🛠 Graduer une LED avec le PWM d'un timer

Objectives

Use the PWM output (MDP in French) of one of the GD32F103's 16-bit timers to gradually vary the brightness of an LED. The GD32F103C8 features up to 10 general-purpose 16-bit timers, 2 advanced timers, and 2 basic timers; the community Arduino core maps analogWrite to the timer channels. You will understand the principle of duty cycle and the clock constraint of APB1 (54 MHz max) / APB2 (108 MHz max).

Hardware

• GD32F103C8T6 / CBT6 Blue Pill compatible board
• External LED + 330 ohm resistor between the output pin and ground (do not rely on the onboard PC13 LED, which is not suitable for visible PWM)
• Timer output pin: PA6 (TIM3_CH1 timer channel on this package) >
• 3.3 V power supply

Complete Code

// pwm_led.ino — GD32F103C8, core CommunityGD32Cores
const uint8_t PWM_PIN = PA6;   // sortie timer (cf. tableau AF du core) > 

void setup {
  pinMode(PWM_PIN, OUTPUT);
  Serial.begin(115200);
}

void loop {
  for (int p = 0; p <= 255; p += 4) {   // rampe montante
    analogWrite(PWM_PIN, p);
    Serial.println(p);
    delay(15);
  }
  for (int p = 255; p >= 0; p -= 4) {   // rampe descendante
    analogWrite(PWM_PIN, p);
    delay(15);
  }
}

Expected Verification

The external LED smoothly ramps up and down in brightness (breathing effect), without abrupt steps. With a logic analyzer or oscilloscope, you see a square wave of constant frequency whose duty cycle follows the 0-255 ramp. The onboard PC13 LED does not blink: it is not on this channel.

Common Errors

• analogWrite has no effect: the chosen pin is not mapped to a timer channel in the core; cross-reference pins_arduino.h from the CommunityGD32Cores with the AF table in the datasheet. >
• LED barely visible or burnt out: missing resistor (pin overvoltage) or too high resistance (LED off); 330 Ω to 1 kΩ to ground is a good range at 3.3 V.
• Confusion between 0-255 / 0-4095: analogWrite expects 0-255 even if the GD32 core's ADC is 12-bit; this is not the same scale as analogRead.
• PWM frequency too low/high: the core's default frequency may be audible or cause flickering; if your core exposes it, adjust the channel frequency, keeping in mind the clock chain (APB1 ≤ 54 MHz, APB2 ≤ 108 MHz).
• 108 MHz misconfigured: the GD32 resets to IRC8M just as the STM32 resets to HSI; switching to 108 MHz requires a correct PLL from the 8 MHz HXTAL crystal, and crystals on some clones are marginal at this frequency. >
• Forgetting HSE_STARTUP_TIMEOUT: for a project ported from ST code, AN001 requires 0xFFFF to avoid startup blocking (the PWM will never start).