PWM

dsPIC30F dsPIC33F dsPIC33E EP/EV/FJ

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The PWM block supports up to 4 complementary PWM channel pairs with independent duty cycle control, dead-time insertion, fault protection, and special event triggers for ADC synchronization.

Block Inputs/Outputs

Block Inputs

Block Outputs

Scaling

This block uses uint16 raw values for all inputs (period, duty cycle, 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 a workspace variable for user scaling:

Note: PTPER is computed from the Period parameter. In center-aligned mode, the block internally halves the period before calculating PTPER, resulting in a halved PWMxmax.

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 Period.

Variable Period (period input enabled):

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

⚠️ Important: The Period input is labeled Period *1/2 because users must provide half the desired period value. The block internally multiplies this by 2 to set the actual timer period.

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

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 (which already accounts for the ×2 factor)
• 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 (CAM = on): The timer counts up then down, effectively doubling the period. The PWMxmax workspace variable is halved at configuration time to account for this. 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 (CAM = off): 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).

Dead Time

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

• Dead time is configured separately for Prescaler A and optionally Prescaler B (dsPIC33E only)

• Each PWM channel can independently select which prescaler to use for active and inactive transitions

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

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

DeadTime_raw = (DeadTime_s × 2 × PWMxmax) / Period_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 6-tab GUI dialog. Parameters shown below are visible to users based on chip capabilities.

Additional Features

PWM-ADC Synchronization

The standard PWM block provides a Special Event Trigger (PxSECMP / SEVTCMP) to synchronize ADC sampling with PWM generation. This is essential for accurate current and voltage sensing in motor control and power conversion applications.

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

Use Case: Standard approach for motor control and power conversion - recommended for most applications.

Configuration Example (Center-Aligned PWM):

% In PWM block:
InitialPeriod = 1/20e3;  % 20 kHz PWM frequency
CAM = 'on';              % Center-aligned mode

% Trigger ADC at PWM valley (center point - low-side FETs ON):
PxSECMP = InitialPeriod / 2;  % Valley = 50% of period

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

Key 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

Duplicate Port Block

When multiple PWM modules are available, you can use “Duplicate Ports for PWM x” to create additional I/O ports for an existing PWM configuration without duplicating the hardware setup.

Usage Examples

Example 1: Basic 3-Phase Inverter Control

% Model setup:
% - 3 PWM channels enabled (1, 2, 3)
% - 20 kHz switching frequency
% - Dead-time: 1 µs
% - Center-aligned mode for reduced EMI

InitialPeriod = 1/20000;  % 50 µs (20 kHz)
CAM = 'on';               % Center-aligned
DeadTimePrescaleA = 1e-6; % 1 µs dead-time

% Block inputs:
% DutyCycle 1..3: Connect to FOC algorithm outputs (0 to PWM1max)

Example 2: Fault Protection

% Configure overcurrent protection:
% Fault A behaviour = 'Use Re-Enable block input after Fault'
% Fault A: PWM1 H/L Pins State = 'H/L ==> 00'  % All outputs low
% Fault A: PWM2 H/L Pins State = 'H/L ==> 00'
% Fault A: PWM3 H/L Pins State = 'H/L ==> 00'

% Connect:
% - Comparator output -> Fault A pin
% - Fault clear logic -> 'Fault A Re-Enable' input
% - 'Flt A' output -> LED or diagnostic logger

Troubleshooting

Period Out of Range Warning

Cause: Requested period exceeds maximum achievable with available prescalers.

Solution: Reduce the period or use center-aligned mode for 2× range extension.

Dead-time Limitation

Cause: Requested dead-time exceeds hardware limits.

Solution: Reduce dead-time value or check FCY (instruction cycle frequency).

No PWM Available Error

Error: “Chip XXXXX does not have this type of PWM peripheral”

Solution: Use appropriate PWM block variant:

• MCHP_PWM_HighSpeed: for dsPIC33C/CH/CK
• MCHP_PWM_HS_FEP: for dsPIC33A
• MCHP_MCPWM: for Motor Control PWM (PIC32/dsPIC33C advanced)

Supported Device Families

Related Blocks

• MCHP_PWM_HighSpeed - High-speed PWM for dsPIC33C/CH/CK
• MCHP_PWM_HighSpeed_Override - Override control for MCHP_PWM_HighSpeed
• MCHP_PWM_HS_FEP - PWM with Fine Edge Positioning (dsPIC33A)
• MCHP_MCPWM - Motor Control PWM (advanced features)
• MCHP_ADC - ADC block (use with PWM special event trigger)

See Also

Datasheets

• dsPIC30F Family Reference Manual
• dsPIC33F Family Reference Manual
• dsPIC33E Family Reference Manual