TUSB1142: gain clarification when LFPS

Hi Alder,

The TUSB1142 supports both linear and limited VOD modes:

1. Linear VOD mode will take whatever LFPS voltage is coming in the receiver (SSTX) and outputs around that same voltage on the transmitter (CTX1/2). Linear VOD does not change the output swing of your signal. It only changes the linearity range of our re-driver. This is the default mode for the 1142.

2. Limited mode can boost the signal peak-peak voltage. This is called limited VOD and is only available on SSRX. This can boost LFPS signals received from the Type-C connector when enabled.

Alder Lim said:

It is a bit unclear on the Linearity VOD, datasheet only mention for SSRX, how about SSTX/CTX1/2 ?

Only SSRX supports the limited mode VOD setting. This is set through the USB_SSRX2_VOD register in I2C mode:

Linear VOD is supported on all transmitters (SSRX, CTX1/2). However, this will not change the output swing on your signal. I drew a diagram to show how this looks:

Alder Lim said:

When in LFPS, was SSTX VOD has the gain of 7dB that i set on my board? or it is linear?

SSTX only supports linear VOD, so the signal is linear. The 7dB of gain is only for superspeed signals and will not affect your LFPS signal.

Alder Lim said:

Does pin strapping mode has USB_CTX12_VOD/LFPS_VOD, will it default to 0h = LINR_L3 (highest)  = 1050mVpp ?

That is correct, but keep in mind that 1050mVpp is not the output swing. That is the input swing that our device will accept without compressing the signal waveform.

Alder Lim said:

The reason i asked this is i measure LFPS Peak-Peak differential output voltage is only 673.6mV. Spec 800mV ~ 1200mV

LFPS voltage swing is largely determined by the LFPS host. Please increase the LFPS peak-peak swing on your USB host to 1V and it will pass.

Best,

Shane