DS90UB941AS-Q1: Relationship PCLK and DSI_CLK

Hi Josef,

Josef Däubler said:

I have a FPD3 serializer/deserializer system with an effectiv pixel clock of 148.5 MHz (2200x1125@60Hz).
In the ub941 data sheet there is this equation:
f_pclk = f_dsi * 12 / n_lanes
For a system with 4 lanes this results in f_dsi = 445.5 MHz and a line rate of 891 Mbps.

You are correct, this would be the correct DSI clock frequency and DSI rate.

Josef Däubler said:

Must the DSI host's PHY run with exactly this rate?
Is it possible to use a higher line rate, e.g. 950 Mbps, when the host adapts sending of sync packets (VSS, VSE, HSS, HSE) to achieve the original timing?

It is typically recommended for the host to run at the corresponding DSI rate, this way the 941AS can run in DSI Reference Clock mode and use the provided DSI clock. Is there a reason you do not want to use this method?

It is possible to run at a higher rate, but there are additional considerations that need to be taken with additional programming of the device. I would need to check with some colleagues on the specifics of what is needed.

Josef Däubler said:

What must be programmed into ub941? I've seen registers DSI_PCLK_DIV_M and DSI_PCLK_DIV_N - are they used to adjust this relationship?

Here is an example script for initializing the 941AS. This uses DSI0, and is for DSI Reference Clock mode:

## Pixel Clock = 148.5 MHz (eg. for standard 1080p)

## DSI clock = PCLK * 12/(# of DSI lanes) 

## DSI clock = 445.5 MHz

## DSI input port 0
## 4 lanes DSI

 
import time

serAddr = 0x18
desAddr0 = 0x58

#RESET 941
board.WriteI2C(serAddr,0x01,0x02)

#Disable DSI
board.WriteI2C(serAddr,0x01,0x08)

board.WriteI2C(serAddr,0x1E,0x01) # Select DSI0
board.WriteI2C(serAddr,0x4F,0x8C) # Set single DSI0 mode - continuous clock mode

board.WriteI2C(serAddr,0x40,0x04) # TSKIP_CNT DSI0
board.WriteI2C(serAddr,0x41,0x05) # TSKIP_CNT DSI0
board.WriteI2C(serAddr,0x42,0x30) # TSKIP_CNT DSI0, TSKIP_CNT (dec)= 24


board.WriteI2C(serAddr,0x40,0x04) # Set HSYNC/VSYNC Polarity DSI0
board.WriteI2C(serAddr,0x41,0x21) # Set HSYNC/VSYNC Polarity DSI0
board.WriteI2C(serAddr,0x42,0x60) # Set HSYNC/VSYNC Polarity DSI0


board.WriteI2C(serAddr,0x40,0x10) # Init DSI Clock Settings (From Section 10.2 of datasheet)
board.WriteI2C(serAddr,0x41,0x86) # Init DSI Clock Settings (From Section 10.2 of datasheet)
board.WriteI2C(serAddr,0x42,0x0A) # Init DSI Clock Settings (From Section 10.2 of datasheet)
board.WriteI2C(serAddr,0x41,0x94) # Init DSI Clock Settings (From Section 10.2 of datasheet)
board.WriteI2C(serAddr,0x42,0x0A) # Init DSI Clock Settings (From Section 10.2 of datasheet)

#Enable DSI
board.WriteI2C(serAddr,0x01,0x00)

Best,

Cameron

Hi Cameron,

thanks for your reply.

The reason for this is the need of flexibility. Our product is a controller box that provides the possibility of connecting a display via FPD Link. One customer has the above mentioned panel, but others may need different. In our box, video timing on the MIPI DSI interface is software programmable, but the line rate is fixed.

So a guide on how to program this into the ds90ub941 would be very helpfull.

Best regards,

Josef

Hi Cameron,

thanks again for your answer.

I'm still wondering if it is also possible to use DSI Reference clock mode also for burst mode?

And it's still not clear to me how PCLK is regenerated from DSI clock. Is there a PLL inside the chip? And what are the above mentioned registers  DSI_PCLK_DIV_M and DSI_PCLK_DIV_N used for? Is there any further description for them available, as there is nothing neither in the data sheet nor the DSI bringup guide.

If only burst mode with external refclk is possible: how to setup this?
In the DSI bringup guide in chapter 2.2 is said:
"Although it is possible to apply different clock rates between REFCLK and DSI clock in this mode, the implications for such a configuration are outside the scope of this document"
Can you may give support for this?

Best,
Josef

Hi Josef,

I need to discuss this with some colleagues, I will get back to you on Monday with an update.

Best,

Cameron