LPC1768 PWM: Registers, Frequency, Duty Cycle & Code

What is PWM?

PWM stands for Pulse Width Modulation.

A PWM signal is a digital signal that repeatedly switches between HIGH and LOW states. Instead of continuously changing the voltage level, PWM controls the amount of time the signal remains HIGH during each cycle.

Two important parameters define a PWM signal:

• Frequency
• Duty Cycle

These parameters determine how the PWM output behaves in an embedded system.

What is PWM Duty Cycle?

The PWM duty cycle represents the percentage of one complete PWM period during which the signal remains HIGH.

The formula is:

For example, if a PWM signal remains HIGH for 25% of its total period, Duty Cycle = 25%.

Common duty-cycle values include:

• 25% → Shorter ON time
• 50% → Equal ON and OFF time
• 75% → Longer ON time
• 100% → Continuously HIGH

In LED applications, increasing the duty cycle generally increases the average power delivered to the LED and therefore its apparent brightness.

In motor-control applications, changing the PWM duty cycle can control the average voltage or power delivered to the motor, depending on the motor driver and overall circuit design.

Understanding LPC1768 PWM duty cycle is therefore important when configuring PWM output for practical embedded applications.

What is PWM Frequency?

PWM frequency represents the number of complete PWM cycles generated per second. It is measured in Hertz (Hz).

For example:

• 1 Hz = 1 cycle per second
• 1 kHz = 1,000 cycles per second
• 10 kHz = 10,000 cycles per second

The relationship between frequency and period is:

The required PWM frequency depends on the application. LED dimming, motor control, servo control, and other applications can require different frequency ranges.

LPC1768 PWM Peripheral

The LPC1768 includes dedicated hardware for PWM signal generation. This allows the microcontroller to generate PWM outputs without requiring the CPU to manually toggle a GPIO pin for every PWM pulse.

This hardware-based approach provides more accurate and consistent timing.

The LPC1768 PWM peripheral includes important components such as:

• PWM Timer Counter
• Prescale Counter
• Prescale Register
• Match Registers
• PWM Match Control Register
• PWM Control Register
• Timer Control Register
• Latch Enable Register

The PWM peripheral can generate multiple PWM outputs depending on the selected PWM channel and the corresponding pin configuration.

LPC1768 PWM Registers

Understanding the LPC1768 PWM registers is essential for register-level PWM programming.

The following registers are commonly involved when configuring PWM.

1. PWMPR – Prescale Register

The Prescale Register (PR) determines how the peripheral clock is divided before the PWM Timer Counter increments.

For example:

LPC_PWM1->PR = 23;

Assume the PWM peripheral clock is PCLK = 24 MHz. The PWM counter frequency becomes:

Therefore, one PWM counter increment represents approximately 1 µs.

This makes timing calculations easier when configuring the PWM period.

2. PWMTC – Timer Counter

The Timer Counter (TC) keeps track of the current PWM timer count.

For example: TC = 0, TC = 1, TC = 2, TC = 3, and so on.

If the PWM counter frequency is 1 MHz, each timer-counter increment represents approximately 1 microsecond.

The Timer Counter works together with the match registers to determine when PWM events occur.

3. PWMPC – Prescale Counter

The Prescale Counter (PC) counts peripheral clock cycles according to the value programmed into the Prescale Register.

When the required prescale period is completed, the Timer Counter is incremented.

The basic timing relationship can be represented as:

This relationship is important when understanding LPC1768 PWM frequency calculation.

4. PWMMR0 – Match Register 0

Match Register 0 (MR0) is one of the most important registers in PWM programming because it establishes the PWM period in the selected PWM mode.

For example:

LPC_PWM1->MR0 = 10000;

If one PWM counter increment equals 1 µs:

So, under these clock and prescaler assumptions, an MR0 value of 10,000 produces a 100 Hz PWM period.

5. PWM Match Registers

The LPC1768 PWM peripheral provides multiple match registers.

Depending on the PWM channel being used, registers such as MR1, MR2, MR3, MR4, MR5, and MR6 can be used to determine individual PWM match positions.

For example:

LPC_PWM1->MR1 = 2500;

If MR0 = 10000 and MR1 = 2500, then the corresponding duty cycle can be calculated as:

This relationship is central to LPC1768 PWM duty-cycle calculation.

6. PWM1MCR – Match Control Register

The PWM Match Control Register (MCR) determines what happens when the PWM timer reaches a configured match value.

Depending on the selected bits, a match event can:

• Generate an interrupt
• Reset the timer
• Stop the timer

For PWM period generation, the timer can be configured to reset when the Timer Counter reaches MR0.

For example:

LPC_PWM1->MCR = (1 << 1);

This configures the corresponding MR0 match action for timer reset.

7. PWM1PCR – PWM Control Register

The PWM Control Register (PCR) is used to enable PWM outputs and configure the supported PWM operating mode.

For example:

LPC_PWM1->PCR = (1 << 9);

The appropriate PCR bit depends on the PWM channel being used.

The PCR therefore plays an important role in enabling the required LPC1768 PWM output.

8. PWM1TCR – Timer Control Register

The Timer Control Register (TCR) controls the PWM timer.

Important functions include:

• Enabling the PWM timer/counter
• Resetting the PWM timer
• Enabling PWM operation

For example:

LPC_PWM1->TCR = (1 << 0) | (1 << 3);

This enables the timer and PWM operation after the required configuration has been completed.

9. PWM1LER – Latch Enable Register

The Latch Enable Register (LER) is used to transfer updated match-register values into the PWM logic at the appropriate period boundary.

For example:

LPC_PWM1->LER = (1 << 0) | (1 << 1);

This enables the updated MR0 and MR1 values to be latched.

The LER is particularly useful when changing PWM parameters during runtime because it helps apply updated match values synchronously with the PWM cycle.

LPC1768 PWM Frequency Calculation

Let’s consider a practical LPC1768 PWM frequency calculation example.

Assume PCLK = 24 MHz and PR = 23.

The PWM counter frequency is:

Therefore, 1 counter increment = 1 µs.

Suppose MR0 = 10000. Then:

So, under these clock assumptions, MR0 = 10000 gives a PWM Frequency of 100 Hz.

This demonstrates how the prescaler and MR0 value determine the PWM period.

LPC1768 PWM Duty Cycle Calculation

Suppose MR0 = 10000 and MR1 = 5000. The duty cycle is:

Therefore, the PWM output has a 50% duty cycle.

For a 25% duty cycle, MR1 = 2500 and MR0 = 10000:

Similarly, for a 75% duty cycle, MR1 = 7500 and MR0 = 10000, giving a resulting duty cycle of 75%.

Therefore, changing the match value while keeping MR0 constant allows the PWM duty cycle to be adjusted.

Basic LPC1768 PWM Program

The following is a simplified LPC1768 PWM C program demonstrating register-level configuration:

#include 

void PWM1_Init(void)
{
    /* Power to PWM1 */
    LPC_SC->PCONP |= (1 << 6); /* PWM peripheral clock configuration */ LPC_SC->PCLKSEL0 &= ~(3 << 12); LPC_SC->PCLKSEL0 |=  (1 << 12); /* Prescaler */ LPC_PWM1->PR = 23;

    /* PWM period */
    LPC_PWM1->MR0 = 10000;

    /* PWM duty cycle */
    LPC_PWM1->MR1 = 5000;

    /* Reset PWM counter on MR0 */
    LPC_PWM1->MCR = (1 << 1); /* Enable PWM1 output */ LPC_PWM1->PCR = (1 << 9); /* Latch MR0 and MR1 */ LPC_PWM1->LER = (1 << 0) | (1 << 1); /* Enable counter and PWM */ LPC_PWM1->TCR = (1 << 0) | (1 << 3);
}

int main(void)
{
    PWM1_Init();

    while(1)
    {
    }
}

This example configures the PWM peripheral for a period and duty cycle based on the assumed clock configuration.

However, the exact output frequency depends on the actual LPC1768 peripheral clock (PCLK) and system clock configuration used by the application.

LPC1768 PWM Pin Configuration

Register configuration alone is not sufficient to obtain a physical PWM signal from the LPC1768.

The selected microcontroller pin must also be configured for the appropriate PWM alternate function using the corresponding pin function registers.

For example, if a particular PWM channel is mapped to a specific LPC1768 pin, that pin must be configured for the PWM function rather than normal GPIO operation.

Therefore, a complete LPC1768 PWM programming workflow generally involves:

1. Enabling power to the PWM peripheral
2. Configuring the PWM peripheral clock
3. Configuring the PWM output pin
4. Setting the prescaler
5. Setting MR0 for the PWM period
6. Setting the appropriate match register for duty cycle
7. Configuring the Match Control Register
8. Enabling the required PWM output
9. Latching the match values
10. Starting the PWM timer

LPC1768 PWM Applications

The LPC1768 PWM peripheral can be used in several embedded-system applications.

LED Brightness Control

PWM can rapidly switch an LED ON and OFF. Changing the duty cycle changes the average power delivered to the LED, which allows brightness control.

DC Motor Speed Control

PWM can be used with an appropriate motor driver to control the average voltage or power supplied to a DC motor.

Servo Control

PWM-based timing signals can be used for servo-control applications when the required pulse timing and frequency are configured according to the servo’s specifications.

Power Control

PWM is also widely used in power electronics and embedded control systems to regulate the average power delivered to a load.

Why Use Hardware PWM in LPC1768?

One of the major advantages of LPC1768 PWM is that PWM generation is handled by dedicated hardware.

With software-based GPIO toggling, the CPU has to continuously manage the timing of HIGH and LOW transitions. This can consume processor time and introduce timing variations.

Hardware PWM allows the peripheral to manage the waveform after the required registers have been configured.

LPC1768 PWM: Frequency and Duty Cycle Relationship

Two values are particularly important when configuring PWM:

• MR0 → PWM period/frequency
• Match register → PWM duty cycle

For example:

This makes MR0 and the selected PWM match register the key parameters for controlling the PWM waveform.

Conclusion

The basic relationship is:

Once these concepts are understood, developers can calculate the required LPC1768 PWM frequency and duty cycle and configure the peripheral using Embedded C.

For practical embedded development, always verify the actual system clock, peripheral clock (PCLK), PWM channel, and pin alternate-function configuration before calculating the final PWM frequency or connecting the output to external hardware.