. The only way to clear this register while in RMII mode is to assert the NRESET

pin of the device.

The receive error counter at register address 19 operates normally in MAC interface modes other than RMII.

7.4

Anomalous PCS error Indications in Energy Efficient Ethernet Mode

When a port is processing traffic with EEE enabled on the link, certain PCS errors (such as false carriers, spurious start-

of-stream detection, and idle errors) and EEE wake errors may occur. There is no effect on traffic bit error rate for cable

lengths up to 75 meters, and minor packet loss may occur on links longer than 75 meters.

Regardless of cable length, some error indications should not be used while EEE is enabled. These error indications

include false carrier interrupts (Section 6.2.20, "Interrupt Status", bit 3), receive error interrupts (Section 6.2.20, "Inter-

rupt Status", bit 0), and EEE wake error interrupts.

Contact Microchip for a script that needs to be applied during system initialization if EEE will be enabled.

7.5

Half-duplex operation in RGMII MAC interface mode

CRS encoding used in RGMII MAC interface mode is missing the following cases:

• Carrier Sense code

• Carrier Extend code

• Carrier Extend Error code

For the corresponding requirements, see the RGMII standard (“Allowable Encoding of RXD, RXDV, and RXERR”). As

a result, Carrier Sense will not be indicated to the local MAC host in any of the conditions listed above. Half-duplex con-

figurations will operate with degraded performance and it is not recommended to use the PHY in this mode with an

RGMII MAC.

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