As a matter of fact, XMODEL is not to blame here. Synopsys Custom WaveView is displaying the waveforms in the "piecewise constant (PWC)" mode, resulting in the staircase waveforms you are seeing. You can get the smooth-looking waveforms simply by switching its display mode to the "piecewise linear (PWL)" mode. Here are the resulting waveforms:

To change the display mode, first select the waveforms by clicking on the signal names on the leftmost column and click the right mouse button. From the pop-up menu that appears, select the "Attributes…". When a dialog window titled "Panel Attributes" shows up, change the display preference from "PWC Waveform" to "PWL Waveform".

You can also make "PWL Waveform" the default display preference. Click the pull-down menu "Configuration→Preferences" and select the tab named "Readers" from the "Preference Settings" window shown below. Then uncheck the box with "Display Logic Real Type Signals in PWC By Default for FSDB File". When you restart the Custom WaveView session, the analog waveforms will be displayed in PWL format by default.

You may apply similar remedies when you encounter this problem with other third-party waveform viewers, such as Cadence ViVA. For Cadence ViVA, select the waveforms by clicking on their signal names located on the left-most column and click "Trace Properties" from the pop-up menu. And from the dialog window that appears, change the value of the "Type/Style" field to "line" as shown below.

For your information, XMODEL does not run time-step based simulations, and the time intervals between the points stored in FSDB or VCD format are not controlled by the simulation timestep. Instead, XMODEL performs true event-driven simulation using functional expressions, of which accuracy does not rely on finely-spaced value-change events. This is why XMODEL is so fast. The points you see with the third-party waveform viewers are created when the waveforms are stored into the FSDB or VCD format files, so that they can best describe the true waveforms when they are plotted as piecewise linear (PWL) waveforms. This is why you get the good looking waveforms back when you switch the display mode to the PWL mode.

As a matter of fact, XMODEL is not to blame here. Synopsys Custom WaveView is displaying the waveforms in the "piecewise constant (PWC)" mode, resulting in the staircase waveforms you are seeing. You can get the smooth-looking waveforms simply by switching its display mode to the "piecewise linear (PWL)" mode. Here are the resulting waveforms:

To change the display mode, first select the waveforms by clicking on the signal names on the leftmost column and click the right mouse button. From the pop-up menu that appears, select the "Attributes…". When a dialog window titled "Panel Attributes" shows up, change the display preference from "PWC Waveform" to "PWL Waveform".

You can also make "PWL Waveform" the default display preference. Click the pull-down menu "Configuration→Preferences" and select the tab named "Readers" from the "Preference Settings" window shown below. Then uncheck the box with "Display Logic Real Type Signals in PWC By Default for FSDB File". When you restart the Custom WaveView session, the analog waveforms will be displayed in PWL format by default.

You may apply similar remedies when you encounter this problem with other third-party waveform viewers, such as Cadence ViVA. For Cadence ViVA, select the waveforms by clicking on their signal names located on the left-most column and click "Trace Properties" from the pop-up menu. And from the dialog window that appears, change the value of the "Type/Style" field to "line" as shown below.

For your information, XMODEL does not run time-step based simulations, and the time intervals between the points stored in FSDB or VCD format are not controlled by the simulation timestep. Instead, XMODEL performs true event-driven simulation using functional expressions, of which accuracy does not rely on finely-spaced value-change events. This is why XMODEL is so fast. The points you see with the third-party waveform viewers are created when the waveforms are stored into the FSDB or VCD format files, so that they can best describe the true waveforms when they are plotted as piecewise linear (PWL) waveforms. This is why you get the good looking waveforms back when you switch the display mode to the PWL mode.