Simulink chalenge: Generate "staircase"-looking waveform and calculate its FFT

Hello,

It's 2nd day I'm searching for a solution how to proceed with FFT on "staircase" waveform. The only approach I've found until now on constructing such waveform is using of "Sample and Hold" block from "DSP System Toolbox"\"Signal Operations". But the problem here is the waveform at the output of "Sample and Hold" block is continuous, and it isn't acceptable by "Buffer" block which is necessary to generate frame for FFT.

Here is staircase waveform (together with original sinusoidal waveform)

And here is my model (staircase was generated before adding "Buffer1" block). Simulation fails and following error message is generated: Error in 'Continuous_Discrete/Buffer1': All sample times must be discrete. No continuous or constant sample times are allowed.