thanks to your help I can now successfully keep the Domino wave running at a stable frequency and maintain the channel cascading information in the Write Shift Register. (Since you told me WSR always reads and writes at the same time, I think I need to rewrite the information back every time after reading out from WSR to decide from which channel my data come, don't I?)
Yes you do. But if you have WSRLOOP=1 in the config register, this is done automatically. So the SR output is visible at the pin and will be fed back into the input.
However I'm still having difficulty in reading out from the DRS cells. I use the ROI readout mode and assume as long as I give a pulse on RSRLOAD the data will come out one by one.
That's not correct. Have a look at Figure 14 of the datasheet. Do you see a single RSRLOAD pulse or many? There is only one RSRLOAD pulse to initialize the readout shift register, then the cells are clocked by SRCLK pulses.
Also I read in the datasheet that WSROUT will give RSR output when DWRITE is low. Sometimes I see some random bits from this output and sometimes I see all zero's. What is the reasonable output I should see from WSROUT, say, when I'm running in the transparent mode with DWRITE low?
A single RSRLOAD pulse loads the RSR with a "1" at the domino stop position and "0" in all other places. A pulse on SRCLK shifts this "1" down the RSR. When it arrives at cell #1023, it will be visible for one clock cycle at WSROUT. The "double" functionality of WSROUT has the following background: Assume you use channel cascading 2x2048. Now the domino wave stopps in cell 1020 of the first channel for example. You have to read cells 1020,1021,1022,1024 of the first channel, then you continue with 0,1,2 on the second channel. But how do you know that you have to switch channels after the first four clock cycles? The SROUT output encodes the stop position (in this case 1020), but it needs 10 clock cycles before the information is available, so you don't have it after four cycles. That's where WSROUT comes into play: Since it outputs RSR bit by bit, it will show three "0", then a "1", when you are at cell 1023. Then you know that you have to switch channels immediately. That's why I output RSR via WSROUT if DWRITE is low.