MCPWM

dsPIC33A AK/AKV dsPIC33C selected PIC32 MK

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MCPWM (Motor Control PWM) offers enhanced features beyond standard PWM blocks, including programmable fault sources with digital comparator inputs, leading-edge blanking, dead-time compensation modes, and extensive trigger output capabilities for ADC synchronization.

Block Inputs/Outputs

Block Inputs

Block Outputs

Scaling

The block supports 4 input datatype representations:

⚠️ Important: The block performs no range checking on inputs. Users must clamp values before the block input (e.g., using a Saturation block) to prevent overflow, regardless of datatype.

Float Inputs (3 options)

Selecting a float datatype accepts either single or double precision floating-point values. PIC32MK has hardware FPU—float inputs are processed efficiently. dsPIC33C uses software emulation—consider uint raw inputs for time-critical loops.

• Float [0 1] (Unipolar): Normalized control where 0.0 represents 0% duty cycle and 1.0 represents 100% duty cycle. The block handles all scaling internally.
• Float [-1 1] (Bipolar): Normalized control where -1.0 and +1.0 represent the duty cycle extremes. Suitable for H-bridge and bidirectional control where the sign indicates direction.
• Float (s) (Physical): Specify duty cycle or period directly in seconds. The block converts to register values automatically.

Uint Raw Input

• Uint raw: Integer values transfer directly to peripheral registers with no runtime computation—maximum efficiency.

The block creates workspace variables for user scaling:

Fixed Period (period input disabled):

$$\text{DutyCycle}_\text{raw} = \text{DutyCycle}_\text{normalized} \times \text{PWMxmax}$$

DutyCycle_raw = DutyCycle_normalized × PWMxmax

Where DutyCycle_normalized is in the range [0, 1] relative to the fixed Max Period.

Variable Period (period input enabled):

When the period is dynamically controlled via block input, both period and duty cycle use the same scaling:

$$\text{Period}_\text{raw} = \text{Period}_\text{normalized} \times \text{PWMxmax}$$

Period_raw = Period_normalized × PWMxmax

$$\text{DutyCycle}_\text{raw} = \text{DutyCycle}_\text{normalized} \times \text{Period}_\text{raw}$$

DutyCycle_raw = DutyCycle_normalized × Period_raw

⚠️ Constraints:

• Period input must always be < PWMxmax (Max Period in GUI)
• Duty cycle normalizes to the actual period input, not PWMxmax
• If ADC triggers from PWM events and drives the scheduler, changing period alters the control loop sample time

Center-Aligned vs Edge-Aligned

Center-Aligned Mode: The PWMxmax workspace variable is halved at configuration time because the timer counts both up and down. This applies to all input types (Uint raw, Float).

No runtime division: Since PWMxmax is already halved, the block passes integer inputs directly to registers without any runtime shift operation. Users simply scale their values to the halved PWMxmax.

Edge-Aligned Mode: The timer counts up only. PWMxmax represents the full period count. Duty cycle updates take effect at the next period boundary.

Asymmetric Center-Aligned Modes

MCPWM supports two asymmetric center-aligned modes:

• Double Update: Duty cycle input is a 2-element vector [val_up val_down]. First value applied during up-count, second during down-count.
• Simultaneous Update: Both up and down values updated at the same time, but can be different.

These modes enable asymmetric PWM waveforms for harmonic reduction and THD optimization.

Dead Time

• GUI configuration: Specify in seconds (e.g., 1e-6 for 1 µs)

• Integer block input: Dead time uses a different scaling base (full period, not halved).

  ⚠️ In center-aligned mode, multiply PWMxmax by 2 for dead time scaling:

$$\text{DeadTime}_\text{raw} = \frac{\text{DeadTime}_s \times 2 \times \text{PWMxmax}}{\text{MaxPeriod}_s}$$

DeadTime_raw = (DeadTime_s × 2 × PWMxmax) / MaxPeriod_s

• Note: Dead time scaling is identical in center-aligned and edge-aligned modes—the factor of 2 compensates for the halved PWMxmax

Dead Time Compensation

When Positive dead time with compensation mode is enabled:

• The block uses comparator feedback to adjust dead time based on current direction
• Compensates for effective duty cycle changes caused by dead time insertion
• Requires proper comparator configuration for zero-crossing detection

Block Parameters

This block uses a multi-tab GUI dialog with many parameters. Multiple instances can be used within a model to configure PWM channels with different characteristics or update at different rates.

Additional Features

PWM-ADC Synchronization

MCPWM provides multiple trigger outputs (TRIG1..3, SEVTCMP) to precisely coordinate ADC conversions with PWM waveform timing.

Option 3: PWM → ADC → Task (Motor Control Standard)

Configuration Example (Center-Aligned PWM):

% In MCPWM block:
Period = 'Center aligned - Symmetric';
Max_Period = 1/20e3;  % 20 kHz PWM frequency

% Trigger ADC at PWM valley (center point - all low-side FETs ON):
Primary_Event_Trigger = 0;  % Valley trigger

% In ADC block:
Trigger_Source = 'PWM1 Special Event';
Sample_Time = 'Inherited';  % ADC rate follows PWM (20 kHz)

Option 4: Multiple Triggers per PWM Period (Advanced Control)

MCPWM supports up to 3 trigger outputs per PWM generator for high-performance control requiring multiple measurements per PWM cycle:

% Center-aligned PWM with 3 trigger points:
Period = 'Center aligned - Symmetric';

% Trigger 1: Before valley (rising edge phase currents)
Trigger_1 = Period * 0.4;

% Trigger 2: At valley (all low-side ON)
Primary_Event_Trigger = 0;

% Trigger 3: After valley (falling edge phase currents)
Trigger_2 = Period * 0.6;

Best Practices

Usage Examples

Example 1: 3-Phase Motor with Dead-Time Compensation

% Configuration:
Period = 'Center aligned - Symmetric';
Dead_Time_Control = 'Positive dead time with compensation mode';

% Dead-time settings:
Dead_Time_High = 1.2e-6;  % 1.2 µs high-side
Dead_Time_Low = 1.0e-6;   % 1.0 µs low-side

% Compensation comparator (current sensing):
% Dead-time automatically adjusted based on current direction

Example 2: Overcurrent Protection with LEB

% Fast overcurrent shutdown with noise immunity:
Fault_Mode = 'Cycle-by-cycle';       % Auto-restart when fault clears
Fault_State = 'All Low';             % Safe state

% Leading-Edge Blanking:
Leading_Edge_Blanking = 'on';
LEB_Duration = 500e-9;               % 500 ns blanking
LEB_Trigger = 'Rising edge PWMxH';   % Blank after high-side turn-on

Troubleshooting

Fault Lockout

Cause: Latched fault mode requires manual clear.

Solution: Send pulse to Fault_Clear input, or change to cycle-by-cycle mode for auto-restart.

False Fault Triggers

Solutions:

• Enable Leading-Edge Blanking with appropriate duration (typically 200-1000 ns)
• Adjust LEB trigger source to match noise characteristic
• Filter fault comparator inputs in hardware (RC filter)

Dead-Time Compensation Not Working

Checklist:

• ✅ Dead Time Control set to “Positive dead time with compensation mode”
• ✅ Comparator configured and connected to current sensor
• ✅ Comparator threshold properly set for zero-crossing detection
• ✅ Comparator polarity matches current direction

Supported Device Families

Related Blocks

• MCHP_PWM_HS_FEP - High-resolution PWM for dsPIC33A
• MCHP_PWM_HighSpeed - High-speed PWM for dsPIC33C/CH/CK
• MCHP_PWM - Standard PWM for dsPIC30F/33F/33E
• MCHP_ADC - ADC with PWM trigger synchronization

See Also

Datasheets

• PIC32MK Family Reference Manual
• dsPIC33C Family Reference Manual
• dsPIC33A Family Reference Manual