Ethernet switch IC/PHY combination and alternative - Page 1
Forum section: electronics / projects

author: shebu18
author_rank: Frequent Contributor
timestamp: June 27, 2018, 06:48:48 am
content: Hello,I am currently working on a project where i need to connect 9xUSB-to-ethernet adapters to an ethernet switch with 8 ports. The components i have chosen for this are the Microchip LAN9512 and the Microchip KSZ8999. I will use capacitors for decoupling the MDI traces.On the 9th port (MII) of the KSZ i want to connect another PHY so that i can access the 8 connection from outside the box via an connector.My questions are:1. Is that 9th port capable to do what i want?2. what PHY should i use?3. Does someone have another recommendation for a 8 port ethernet switch IC with an aditional mac for external access?Thanks,Cristian

author: Scrts
author_rank: Frequent Contributor
timestamp: June 27, 2018, 01:28:29 pm
content: Hello,the idea seems to be fine, but what's your architecture? Maybe it's worth having an 8 port USB switch and then only one USB-to-Ethernet chip? Other than that - what is the expected USB bandwidth all together? Your switch is 100Mbps, so you can load about 8Mbps max for each. If not, you need 1Gbit switch.

author: ajb
author_rank: Super Contributor
timestamp: June 27, 2018, 03:34:35 pm
content: Yes, you can almost always use an MII port on a switch to connect to either a MAC or a PHY. Pay careful attention to the pin strapping required to configure it correctly, as well as the available clocking options (both the switch and the external PHY need a clock, and depending on the parts you choose there are different options for how you can arrange this from a single crystal). It's a good idea to include pullup and pulldown options for all of the strapping pins on your first board so that you have full control over the configuration. Also, many switches have I2C/SPI ports that can be used for management (and often expose more features than the traditional SMI does), so you can bring those pins out in case you want to hook them up to an MCU to poke at the switch, read status, etc.As for other switch options, there aren't many manufacturers I know of that are conducive to low-quantity production. Microsemi parts are available at qty 1, but they quickly get into pretty hefty managed switches that are going to be a lot more work to bring up and require an NDA to even see the datasheet.As far as PHYs, there are tons of options, just make sure it supports the right flavor of MII (MII or RMII for 100Mb) and has a form factor you like. As with the switch, it's a good idea to bring out all of the strapping pins so that you can change the configuration if necessary as you bring the board up.

author: shebu18
author_rank: Frequent Contributor
timestamp: June 27, 2018, 04:37:20 pm
content: Thank you @ajb and @Scrts for your reply.The architecture is something like this: 8 boards (USB 2.0) each with its USB to ETH converter that are connected to an 8 port switch. The use for the 9th port is that i can access the network of 8 from outside the box. The RJ45 connector will be located on the side panel of the box. What i want from the switch is relatively simple, it needs to build an unmanaged network with 9 connections. My wish would be to not configure the switch, plug and play.The 100Mbit is enough for what i need.What scared me on the KSZ8999 was the management interface (I2C). I thought that i need more than just the straps to configure the switch. It looks like i can leave it floating.I find it normal to add both strapping options (PU/PD).I found a switch from Marvell that fills all my options but i do not think that i can get a small quantity from them (<5) and did not find it on the commercial sites.

author: Scrts
author_rank: Frequent Contributor
timestamp: June 27, 2018, 06:19:25 pm
content: I thought you have one board with all those USBs, but if you have 8 boards, then why don't you just route the ethernet cable to an off-the-shelf 16-port switch? You won't beat the price of that.

author: shebu18
author_rank: Frequent Contributor
timestamp: June 27, 2018, 06:57:58 pm
content: I am shure about that, but the boards with usb are inserted in a socket, so the usb to eth converter and switch must be on the same board.

author: Heartbreaker
author_rank: Unknown
timestamp: June 27, 2018, 07:30:06 pm
content: Quote from: Scrts on June 27, 2018, 01:28:29 pmSorry, this will not work as the 8 USB-to-Ethernet devices all connects to their own USB host from each of the 8 boards. Replacing these 8 devices with a single USB switch put 8 USB hosts in the same spanned USB tree. There is a distinct host- and device-end (up- and down-stream ports) to a USB switch - it is not symmetrical like e.g. an Ethernet switch./Sverre

author: ajb
author_rank: Super Contributor
timestamp: June 27, 2018, 08:11:13 pm
content: Quote from: shebu18 on June 27, 2018, 04:37:20 pmUnmanaged switches can usually be put into any typical configuration just by pin strapping, but again, the management interface will give you more flexibility and allow you to read out status. Some un- or lightly-managed switches can load their configuration from an external EEPROM. I would recommend at least bringing the management interface out to pads, that way you can easily wire them up to an Arduino or maybe a Bus Pirate in case you want to read out the switch status for troubleshooting purposes.

author: shebu18
author_rank: Frequent Contributor
timestamp: June 28, 2018, 05:01:16 am
content: I will bring out every pin of the eth switch and the ones with PU/Pd options will have both possibilities. I hope that the chosen switch will work without the use of the management interface. I will also add the possibility to add a EEPROM.@Heartbreaker: I do not want to replace the USB to ETH adapter. Each board will have its adapter and then the adapters eth side is connected to the switch.In the search for a PHY for the 9th port i saw that they also have a management interface but they also work if not managed, this is good for what i want.