Problem with Ethernet controller ENC424J600

Question

I have recently produced a number of prototypes using the Microchip ENC424J600 Ethernet IC.

I can't a full circuit diagram as this is a commercial product but I have reproduced the relevant part of the circuit below. What I am seeing is that as soon as I apply power the green LED lights, indicating a good link, even without an Ethernet cable plugged in.

If I am to believe the data sheet this LED should not require me to have good communications with the micro that is driving it and this LED should work provided it is powered, the oscillator is running and the Ethernet cable is plugged in and connected to a suitable device such as a hub.

Does anyone have experience with this IC and can suggest what I may be doing wrong.

If I have understood the data sheet correctly this is a typical application circuit

As as been pointed out in comments the data sheet does not show the TPIN transformer with the centre tap pulled up to 3V3. My original circuit does not have this connection. I added this because it is shown on the circuit diagram of Microchips demo board for this IC. The LED performs in the same way in either case.

I have also scoped the OSC1 pin to confirm it has a clean signal.

Answer 1

From the datasheet:

Both LEDs automatically begin operation whenever power is applied, a 25 MHz clock is present and the Ethernet magnetics are present and wired correctly. A connection to the host microcontroller via the SPI or PSP interface is not required. LEDA and LEDB can, therefore, be used as a quick indicator of successful assembly during initial prototype development.

Then it talks about another "bit" in a configuration register, I'd imagine that this would change it to something you'd expect, but you have to connect to it with a micro to write to this register.

STRCH: LED Stretching Enable bit 1 = Stretch transmit, receive and collision events on LEDA and LEDB to 50 ms 0 = LEDA and LEDB outputs show real-time status without stretching

There is also a register that lets you monitor the status and led state. REGISTER 8-2: EIDLED: ETHERNET ID STATUS/LED CONTROL REGISTER

The best way to verify systems that are being prototyped is to test all the pieces one by one. Start with the spi, see if you can write and then read configuration registers. Then test the ethernet, send a small packet and see if it shows up in the buffer of the phy chip.

If you can't read a register on the spi side then you have a problem with the chip or the setup, and you'll need to do some schematic and hardware troubleshooting. With any design, first check power, check both DC and AC levels (ie, switch the meter to ac, if there is too much ripple you might have problem. Also with power, check the startup conditions. I've had cheap power supplies that had an overvolt condition on startup that was messing with my design. Then check the clock signal.

If your pulling this design directly from the manufacturers example, it might be a good idea to compare this design to the manufactures. If you assembled it yourself examine the pins and check for solder bridges. Also, if there is some way for you to measure the chips temperature or current, make sure it's reasonable. If that doesn't work, replace the chip and follow stringent ESD and package handling guidelines (humidity can kill chips, esp when soldering).