PWM HighSpeed

dsPIC33C CH/CK/CDV dsPIC33E GS dsPIC33F selected

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This block is designed for demanding motor control and power conversion applications requiring precise timing, independent channel control, and high switching frequencies.

Block Inputs/Outputs

Block Inputs

Block Outputs

Scaling

This block uses uint16 raw values for all inputs (period, duty cycle, phase, trigger). User scaling is required.

⚠️ 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.

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 (unless Immediate Update is enabled).

Double Update Mode

When Double Update is enabled (requires Center-Aligned Mode + Independent Period):

• Duty cycle input becomes a 2-element vector: [val_up val_down]
• First value applied during up-count, second during down-count
• Enables asymmetric PWM waveforms for harmonic reduction

Workspace variables for double update:

• PWMxmax_H — Maximum value for high-side (up-count)
• PWMxmax_L — Maximum value for low-side (down-count)

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

Block Parameters

This block uses a custom 5-tab GUI dialog. All parameters shown below are visible to users.

Output Modes

Additional Features

PWM-ADC Synchronization

Synchronizing ADC sampling with PWM switching is essential for accurate current measurement in motor control and power conversion.

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

Configuration using Initial ADC Trigger:

% In PWM HighSpeed GUI, Block Input and Initialisation tab:
Initial_ADC_Trigger = 0;  % Trigger at valley (center-aligned)
% Or for edge-aligned:
Initial_ADC_Trigger = Max_Period / 2;  % Trigger at mid-period

% In ADC block:
Trigger_Source = 'PWM Trigger 1';
Sample_Time = 'Inherited';

Benefits:

• Synchronized current sensing with PWM switching
• Sample when low-side FET is ON (minimal switching noise)
• Control loop rate automatically matches PWM frequency

Best Practices

Usage Examples

Example 1: 3-Phase Motor Control (Complementary Mode)

% Main tab:
PWM_HL_mode = 'Complementary';
Independent_Period = false;      % Master time base
Independent_Duty_Cycle = true;   % Per-channel control
Center_Aligned_Mode = true;
Immediate_Update = false;        % Synchronized updates
Double_Update = false;

% Max Period for 20 kHz PWM:
Max_Period = 1/20000;  % 50 µs

% Dead Time tab:
Dead_Time = 1e-6;      % 1 µs
Alt_Dead_Time = 1e-6;  % 1 µs

Example 2: Asymmetric Center-Aligned (Double Update)

% Main tab:
Independent_Period = true;       % Required for Double Update
Center_Aligned_Mode = true;      % Required for Double Update
Double_Update = true;            % Enable asymmetric PWM

% Duty cycle input becomes [val1 val2] for up/down count
% Use case: Harmonic reduction, EMI optimization

Troubleshooting

Double Update Mode Requirements

Double Update requires:

• Independent Period = enabled
• Center Aligned Mode = enabled

If conditions not met, double update is automatically disabled.

Immediate vs. Synchronized Update

• Immediate Update = off (default): Updates at period boundary. Eliminates pulse width glitches. Recommended for motor control.
• Immediate Update = on: Updates immediately. Use for fast transient response but may cause glitches.

Supported Device Families

Related Blocks

• MCHP_PWM - Standard PWM for dsPIC30F/33F/33E
• MCHP_PWM_HighSpeed_Override - Override control for this block
• MCHP_PWM_HS_FEP - PWM with Fine Edge Positioning (dsPIC33A)
• MCHP_MCPWM - Motor Control PWM (advanced fault handling)
• MCHP_ADC - ADC with PWM trigger synchronization

See Also

Datasheets

• dsPIC33F Family Reference Manual
• dsPIC33E/EP Family Reference Manual
• dsPIC33C Family Reference Manual