PWM / Timer Calculator

PWM (Pulse Width Modulation) is used to control power delivery by switching a digital signal on and off rapidly.

Formulas:

• Edge-Aligned Mode: f = Clock / ((PSC + 1) * (ARR + 1))
• Center-Aligned Mode: f = Clock / ((PSC + 1) * 2 * (ARR + 1))
• Duty Cycle %: (CCR / (ARR + 1)) * 100

Usage: Enter your MCU's timer clock frequency, select the alignment mode, and set the Prescaler and Period Count to calculate the resulting PWM frequency and resolution.

When you need it: Setting an LED dimming or motor-drive frequency, generating a servo pulse, or synthesising an analog voltage by low-pass filtering the output. On an MCU the real task is picking a timer prescaler (PSC) and auto-reload (ARR) that hit your target frequency, then a compare value (CCR) for the duty.

Worked example: A 72 MHz timer clock targeting 20 kHz PWM needs ARR + 1 = 72e6 / 20000 = 3600, so PSC = 0, ARR = 3599. For 25% duty, CCR = 0.25 × 3600 = 900. The available duty resolution is timer_clk / f = 3600 steps (~11.8 bits).

Tips & gotchas:

• Resolution and frequency trade off: at a fixed timer clock, doubling the PWM frequency halves the number of duty steps.
• Higher frequency reduces audible whine and output ripple but raises switching losses in the driver/MOSFET.
• Use center-aligned mode for motor drive (symmetric edges, lower harmonics) and insert dead-time on complementary half-bridge outputs.
• To make an analog voltage, size the RC filter so fc ≪ f_PWM; ripple falls ~20 dB/decade below the PWM frequency.