Components for automotive one-pair Ethernet
Conventional Ethernet connections use multiple twisted-pair cables. Automotive Ethernet however communicates full duplex via one pair of unshielded twisted wires only. This saves space, weight and complexity in the wiring. Galvanic decoupling of single nodes is done with low cost capacitors instead of a HF-transformer. The main building blocks for an automotive Ethernet application are shown in the image above and listed below:
- Microcontroller with Ethernet MAC
- Ethernet PHY
- Crystal (XTAL) or Clock Oscillator (XO) for Ethernet PHY
- Low Pass Filter
- ESD Protection
- Common Mode Choke, L1 in the image obove
- Coupling Capacitors
The Ethernet MAC (Media Access Controller) is a peripheral inside a microcontroller for Ethernet communication. The table lists some popular automotive microcontroller families with on-chip Ethernet MAC.
|Infineon||AURIX 2G, SAK-TC3xx||up to dual (TC37xEXT) Gbit MAC, RGMII/RTBI, RMII, MII, A/V-traffic, PTP|
|STMicroelectronics||SPC584B, EC, NG||up to dual MAC, 10/100 MBit, RMII, MII, A/V-traffic|
|STMicroelectronics||SPC58NH||up to dual MAC, 1×10/100Mbit + 1xGbit|
The PHY translates the signals of the Media Independant Interface (MII) or the Reduced Media Independant Interface (RMII) to the physical signal layer. In our case this is one twisted-pair of copper wires.
The crystal determines the clock frequency and is part of an analog oscillation circuit. To ensure reliable oscillations in the complete temperature range the crystal has to be carefully selected according to the datasheet of the PHY. Common frequency values are multiples of 25 MHz.
Our crystal suggestions for customer evaluation are from
DIODES, series FHQ, FLQ, FYQ, XRQ,
Kyocera, series CX3225xA,
EPSON, series FA-238A.
Crystal oscillators integrate the crystal and a matched oscillation circuit and provide reliable square wave output. Our suggestions for a clock oscillator are from
DIODES, series FDQ, FKQ, FNQ, and KDQ,
Kyocera, series MC2016Z, MC2520Z, MC3025Z.
Low pass filters for high frequency can be implemented with discrete passive components or with integrated passives in silicon. STMicroelectronics offers integrated filters. The type-names start with EMIF. A filter for the first generation of OPEN Alliance PHYs from Broadcomm was the EMIF02-02OABRY. Successive generations integrated the filter in the PHY so that no external filtering is necessary any more.
Protection devices for high frequency data lines feature fast response and ultra low capacitance. Have a look at
Manufacturer DIODES, type DRTR5V0U2SR, protection diodes for high speed data lines, AEC-Q, no PPAP,
Manufacturer STMicroelectronics, type USBLC6-2SC6Y, automotive very low capacitance dual data line ESD protection device.
Manufacturer TDK, type ACT45L-201-2P-TL000, only for existing designs, not recommended for new designs,
Manufacturer TDK, type ACT1210L-201-2P-TL00,
Manufacturer Murata, type DLW32MH101XT2, 1000Base-T1 common mode choke, AEC-Q200, fulfills the OPEN Alliance specifications according to Murata,
Manufacturer Yageo/Pulse, type AE5002, 1000Base-T1 common mode choke, AEC-Q200.