TUSB546-DCI: AC coupling in a USB MUX architecture

Ambroise Chaigne

Prodigy 20 points

Part Number: TUSB546-DCI
Other Parts Discussed in Thread: TUSB8044A, , TUSB8020B, TUSB542

Tool/software:

Hello,

I'm implementing the following architecture:

USB 3.X Host (COM-Express) <--> one USB 3.X bus <--> (upstream) TUSB8044A (downstream) <--> four USB 3.X buses [...]

The four TUSB8044A downstream buses are routed as follows:

Downstream bus #1: routed to TUSB546-DCI and then to USB-C connector
Downstream bus #2: routed to TUSB8020B and then to two USB-A connectors
Downstream bus #3: routed to TUSB542 and then to USB-C connector
Downstream bus #3: routed to TUSB542 and then to USB-C connector

I wonder where to place the AC coupling capacitors on the SuperSpeed USB signals. Reading the TI component datasheets, so far I placed them as follows:

1x100nF series capacitor on each TX signal between the USB connector and the re-driver (TUSB546-DCI/TUSB542, for USB-C connectors) or the TUSB8020B (for USB-A connectors)
1x100nF series capacitor on each RX/TX signal between the re-driver/the TUSB8020B and the TUSB8044A
No capacitor on the updstream side of TUSB8044A

Could you guide me through this, are these capacitors positioned correctly?

2 days ago

0 Vishesh Pithadiya 2 days ago

TI__Mastermind 38405 points

Hi,

Are you able to share a block diagram/ schematic?

0 Vishesh Pithadiya 2 days ago in reply to Vishesh Pithadiya

TI__Mastermind 38405 points

From your description this seems correct, but Id like to doublecheck with a visual.

0 Ambroise Chaigne 2 days ago in reply to Vishesh Pithadiya

Prodigy 20 points

To make it simpler, each wire actually represents a differential pair.

0 Vishesh Pithadiya 2 days ago in reply to Ambroise Chaigne

TI__Mastermind 38405 points

Yes, this downstream AC coupling scheme looks ok.

Can you confirm you do not need AC coupling on the upstream port of the hub?

0 Ambroise Chaigne 18 hours ago in reply to Vishesh Pithadiya

Prodigy 20 points

I'm not really sure whether I need it or not. The upstream side on the TUSB8044A is structured as follows:
TUSB8044A --> Board-to-Board connector --> Base Board (only routing from one connector to another) --> Board-to-Board connector --> Com-Express board

Could you help me understanding the situation better to decide if AC coupling is needed on the upstream port? Which parameters should I consider?

+1 Vishesh Pithadiya 17 hours ago in reply to Ambroise Chaigne

TI__Mastermind 38405 points

Hi,

All USB connections of TX and RX should have 75-265nF of capacitance from end to end.

The reason we have 100nF only on the outgoing TX is because this will connected to an RX with no capacitance. The outgoing RX will connect to a device TX which will have 100nF of capacitance.

Ambroise Chaigne said:
TUSB8044A --> Board-to-Board connector --> Base Board (only routing from one connector to another) --> Board-to-Board connector --> Com-Express board

The should be 75-265nF between the SoC and the hub for TX and RX connections for USB3.x.

0 Ambroise Chaigne 2 hours ago in reply to Vishesh Pithadiya

Prodigy 20 points

Thank you very much for your answer!

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TUSB546-DCI: AC coupling in a USB MUX architecture