DS90UB960-Q1 Length matching issue

[newbie_hs]

Dear Team,

We are using DS90UB960-Q1 n our design. The layout guideline(Page no 178) says ". Minimize intra-pair and inter-pair length mismatch within a single CSI-2 TX Port (recommended <= 5 mils)."

Our layout team is not able to achieve this. The best they can achieve is 250mils.

May I know is this fine.

Do we able to run the system with this.

How do I calculate the worst mismatch allowed

 

[FvM]

Don't know how they arrive at 5 mils mismatch. MIPI spec specificies maximal delay skew between clock and data of 1/50 UI, which is 12.5 ps at 1.6 GBPS. Corresponds to about 75 mils with usual FR-4 propagation speed. 250 mils is definitely beyond limits.

 

[newbie_hs]

This is very informative.
I think TI have some confusion in this. Below is the guidance they provided in the reference design.
Here it is 10mils and in datasheet it is 5mils.

--- Updated Dec 12, 2023 ---

I tried to back calculate how you arrived 75mils.Please let me know it is fine or not.

FR4 propagation speed = 6inch/ns
Speed = Distance/time
Distance = Speed*time
=6inch/ns * 12.5ps
=0.075inch
=75mil

 

[D.A.(Tony)Stewart]

Show your layout and PCB design rules, laminate materials. You must meet the specs.

You will be disappointed with std FR4.
I'd recommend Getek ML200C Er= 3.8 Tpd =5.42 ps/mm

 

[FvM]

FR4 propagation speed = 6inch/ns
Speed = Distance/time
Distance = Speed*time
=6inch/ns * 12.5ps
=0.075inch
=75mil

Yes.
The 1/50 UI skew limit has to be shared by the total transmission path, TX, RX PCB and cable. 75 mil describes the case where RX and TX are on the same PCB.

 

[newbie_hs]

My RX and TX are on same PCB

For RX UFL connectors are used.

Cameras will be connected to this RX connector. SERDES cameras are used.
These cameras are sitting in the back of our electric bike

 

[marce]

I would worry about not achieving the correct skew, during layout... If it is not achieved then the layout is incorrect and needs changing.