DS90UB948-Q1: The screen flickers black screen from time to time

Hello Amy,

can you provide schematics how are you connecting GPIO2 on the SER and on the DES?

Also please provide which configurations are you applying for those GPIOs?

Additionally, when or why is LOCK getting unstable? Any special condition?

Hi Hamzeh,

Thanks for your response. 

Below is the information provided by the customer.

Now we can ignore the jitter of GPIO2, because we found that the flash screen is accompanied by the unlock and relock process by observing the lock pin of 948.

The problem now is that 948 will occasionally unlock.

By replacing different hosts, harnesses and screens, it is found that the 948 unlock cannot be eliminated. However, when we change the video frame rate output by the host from 60Hz to 59Hz, the picture is stable and 948 unlock phenomenon is hardly observed.

Could you lease help us analyze the possible causes.

Best,

Amy

Hello Amy,

thanks for the new details.

Can you please get more details about the signal being transferred? Which resolution/frame rate/ blanking, and what is the pixel clock?

Are you using single FPD-Link lane or dual lane?

What is the paired SER?

Did you test using the pattern generator option from the SER?

Hi Hamzeh,

Below is the information provided by the customer.

resolution :1920x1080

frame rate :60Hz

blanking: hsw=44，hbp=148，hfp=88；vsw=5，vbp=36，vfp=4；

hblank=44+148+88=280；vblank=5+36+4=45；

htotal=1920+280=2200；vtotal=1080+45=1125；

pclk：2200*1125*60=148.5MHz

FPD-LINK：Dual link mode

paired SER: DS90UB941

Serializer PG (pattern generator) test: When using the internal clock (0x56 register is set to 0x02) and PG's self-generated timing (0x65 register is set to 0x04), there is no problem, and the test pattern display is stable.

If there is any missing information, please do not hesitate to ask.

Best Regards,
Amy

Hello Amy,

thanks for the details.

From the above test, it looks like the issue is caused by the video source signal, either CLK is sometimes not stable or jittery, or the timing is not stable.

For more accurate diagnostics, can you please test the PG using 3 different settings as per this AppNote:

1) Using Internal CLK and Internal timing ===> 0x65= 0x04 ------ This one you have tested already, so no need to repeat!

2) Using External CLK and External timing ===> 0x65= 0x00

3) Using External CLK and Internal timing ===> 0x65= 0x0C

Hi Hamzeh,

Below are the results of this customer's testing:

When 0x56=0x02 (internal reference clock mode):

0x65=00 (external timing): black screen without display.

0x65=04 (internal timing, internal frequency division clock): stable without flashing.

0x65=0C (internal timing, external pixel clock): black screen without display =========> We did not attach a reference clock to the 941.

When 0x56=0x00 (DSI reference clock mode):

0x65=00 (external timing): there is a display but it will flash.

0x65=04 (internal timing, internal frequency division clock): stable without flashing.

0x65=0C (internal timing, external pixel clock): there is a display but it will flash.

From the above:

1) When 0x65=0x04, regardless of how 0x56 is configured, it is stable and does not flash.

2) When 0x56=00 (Reference Clock Select DSI), the screen will splash regardless of 0x65=00 or 0C. Should all indicate that it is a problem with the DSI clock?

Hello Amy,

yes, you are correct. These results indicate an issue with the DSI clock. Please make sure the DSI is stable and fulfills our jitter specs.