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The lowdown on powering Ethernet (Part 1)

30 Jun 2015  | Jones Mike

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10Base-T operation differs during quiet periods, since when no traffic is present, the PHY transmitter does not transmit out any IDLE symbols. Instead, it sends out a single link pulse approximately every 16ms, designed simply to keep the link alive. The power consumption of the PHY itself during a quiet period in 10Base-T operation will not reduce significantly, but the current consumed externally in the transformer will reduce to negligible, saving around 70mA per PHY compared to full traffic.


Cable length, driver strength
For Ethernet conformity, the PHY transmitter must adhere to fitting within the defined limits of the mask (figure 3).


Figure 3: 100Base-TX IEEE802.3 Output Eye Diagram.


This waveform is designed to ensure that the PHY is capable of operating up to a minimum 100m of CAT5 grade cable. As a consequence, the PHY output drive strength is fixed at this criterion, consuming maximum power, independent of the actual length of cable connected. There was no provision to adaptively adjust the drive strength dependent on the cable length, making this a major obstacle in the original IEEE 802.3 specification with respect to energy saving.

However, in practice, many applications do not require the capability of 100m-cable reach and can guarantee a much shorter length. A simple change to the circuit can reduce the PHY transmitter current drive, typical set by a resistor, from the standard ±1V amplitude of the 100Base-TX signal down by up to 50 per cent and still operate error free over a 10-20m reach (typical for automotive networks). For example, doubling the resistance will half the typical 40mA 100BT drive current to around 20mA per port. Longer cable reach could be achieved while operating at reduced current drive by installing higher quality cable e.g., CAT6 or above, that exhibits lower attenuation. System costs, however, are increased.

The use of cable diagnostics features, such as Micrel's LinkMD, offer has the ability to measure the connected cable length, using time domain reflectrometry techniques. This allows designers to intelligently adjust the drive strength according to the cable length, thereby improving the power consumption efficiency. An additional benefit of reducing the PHY current drive strength is the reduction in electromagnetic interference (EMI) radiated emissions.


Supply voltage
Another area important to explore when ensuring maximum energy efficiency is the power management of the Ethernet device. Many modern devices operate using a single voltage, typically 3.3V, and provide internal regulation for core voltage(s). This provides the customer with a simpler implementation – but has a significant impact on power efficiency. If possible (often not unfortunately), disable the internal linear regulator and supply the lower core voltage externally (often already available on the board). Micrel's KSZ8863/73 3-Port Ethernet switch offers an example (table)


Table: KSZ8863/73 Typical Current Consumption.


Using the internal regulator, total Power Consumption is 3.3V × 121mA = 400mW. But if one disables the internal regulator and operates using external 1.8V supply, then the total Power Consumption is 3.3V × 21mA + 1.8V × 100mA = 249mW. This improves power efficiency by an impressive 38 per cent.

Power consumption in electronic applications has increasingly been viewed as critical, with worldwide legislation forcing manufacturers to improve energy efficiency. As we have seen here, power consumption can be reduced during normal operation, but this is only once aspect of achieving Energy Efficient Ethernet. Not only is the power consumed when devices are in operation but also during 'standby' periods. Power savings during standby operation will be covered in Part 2 of this series.


About the author
Jones Mike has over 25 years of experience in high-tech design in the semiconductor industry. He is currently based in Newbury, U.K., where he is am Product Marketing Director, responsible for Micrel's Automotive and Industrial LAN Solutions. Prior to Micrel, he worked for several high tech companies, in various engineering roles, including as principal engineer at BT and Fijitsu Telecommunications where he gained more than a decade in design experience in SONET, SDH and PDH systems. He also held position of senior FAE and Product Marketing Manager with Micrel for fourteen years prior to his current position.


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