USB to fiber (100BASE-FX) interface - Page 1
Forum section: electronics / projects

author: bonelli
author_rank: Contributor
timestamp: February 14, 2025, 10:42:22 am
content: Hello,I want to integrate in my hardware (embeded linux on rpi compute module) an USB to fiber interface. The USB host should recognize it as a ETH port.I tried with an external commercial device (packaged), it works greatly. The USB is recognized as an ETH from the linux. The same converter is used at the other end of the fiber, and I can ping the rpi from any computer.Now I have to integrate it on my PCB. I'm looking for the good chip to do that.I opened the commercial interface and found an ASIX AX88722C device, it is a USB to ETH converter with fiber support. A standard 1x9 SC fiber transceiver is wired on the chip. Unfortunately this device is hard to source in Europe, and I'm looking for a stable supplier for 5-10 years at least.I don't find any single chip solution. Other options are to use any classical USB to ETH converter (Microchip seems to have multipe references) and then a ETH to fiber converter (DP83849 seems to be the best choice for that conversion - app note TIDA-00306).Does anyone have any ideas or solutions for my quest?Thanks !

author: aeg
author_rank: Regular Contributor
timestamp: February 15, 2025, 07:25:45 am
content: Instead of using that monster DP83849 as a media converter... just turn off the built in PHY of your USB-Ethernet bridge chip, and connect a fiber PHY to the MII/RMII of your bridge chip.

author: fchk
author_rank: Frequent Contributor
timestamp: February 15, 2025, 09:12:09 am
content: Does it need to be 100FX? 100FX was never common and is slowly dying out. Maintaining 10 years of supply will be extremely difficult.Gigabit optical fiber is much more widespread these days, and SFP cages for transceivers would be the most universal solution. There are dozens of vendors for SFP transceiver for multimode and single mode fiber.Why do you need fiber in the first place? Length?What operation temperature ranges do you need?

author: bonelli
author_rank: Contributor
timestamp: February 17, 2025, 01:36:42 pm
content: In need ~300m between the 2 devices, galvanic isolation andabsolutely no EMI radiations. Any copper cable is prohibited.Temperature could be negative in some cases. I also have a power requirement : hundreds of mW are acceptable, 1W is a little bit too much. 100Mb transceivers are generally more energy efficient than Gb ones.I'm not familiar with fiber components, but the SFP could be a very good way to solve my problem. Actuelly I have 3 different use cases :- Nominal : my product will be used without ETH and without fiber. In this case, lowering the power consumption would be great.- Specific usage : it could be only used only with a fiber link.- Development, maintenance and test : an ETH link is always helpful.SFP will be a simple way to simply solve all use cases, pluging the right module (when needed) in the SFP cage.The Broadcom AFBR_57E6APZC SFP/fiber module looks to fit well :https://www.broadcom.com/products/fiber-optic-modules-components/industrial/industrial-fiber-optic-transceivers/sfp/afbr-57e6apzcWhen the copper link is needed, I can use a module like that :https://www.mouser.fr/datasheet/2/18/2/SFP_1000BASE_T_CX_C_Datasheets_EN-3524588.pdfI only have USB2 available, so I need something to convert USB2 to the right format. I'm not sure if I can use any ETH bridge with integrated PHY, for example the LAN9514 :https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/00002306A.pdfDo you think it could work ?

author: fchk
author_rank: Frequent Contributor
timestamp: February 17, 2025, 03:54:41 pm
content: Please note that there are three types of transceiver modules:* SFP: 1 GBit/s* SFP+: 10 GBit/s* SFP28: 25 GBit/sThe three types are interchangeble from a mechanical and electrical way, but not on a signal level. In general, you can't expect a SFP module to work in a SFP+ slot, except the manufacturer states otherwise. SFP Modules require Gigabit SFI interface.This is the spec:https://members.snia.org/document/dl/26184The easiest way here would be using the Pi's Ethernet port and a Gigabit Switch with SGMII interface (e.g. KSZ9477). SGMII is required for SFP modules.Alternative solution:Microchip LAN9250https://www.microchip.com/en-us/product/lan9250This is a 10/100 MAC/PHY controller that supports 100FX Fiber. Host interface is either CPU bus or SPI. Raspberry Pi has multiple SPI interfaces, so this would be possible from a hardware standpoint. LAN9250 is basically supported in Linux via smc911x driver. However, I don't know whether SPI support has been added yet.

author: bonelli
author_rank: Contributor
timestamp: February 18, 2025, 03:43:10 pm
content: Thanks for the spec.Apparently, the smc911x linux driver only supports CPU bus. Even if I found help to implement the SPI version, it will be too much bandwidth limited.The problem is that there is no ETH port on the rpi compute module. I have to deal with the USB2 IF.Another option could be to use a DP83869 converter. According to the following app note I could use it in 2 modeshttps://www.ti.com/lit/an/snla318/snla318.pdf1/ Using a LAN7801 (USB to RMII) on RMII port2/ Using LAN9414 (USB hub + ETH PHY) on copper portWith LAN9414 ETH output connected instead the RJ45 and magnetics :Solution 2 spare me a silicon chip (In all cases I need an USB hub somewhere). Solution 1 (with another chip for USB hub) is more energy efficient.I could route the MODEDEF signals from SFP to rpi and write a script on the linux to check the SFP module presence, read the type (RJ45 or fiber) and reconfigure the DP83869 consequently. I also have to route the LOS signal from SFP, and redirect it to the DP83869 SD pin with an inverter.Any opinion on theses solutions ?I'm confused with 2 things :- When using an optical SFP module, TI indicates to configure the chip in 100Base-FX, not SGMII.- When using a RJ45 SFP module, I don't know if I should use "RGMII to SGMII" or "RGMII to 100Base-FX". From the host point of view, it should be no difference between fiber and RJ45 SFP modules. So if 100Base-FX will work for fiber modules, it should wotk for RJ45 modules. Am I wrong ?

author: fchk
author_rank: Frequent Contributor
timestamp: February 19, 2025, 09:22:05 am
content: Quote from: bonelli on February 18, 2025, 03:43:10 pm? Do you still use CM3? CM4 and CM5 do have Gigabit Ethernet onboard (MAC+PHY) and an external PCIe lane to work with. CM5 also has got two USB3 interfaces.

author: fchk
author_rank: Frequent Contributor
timestamp: February 19, 2025, 09:30:53 am
content: Quote from: bonelli on February 18, 2025, 03:43:10 pm"Optical module" and "SFP" are two different things. SFP-Modules as defined in the spec don't use 100Base-FX and are NOT compatible with it. 100Base-FX is ONLY for special modules designed for 100Base-FX.Quote from: bonelli on February 18, 2025, 03:43:10 pmYes. RJ45-SFP modules are not designed to work with 100Base-FX. They are designed to use SGMII, the Serial Gigabit Media Independent Interface. It carries the "Gigabit" in it.And I repeat myself: 100Base-FX is basically dead.