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Attached is a small section of USB to Ethernet converter design. RJ 45 Connector has one dual LED (see attached) Green and Yellow, backward connected, and drive via the LAN9500 chip pin27. enter image description here

As per design look like only Green LED will trun on when nLNKA_LED/GIO9 goes low. and when nLNKA_LED/GIO9 goes high no LED will turn on. then how the yellow LED will turn on here?

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  • \$\begingroup\$ You have forgotten to tell us what the LEDs are supposed to indicate. For example, if the 3.3 V falls to zero it might light and indicate a PSU fault. Put the information into the question. What is the supply voltage on the LAN9500? \$\endgroup\$ – Transistor Aug 22 '17 at 18:00
  • \$\begingroup\$ supply voltage is 3.3V, This pin is driven low (LED on) when a valid link is detected. This pin is pulsed high (LED off) for 80 mS whenever transmit or receive activity is detected. This pin is then driven low again for a minimum of 80 mS, after which time it will repeat the process if TX or RX activity is detected. Effectively, LED2 is activated solid for a link. When transmit or receive activity is sensed, LED2 will function as an activity indicator. \$\endgroup\$ – Short Circuit Aug 22 '17 at 18:03
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    \$\begingroup\$ All that information belongs in the question as I suggested so that the readers don't have to trawl through the comments to find all the relevant info. \$\endgroup\$ – Transistor Aug 22 '17 at 18:07
  • \$\begingroup\$ link of design is ww1.microchip.com/downloads/en/DeviceDoc/9500ai_sch.pdf \$\endgroup\$ – Short Circuit Aug 22 '17 at 18:10
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    \$\begingroup\$ @MatsK: Because OP has twice added relevant information in the comments instead of in the question. Read the comment history. \$\endgroup\$ – Transistor Aug 24 '17 at 17:05
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It won't ever turn on. The supply for the chip is 3.3V so the output will be either 0V or 3.3V. It's effectively the same as if the yellow LED die was not present.

Still better than an eval board I have which has the link LED connected to the power supply through a resistor. You can plug a pencil into the jack and the light still turns on when power is applied.

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  • \$\begingroup\$ Looks like you got fault eval board \$\endgroup\$ – Short Circuit Aug 22 '17 at 18:06
  • \$\begingroup\$ @ShortCircuit The board faithfully follows the OrCAD schematic, so it's the design that's questionable. \$\endgroup\$ – Spehro Pefhany Aug 22 '17 at 19:34
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The schema doesn't make sense, from what I have seen as common practice.

The only way I can think of is that "nLNKA_LED/GPIO9" is either 0 volt or 6,6 volt. That way the current will either flow through the green or the yellow LED.

A common practice when you have two LED's in opposing directions, you normally have two I/O pin connected to either side.

This way you can have either the green or the yellow lit with just two wires.

The way you do it is:

Pin 9  Pin 10   LED lit
  0      0      No light
  1      0      Green
  0      1      Yellow
  1      1      No light
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  • \$\begingroup\$ Reference design show only one Io connected to this LED's. ww1.microchip.com/downloads/en/DeviceDoc/9500ai_sch.pdf \$\endgroup\$ – Short Circuit Aug 22 '17 at 18:11
  • \$\begingroup\$ Mmmm and then one of the LED's (Yellow) will newer produce light! as @Spehro Pefhany answered. \$\endgroup\$ – MatsK Aug 22 '17 at 18:15
  • \$\begingroup\$ Oops there is a way!!! If the status (voltage level) on "nLNKA_LED/GPIO9" can be either 0volt or + 6,6volt, then will either the green or yellow light up. I will adjust my reply accordingly. \$\endgroup\$ – MatsK Aug 22 '17 at 18:18

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