TUSB501: Why is Vcm-RX different for each re-driver?

Tamio Yokokura

Part Number: TUSB501
Other Parts Discussed in Thread: TUSB522P, TUSB551

I have three question as follows;

[1] When I confirm Vcm-RX of three re-driver, which are TUSB501, TUSB522P and TUB551, Vcm-RX is different for each re-driver. Why is Vcm-RX different for each re-driver?

TUSB501 : Vcm-RX = 3.3V
TUSB522P : Vcm-RX = 0.7V
TUSB551 : Vcm_RX = 0V

[2] Can TUSB501 and TUSB551 receive the correct swing voltage when Vcm-RX=3.3V (=Vcc) or Vcm-RX=0V (=GND)?

[3] Can customer place the resister as shown at the following figure? If yes, what is the range of resister?

Thanks

Tamio

over 4 years ago

0 Jerry Madalvanos over 4 years ago

TI__Mastermind 23011 points

Hello Tamio,

1. Vcm_Rx is different for each receiver for the re-drivers as a design choice.
2. They are still able to receive the correct swing voltage. The reason we list the Vcm_Rx is that if there is anything between your re-driver and your next re-driver, for example, a mux, you must be sure the Vcm of the mux matches the receiver Vcm, or you must AC couple the output of the mux.
3. You can, but you do not need to place those there. If you are trying to correct some Vcm with those resistors, you do not need to add them in.

Best,
Gerasimos

0 Tamio Yokokura over 4 years ago in reply to Jerry Madalvanos

TI__Expert 3560 points

As shown in my first thread, when adding a resistor between Rxp / Rxn and GND, what is the recommended value?

I can 't find out the specification regarding this resistance in USB specification. Please tell me the section it is explained.

Thanks

Tamio

0 Jerry Madalvanos over 4 years ago in reply to Tamio Yokokura

TI__Mastermind 23011 points

Tamio,

You can add a 100K pull-down resistor. This isn't necessary, but can lower your common-mode voltage.

Best,
Gerasimos

0 Tamio Yokokura over 4 years ago in reply to Jerry Madalvanos

TI__Expert 3560 points

Gerasimos

If a 100kohm pull-down resistor is specified by USB specification, please teach me where I should look at it in the specification.

Thanks

Tamio

0 Jerry Madalvanos over 4 years ago in reply to Tamio Yokokura

TI__Mastermind 23011 points

Tamio,

It is not specified by USB specification. It is only to bias Vcm. This is not necessary, and you should not need to add these resistors there.

Best,
Gerasimos

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TUSB501: Why is Vcm-RX different for each re-driver?