DS90UB941AS-Q1: External clock questions

Hi Link,

Link Huang said:

When is an external clock source needed?

An external clock is used as a reference for the FPD-Link serial stream rates in the absence of DSI clocks (high speed channel is 35 bits * PCLK).

For example, if an external clock is input at 33 MHz, the forward serial stream rate is 1.155 Gbps per lane.

Link Huang said:

• Can this device work without any external oscillator or crystal?

The internal clock inputs are used for built-in self-test and Pattern Generation also meant for testing purposes. These are not meant for production, since they have a very large tolerance.

For end-to-end setups regarding the FPD-Link, it is recommended to provide a clock input from the source (external reference clock input) that meets the timing and specifications of the intended resolution/framerate.

An external clock input pin can input any oscillator which meets automotive quality standards and the jitter requirements of the PCLK for the 941AS device. [7.7 Recommended Timing for External Clock Reference from DS90UB941AS-Q1 Datasheet]

Best,

Miguel

Hello, Miguel,

Sorry for late reply because my TI registration has some problem.

Do you mean, even though 941AS could get DSI clock from uP by DSI bus interface, an external oscillator is still needed to pursue a good timing quality?

Thanks.

Hi Miguel,

Thank you for your reply，But please forgive me, I still don’t understand.

• Do you mean that if there is no DSI_CLK input, an external oscillator is required for the device to work properly?
• May I use the internal oscillator to make the device work without the DSI_CLK input?
• How to determine the conditions for using internal and external oscillators？

BR，

Link

Hi Link,

No problem, I may try to clarify this further;

Link Huang said:

Do you mean that if there is no DSI_CLK input, an external oscillator is required for the device to work properly?

No, DSI_CLK will always be there whether the external oscillator is connected or not. The external oscillator is only used when it is in independent mode or splitter mode.

Link Huang said:

May I use the internal oscillator to make the device work without the DSI_CLK input?

Yes, you may use internal oscillator with/without the external DSI_CLK.

Link Huang said:

How to determine the conditions for using internal and external oscillators？

It depends on the application that they intend to use. First, are they using independent or splitter mode? If it is neither, then they do not need the external clock for the REFCLK0 and REFCLK1. They can simply use the external DSI_CLK. 

Best,

Miguel

Hello, Miguel,

In our application, 941 has 2 FPD LINK outputs, which are connected to 2 displays. They have the same resolution, frame rate and color depth, but the displays' content are different. Can it be called "independent or splitter mode"?

Thanks.

Hi Link,

yufeng.zhang said:

In our application, 941 has 2 FPD LINK outputs, which are connected to 2 displays.

For clarification - does your 941 have one input stream to DSI0, or are there two input streams to DSI0 and DSI1?

If you have different content on both displays, and not coming from the same image, it is independent mode (2 inputs, 2 displays).

If you need more clarification, please see 8.4.4.1 Left/Right 3D Format Support and 8.4.5 Independent 2:2 Mode of DS90UB941AS-Q1 DSI to FPD-Link III Bridge Serializer datasheet (Rev. C) (ti.com).

Best,

Miguel

Hello, Miguel,

As specified in 8.4.4.1 and 8.4.5, the work mode that needs the external REFCLK is only as showed in "Figure 8-18. Independent 2:2 Mode With External Reference Clocks" , even in that use case, it also could be replaced with "Independent 2:2 Mode With DSI Reference Clock" as showed in Figure 8-17. So, I have not found any use case which must apply external REFCLK. Is that the fact?

Thanks.

Hi Link,

yufeng.zhang said:

2 FPD LINK outputs, which are connected to 2 displays. They have the same resolution, frame rate and color depth, but the displays' content are different.

yufeng.zhang said:

So, I have not found any use case which must apply external REFCLK. Is that the fact?

Correct, as long as the two DSI inputs have their own DSI0_CLK and DSI0_CLK coming into the 941, there is no need for the external REFCLK.

Please confirm if the source data has its own DSI0_CLK P/N and DSI1_CLK P/N connection coming to the 941.

Best,

Miguel

Hello, Miguel,

In my application: 2 FPD LINK outputs, which are connected to 2 displays. They have the same resolution, frame rate and color depth, but the displays' content are different.

Even though just one DSI input was used, can I use the solution like "Figure 8-15. Splitting at Serializer Option"? As showed in the figure, it also does not need external REFCLK.

Thanks.

Hi Link,

yufeng.zhang said:

Even though just one DSI input was used

Do you mean to say, there is one DSI input to the 941?

yufeng.zhang said:

can I use the solution like "Figure 8-15. Splitting at Serializer Option"? As showed in the figure, it also does not need external REFCLK.

Correct, if the incoming data on the DSI input also includes "DSI_CLK", then it does not require the external REFCLK option, considering that both displays have the same video timings from the input source. Please pay close attention to the configuration described in the paragraph above the figure that describes programming the images and the default values.

Best,

Miguel