TUSB1146: USB VESA spec

Fred

Please note the 8.5dB being supported by the host is only for Type-A connector. Different connector will have different loss support requirement, please see this link for more detail, https://www.usb.org/sites/default/files/USB_3.1_Loss_Budget_Rev_1.0_-_2015-03-02.pdf.

If the total PCB loss is 20dB, and the redriver RX compensates 12dB, then there is 8dB loss left which can be compensated by a USB-compliant host. But there are other factors impacting the actual compliance testing and we have to look at it in detail.

Thanks

David

Hi David,

thanks but I notice that type-C port for gen2 is also 8.5dB, same as  type-A.

I understand there're other factors, but I mean in general this is what this spec means right?

if the CPU TX/RX design did follow the USB spec, then within 8.5dB loss is suppose to be ok.

Fred

The spec is looking at the loss a host can support without a redriver. So for example, if the host TX is sending high speed signal outside the redriver linearity range with the 12dB EQ, then depend on the host TX signal characteristic, there could be compression and you may or may not pass the TX compliance. 

If the redriver is a linear redriver, then the only compensation is the RX EQ. So for RX compliance, the redriver TX will not able to compensate the 12dB and if the host RX can't compensate either, then you may also fail the RX compliance.

Thanks

David 

Hi David,

so this spec is only for the case without redriver, got it.

I want to follow up on RX part, 

so if we use redriver to compensate 12dB, 

since the host RX only claim for 8.5dB  (USB spec),  so if the loss is higher  , the host can't guarantee to compensate it.

then we may have to look into CPU's datasheet to check the actual EQ, am I right?

Fred

This is correct, and we have couple choices

• Use limited redriver or retimer
• For linear redriver, reduce the loss
• For linear redriver, place it closer to the host

Thanks
David

so limited redriver  has de-emphasis and boost that linear doesn't have ?

Hi David,

thanks a lot , fully understand!