How do I control probe start/stop in ncsim from embedded CPU?

The simplest way would be to use Tcl breakpoints to execute the probe commands when certain signal values are observed. For example, at the start of simulation, create the probe for the signals you want to debug, and give the probe a name, then immediately disable it (unless you want to probe at time 0):

probe -create -shm -name myprobe <list_of_signals>
probe -disable myprobe

Then, create breakpoints to turn the probe on and off:

stop -create -execute {probe -enable myprobe} -continue -condition {[value top.sig_to_start_probing] = 1'b1}
stop -create -execute {probe -disable myprobe} -continue -condition {[value top.sig_to_start_probing] = 1'b0}

There are many other types of breakpoints so you'll need to use your imagination to decide which is the most appropriate for your scenario.

I would also suggest thinking about whether it's better to check the encryption result using some zero-time method in the testbench instead of having a simulated CPU to crunch the data, your tests would finish much quicker and be much easier to debug!

Thank you. 

Hello Stephen,

I have provided the following sequence of commands in my tcl

probe -create INST_CHANNEL -depth all -shm -database waves -name dumps
probe -disable dumps
stop -create -execute {probe -enable dumps} -continue -condition {[value tb_top.START_DUMP] = 1'b1}
stop -create -execute {probe -disable dumps} -continue -condition {[value tb_top.START_DUMP] = 1'b0}

When the above tcl script is not provided ( and without any dump or with a full dump ) the simulation completes in a few ms.

When the above tcl script is provided the sim halts at "Created stop 2" and does not proceed.

The default state of START_DUMP is 0 and toggles 1 to 0 and again 1 to 0 inbetween the sims.

I would want the Scope INST_CHANNEL to be dumped only when START_DUMP is 1

Would the above seq of commands really work .. 

Thanks Stephen,

I got it working .. I missed the "run"