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Design USB hub circuit #19
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The backplane needs a USB hub for connecting the BMCs in each compute tray to the RMC. HUB IC selectionThe reference rack design houses up to four computer trays so we need at least four downstream ports:
TUSB2046x variants seem to be the only active components that are still well stocked. TUSB2046IBVF is the most accessible package so I'm basing this design around that. TUSB2046IBVF Peripheral ComponentsSpeed IdentificationUSB 2.0 full-speed should be sufficient for our needs. The resistor values are selected as described in this page. Full-speed devices require the following resistors on the data lines:
Update 2022-10-21 |
The electronics management (fan control, ...) also needs a microcontroller which acts as a USB device for management and data collection. Therefore, the hub controller should have at least 5 downstream ports. Additionally, if there's a desire to chain multiple racklets together and share a single RMC, one additional downstream port is required bumping the number of required downstream ports to 6. Based on these additional requirements TUSB2077APTR seems like a suitable HUB controller. |
Over-current ProtectionThe USB 2.0 specification mandates that all USB hosts and hubs implement over-current protection circuitry (see section 7.2.1.2.1). This could be implemented in a few ways:
[MIC2077-2YM] is the most affordable compatible (active-low enable and overcurrent signals) and is priced at around 2.23€. The 7 downstream ports would require two of these costing 4.46€ in total. PTC fuses like MF-MSMF050-2 are priced at around 0.30€ giving a total cost of around 2.10€ for all 7 downstream ports. |
The backplane needs a USB hub for connecting the BMCs in each compute tray to the RMC.
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