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I have a problem with a relatively simple board which consists of an ENC28J60 Ethernet controller and an ATmega168.

The circuit around the ENC28J60 is straight out of the datasheet and I have the same circuit built up in a prototype with DIP parts where it works, but in my SMD version the ENC sucks down 40 mA @ 3.3V and it gets pretty hot.

The datasheet mentions that a relatively high current is sunk by the two TPOUT pins and that the total current is around 180 mA, but there is no mention of how much current the ENC itself will pull out of VDD.

The oscillator runs perfectly and CLKOUT is 6.25 MHz, so the chip is not completely dead.

I've measured the voltage at every pin and:

  • Every VSS pin is 0V
  • Every VDD pin is 3.3V
  • The oscillator and CLKOUT out are good.
  • VCAP is 2.65V
  • RBIAS is at 1.2V.
  • !RESET is 3.3V.
  • SPI looks ok.

Yet, I can't seem to raise the chip over SPI from the AVR and it gets pretty damn hot.

I've tried yanking !RESET low and that doesn't affect the power usage. Any clues?

Notice that the pin numbers for OSC1, OSC2, VDDOSC and VSSOSC are wrong in the schematic, I fixed that bug after exporting that version of the PDF however it is no longer available online.

OK, here's the real list of problems with my design that led to my problem:

  1. My 3.3V regulator is dumping waaay too much power, so it gets too hot.
  2. The ENC28J60 eats lots of power, even when working normally, so that gets hot too.
  3. I messed up and forgot to double check the Eagle library pinout, so I had to do some board hacking to fix that.
  4. The RX center tap should not be connected.
  5. I forgot to update my Makefile to build for the Mega168 part.

Problem number 5 was the real source of my problem, #1 & #2 kept me looking at the hardware for far too long, which is doubly embarrassing because I make my living writing software.

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    \$\begingroup\$ Are you measuring with a scope or DMM? A scope will show you the noise and any glitches on the power supply. A DMM smooths everything out and you won't see it. \$\endgroup\$ – Robert Oct 26 '10 at 14:54
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The Center Tap of the receive magnetics should not be connected to R5 and R6.

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  • \$\begingroup\$ You might try removing the Ethernet magnetics just to eliminate part of the circuit. Also remove the LEDs. At this point, your chip might be gone. \$\endgroup\$ – Robert Oct 26 '10 at 12:52
  • \$\begingroup\$ You are right about the center tap on the receive side, I don't know how I could make that mistake, but that shouldn't do anything other than screw up my reception, there is no way that should give a huge power drain. \$\endgroup\$ – dren.dk Oct 26 '10 at 13:21
  • \$\begingroup\$ I'll try lifting the resistors for the LEDs, though the datasheet does mention that the LEDs are used to program some defaults, so they are not completely without side effects. \$\endgroup\$ – dren.dk Oct 26 '10 at 13:23
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You have some pins wrong.

The datasheet has pin 23 as Vssosc. The SCH has is as OSC2.

The datasheet has pin 24 as OSC2. The SCH has is as pin 22.

Pin 25 in the datasheet is Vddosc. The SCH has Vssosc.

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  • \$\begingroup\$ Nice catch, that was an old schematic and I already fixed the bug, but but I forgot to mention it in my question. Sorry. \$\endgroup\$ – dren.dk Oct 26 '10 at 12:22

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