DS90UB933-Q1: The relationship between back channel and forward channel

Hi David,

Thank you for your patience. 

Huang David said:

For 933, my understanding is that if  there is no PCLK input, and 934 side can not be locked. What about the back channel now?  Can the back channel work without the PCLK input?  And Why?

The back channel will still be able to send control data (I2C and GPIO) from the deserializer to the serializer when there is no PCLK present. This is because the back channel and the forward channel operate independently from each other. The back channel uses the internal oscillator from the deserializer to drive communication, so even without a lock on the forward channel, control data can still be sent from the deserializer to the serializer using the back channel. 

Huang David said:

And is there a lock status register or pin to check the back channel lock status?

There is no feature that individually reports successful back channel communication. Without a lock on the forward channel, there will be no forward channel communication. When there is no forward channel communication, there is no way to send control data bidirectionally between the serializer and the deserializer. Control data can only be transmitted over the back channel, from the deserializer to the serializer. To confirm control data communication between the serializer and the deserializer, there would need to be a way to transfer data bidirectionally over the link. Without a forward channel lock, this is not possible.

Regards,

Kenneth

Hi David,

Huang David said:

Could you help confirm is there any typo or there is indeed a internal oscillator in deserializer?

Yes, there is an internal oscillator in the referenced deserializer, the DS90UB934-Q1. This internal oscillator is mentioned in section 5.4.6.4.2 in the datasheet:

Huang David said:

What's is the purpose of the internal oscillator and what is the use of internal oscillator?

As shown by the screenshot above, the purpose of the internal oscillator is to generate the back-channel reference clock. 

Regards,

Kenneth

Hi David,

The internal oscillators of both devices have similar functions, but are configured in different ways.

Both internal oscillators are used to initially establish the FPD-III link before switching to the clock source mode that the device is configured for. In the case of the 933, the device will use the internal oscillator as the PCLK signal if nothing is connected to the PCLK pin. In the case of the 953, the device will use the internal oscillator (the internal always-on (AON) clock) as the clock signal if the device is configured for the non-synchronous internal clock mode. 

Regards,

Kenneth

Hi David,

Huang David said:

So for 933, if the external PCLK is applied to the device, that means the internal oscillator will shutdown and the device shift to external PCLK as the clock sources to work?

For the DS90UB933-Q1, if an external PCLK signal is applied to the device, the device will switch to using this signal as the forward channel clock source instead of using the internal oscillator as the forward channel clock source. The internal oscillator does not shut down when the device is not using it as the forward channel clock source. The internal oscillator remains powered when it is not being used as the forward channel clock source, because it is used to send I2C and GPIO data. 

Huang David said:

For the 953, if the device is configured as sync mode of non-sync external clock mode, the AON will not work? right?

If not work, why called always-on?

If the DS90UB953-Q1 is operating in synchronous clock mode, the AON internal oscillator will not be used for forward channel communications, but it will still be operational. As previously mentioned, this internal oscillator does not get powered down when it is not being used as the forward channel clock source, because it is still used for other functions, such as sending I2C and GPIO data. 

Regards,

Kenneth