SN75DPHY440SS: Connecting multiple SN75DPHY440SS based MIPI cables in series?

Thomas Lau54

We have created a DPHY MIPI CSI FPC extension cable based on the SN75DPHY440SSRHR design:

The track length between the Connector to the Camera and SN75DHY440 retimer is ~200 mm

The track length between the Connector to the Processor and SN75DHY440 retimer is ~35 mm (see below)

From Camera ------------200 mm --------------------- SN75DPHY440 Re-timer -------------------------- 35 mm------------------------------- To Processor

We have tested images with 1280x800 (resolution) with 120 fps frame rate with only 2-lane MIPI connected, with just a single MIPI Extension cable everything works.

But as we trying to connect 2x MIPI extension cable in series together, the processor started to lose sync with the frame and in some cases not able to pick up any frame at all.

As I do not have a decent scope to measure the MIPI bus, making debugging rather difficult, wondering if someone else have tried similar design and could shed some light?

Thanks

Thomas

over 7 years ago

0 Michael.Walker over 7 years ago

TI__Mastermind 19720 points

Hello,
Your issue sounds like a signal integrity issue. What is the DSI clock frequency? What EQ setting are you using? have you tuned EQ with both extension cables? What are the lengths of the 2 extension cables?

0 Thomas Lau54 over 7 years ago in reply to Michael.Walker

Prodigy 40 points

Hi Michael

What is the DSI clock frequency?

The MIPI clock frequency at 400MHz, (the camera system clock is set to 24MHz)

What EQ setting are you using? have you tuned EQ with both extension cables?

I have tried both 0db and 5db with different combination, but it doesnt seem to make much different.

What are the lengths of the 2 extension cables?

Each cable has total length of 235mm (see below)

From Camera ------------200 mm --------------------- SN75DPHY440 Re-timer -------------------------- 35 mm------------------------------- To Processor

Thanks in advance

Thomas

Michael.Walker said:
Hello,
Your issue sounds like a signal integrity issue. What is the DSI clock frequency? What EQ setting are you using? have you tuned EQ with both extension cables? What are the lengths of the 2 extension cables?

0 Michael.Walker over 7 years ago in reply to Thomas Lau54

TI__Mastermind 19720 points

Does the processor lose sync for all images such as just a plain white image or is it only for certain images? The inter-pair skew of the cables could be causing the problem you could test the inter-pair skew with a scope by viewing the skew between the data and the clock.

0 Thomas Lau54 over 7 years ago in reply to Michael.Walker

Prodigy 40 points

Hi Michael

Yes, the processor basically lost sync for all images and the driver just gave up after a few re-tires, I am also suspecting there might be some inter-pair skew issue, as in the current design, there is up to 0.3mm of different in track length between MIPI Clock and data lines at the extension cable.

However as we didtn have a proper scope to measure it, its hard to validate at this stage.

Cheers,

Thomas

0 Michael.Walker over 7 years ago in reply to Thomas Lau54

TI__Mastermind 19720 points

The inter-pair skew can definitely cause what you are seeing. can you fix the intra-pair skew as best you can?

0 Thomas Lau54 over 7 years ago in reply to Michael.Walker

Prodigy 40 points

Hi Michael

I am updating the PCB for a better track length, shall see how its perform once I got the board.

Cheers

Thomas

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SN75DPHY440SS: Connecting multiple SN75DPHY440SS based MIPI cables in series?