mcpwm: can't apply the same delay module to multiple generators

This is a hardware limitation, one delay module can only be used by one generator at one time.

Closes https://github.com/espressif/esp-idf/issues/11327

docs/en/api-reference/peripherals/mcpwm.rst

@@ -407,13 +407,29 @@ Dead Time
 
 In power electronics, the rectifier and inverter are commonly used. This requires the use of rectifier bridge and inverter bridge. Each bridge arm has two power electronic devices, such as MOSFET, IGBT, etc. The two MOSFETs on the same arm can't conduct at the same time, otherwise there will be a short circuit. The fact is that, although the PWM wave shows it is turning off the switch, but the MOSFET still needs a small time window to make that happen. This requires an extra delay to be added to the existing PWM wave that generated by setting `Generator Actions on Events <#generator-actions-on-events>`__.
 
-The dead-time driver works like a *decorator*, which is also reflected in the function parameters of :cpp:func:`mcpwm_generator_set_dead_time`, where it takes the primary generator handle (``in_generator``), and returns a generator (``out_generator``) after applying the dead-time. Please note, if the ``out_generator`` and ``in_generator`` are the same, it means we're adding the time delay to the PWM waveform in a "in-place" fashion. In turn, if the ``out_generator`` and ``in_generator`` are different, it means we're deriving a new PWM waveform from the existing ``in_generator``.
+The dead time driver works like a *decorator*. This is also reflected in the function parameters of :cpp:func:`mcpwm_generator_set_dead_time`, where it takes the primary generator handle (``in_generator``), and returns a new generator (``out_generator``) after applying the dead time. Please note, if the ``out_generator`` and ``in_generator`` are the same, it means we are adding the time delay to the PWM waveform in an "in-place" fashion. In turn, if the ``out_generator`` and ``in_generator`` are different, it means we're deriving a new PWM waveform from the existing ``in_generator``.
 
 Dead-time specific configuration is listed in the :cpp:type:`mcpwm_dead_time_config_t` structure:
 
 - :cpp:member:`mcpwm_dead_time_config_t::posedge_delay_ticks` and :cpp:member:`mcpwm_dead_time_config_t::negedge_delay_ticks` set the number of ticks to delay the PWM waveform on the rising and falling edge. Specifically, setting both of them to zero means to bypass the dead-time module. The resolution of the dead-time tick is the same to the timer that is connected with the operator by :cpp:func:`mcpwm_operator_connect_timer`.
 - :cpp:member:`mcpwm_dead_time_config_t::invert_output`: Whether to invert the signal after applying the dead-time, which can be used to control the delay edge polarity.
 
+.. warning::
+
+    Due to the hardware limitation, one delay module (either `posedge delay` or `negedge delay`) can't be applied to multiple MCPWM generators at the same time. e.g. the following configuration is **invalid**:
+
+    .. code:: c
+
+        mcpwm_dead_time_config_t dt_config = {
+            .posedge_delay_ticks = 10,
+        };
+        // Set posedge delay to generator A
+        mcpwm_generator_set_dead_time(mcpwm_gen_a, mcpwm_gen_a, &dt_config);
+        // NOTE: This is invalid, you can't apply the posedge delay to another generator
+        mcpwm_generator_set_dead_time(mcpwm_gen_b, mcpwm_gen_b, &dt_config);
+
+    However, you can apply `posedge delay` to generator A and `negedge delay` to generator B. You can also set both `posedge delay` and `negedge delay` for generator A, while letting generator B bypass the dead time module.
+
 .. note::
 
     It is also possible to generate the required dead time by setting `Generator Actions on Events <#generator-actions-on-events>`__, especially by controlling edge placement using different comparators. However, if the more classical edge delay-based dead time with polarity control is required, then the dead-time submodule should be used.