Datasheet

DP83849IF
www.ti.com SNOSAX8D –JUNE 2009–REVISED APRIL 2013
5.5.2 LED Direct Control
The DP83849IF provides another option to directly control any or all LED outputs through the LED Direct
Control Register (LEDCR), address 18h. The register does not provide read access to LEDs.
5.6 HALF DUPLEX vs. FULL DUPLEX
The DP83849IF supports both half and full duplex operation at both 10 Mb/s and 100 Mb/s speeds.
Half-duplex relies on the CSMA/CD protocol to handle collisions and network access. In Half-Duplex
mode, CRS responds to both transmit and receive activity in order to maintain compliance with the IEEE
802.3 specification.
Since the DP83849IF is designed to support simultaneous transmit and receive activity it is capable of
supporting full-duplex switched applications with a throughput of up to 200 Mb/s per port when operating
in either 100BASE-TX or 100BASE-FX. Because the CSMA/CD protocol does not apply to full-duplex
operation, the DP83849IF disables its own internal collision sensing and reporting functions and modifies
the behavior of Carrier Sense (CRS) such that it indicates only receive activity. This allows a full-duplex
capable MAC to operate properly.
All modes of operation (100BASE-TX, 100BASE-FX, 10BASE-T) can run either half-duplex or full-duplex.
Additionally, other than CRS and Collision reporting, all remaining MII signaling remains the same
regardless of the selected duplex mode.
It is important to understand that while Auto-Negotiation with the use of Fast Link Pulse code words can
interpret and configure to full-duplex operation, parallel detection can not recognize the difference between
full and half-duplex from a fixed 10 Mb/s or 100 Mb/s link partner over twisted pair. As specified in the
802.3u specification, if a far-end link partner is configured to a forced full duplex 100BASE-TX ability, the
parallel detection state machine in the partner would be unable to detect the full duplex capability of the
far-end link partner. This link segment would negotiate to a half duplex 100BASE-TX configuration (same
scenario for 10Mb/s).
Auto-Negotiation is not supported in 100BASE-FX operation. Selection of Half or Full-duplex operation is
controlled by bit 8 of the Basic Mode Control Register (BMCR), address 00h. If 100BASE-FX mode is
strapped using the FX_EN pin, the AN0 strap value is used to set the value of bit 8 of the BMCR (00h)
register. Note that the other Auto-Negotiation strap pins (AN_EN and AN1) are ignored in 100BASE-FX
mode.
5.7 INTERNAL LOOPBACK
The DP83849IF includes a Loopback Test mode for facilitating system diagnostics. The Loopback mode is
selected through bit 14 (Loopback) of the Basic Mode Control Register (BMCR). Writing 1 to this bit
enables MII transmit data to be routed to the MII receive outputs. Loopback status may be checked in bit 3
of the PHY Status Register (PHYSTS). While in Loopback mode the data will not be transmitted onto the
media. To ensure that the desired operating mode is maintained, Auto-Negotiation should be disabled
before selecting the Loopback mode.
5.8 BIST
The DP83849IF incorporates an internal Built-in Self Test (BIST) circuit to accommodate in-circuit testing
or diagnostics. The BIST circuit can be utilized to test the integrity of the transmit and receive data paths.
BIST testing can be performed with the part in the internal loopback mode or externally looped back using
a loopback cable fixture.
The BIST is implemented with independent transmit and receive paths, with the transmit block generating
a continuous stream of a pseudo random sequence. The user can select a 9 bit or 15 bit pseudo random
sequence from the PSR_15 bit in the PHY Control Register (PHYCR). The received data is compared to
the generated pseudo-random data by the BIST Linear Feedback Shift Register (LFSR) to determine the
BIST pass/fail status.
Copyright © 2009–2013, Texas Instruments Incorporated Configuration 37
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