Having trouble getting a Texas Instruments DP83848J working in 10BASE-T mode.

The link partner and local status register agree that the link is in 10Mbps full-duplex mode. However, instead of receiving 4-bit 2.5Msps data on RXD[3..0] as required by the MII spec, a logic analyzer shows 1-bit 10Msps data. The only Ethernet standard I can find that describes that type of data is GPSI, which is not part of the DP83848J feature set.

Any idea how to configure the DP83848J to output data compliant with the 10 Mb/s MII Timing spec?

  • \$\begingroup\$ Uh oh - I was about to build a board with one of these on it! Looks like some advice is in order. The Microchip PIC32 Ethernet Starter Kit has one on it, if you're interested in an application circuit. \$\endgroup\$ Commented Feb 24, 2011 at 19:11
  • 1
    \$\begingroup\$ We used a NatSemi development module that provided the crystal and magnetics and all, we'll copy that circuit when we flatten it onto our board. We actually ran into lots of trouble making this chip work at all, but every problem we had was related to the driver for the MAC (it was a nios2 softprocessor in an Altera FGPA)... except for this one. DP83848J works quite nicely in 100Base-T mode. I'm wondering if anyone at all has gotten in working in 10Base-T, but my project doesn't actually NEED 10Mbps, using a slower speed was mainly a troubleshooting idea for our other problems. \$\endgroup\$
    – Ben Voigt
    Commented Feb 24, 2011 at 22:14
  • \$\begingroup\$ @reemrevnivek: But naturally we'd like to have 10Base-T compatibility also if possible. Thanks for placing the bounty. If you ever want a bounty on a problem over at stackoverflow, just let me know. Over there I've got plenty of rep to spare. \$\endgroup\$
    – Ben Voigt
    Commented Feb 24, 2011 at 22:15
  • \$\begingroup\$ No problem! You asked a good question, and it was going unanswered. \$\endgroup\$ Commented Feb 24, 2011 at 22:18
  • \$\begingroup\$ What are the TX_CLK/RX_CLK frequencies ? 25MHz, 2.5MHz or 50MHz ? \$\endgroup\$
    – Grabul
    Commented Dec 16, 2015 at 1:40

1 Answer 1


You are probably in RMII (reduce MII mode). Check this registers:

7.2.5 RMII and Bypass Register (RBR)

And this pin:


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    \$\begingroup\$ According to the datasheet page 76, RMII uses two data bits. But RXD[1] is not toggling. So it's not RMII mode. Or is the datasheet missing some information (I can't find any waveforms for RMII 10 Mbps mode)? \$\endgroup\$
    – Ben Voigt
    Commented Feb 25, 2011 at 19:18
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    \$\begingroup\$ @Ben Voigt At 10Mbps, RMII is still running at 50MHz, but each bit is repeated 10 times, look for "rmii_rev12.pdf" \$\endgroup\$
    – Grabul
    Commented Dec 16, 2015 at 1:31

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