This blog post describes how to generate a PWM with variable duty cycle.the processor we used is NXP LPC 4088 .

Steps for PWM Generation using Cortex M4

1. Power: In the PCONP register, set bit PCPWM1
2. Configure pins P2.0 to P2.5 for PWM using IOCON_P2_0 to IOCON_P2_5.
3. Select the single edge mode of PWM
4. Select the clock source and clock value for PWM (peripheral clock).
5. Set match value for PWM match channel 0 (Frequency of PWM ie period)
6. Select and update the duty cycle for each channel (Match registers)
7. Enable PWM Channel Output
8. Reset the counter to zero

PWM Generation using Cortex m4- C source code

#include "LPC407x_8x_177x_8x.h"

#define PWMPRESCALE 30		//30 PCLK cycles to increment TC by 1 i.e 1 Micro-second 

void init_pwm(void);
void delay_ms(long ms) ;

int main(void)
{
		init_pwm();
		while(1);
}

void init_pwm(void)
{
	//1. set bit PCPWM1 
		LPC_SC->PCONP |= (1 << 6); 										/* enable power to PWM1*/
	
	//2. Configure Pins P2.0 to P2.5 for PWM using IOCON_P2_0 to P2_5
		LPC_IOCON->P2_0 =  1;        									/* Pin P2.0 to P2.6 used as PWM */
		LPC_IOCON->P2_1 =  1;  
		LPC_IOCON->P2_2 =  1;  
		LPC_IOCON->P2_3 =  1;        									
		LPC_IOCON->P2_4 =  1;  
		LPC_IOCON->P2_5 =  1;  

		LPC_PWM1->PCR  = 0x0; 									// Step-3. Select Single Edge PWM - by default its single Edged so this line can be removed
		LPC_PWM1->PR   = PWMPRESCALE-1; 				// Step-4. 1us resolution (30MHz/30)=1MHz
		LPC_PWM1->MR0  = 10000;      						// Step-5. 10ms=10000us period duration ie 100Hz
		
		//6.Select the duty cycle for each channel
		LPC_PWM1->MR1  = 9500; 									// 9.5ms - pulse duration i.e Duty=95%
		LPC_PWM1->MR2  = 8000; 									// 8.0ms - pulse duration
		LPC_PWM1->MR3  = 6000; 									// 6.0ms - pulse duration
		LPC_PWM1->MR4  = 3000; 									// 3.0ms - pulse duration
		LPC_PWM1->MR5  = 1500; 									// 1.5ms - pulse duration
		LPC_PWM1->MR6  = 500; 									// 0.5ms - pulse duration i.e Duty=5%
		LPC_PWM1->LER  = 0x7F; 									// Update Period and Duty cycle
		
		//7.Enable PWM Channel Output		
		LPC_PWM1->PCR  = (0x3F<<9); 						// enable PWM1 to PWM6 output
		LPC_PWM1->TCR  = (1<<0) | (1<<3); 			// enable counters and PWM Mode
		
		//8.Reset the Counter
		LPC_PWM1->TCR  = (1<<1) ; 							// Reset PWM TC & PR
}

void delay_ms(long ms) 					// delay 1 ms per count @ CCLK 120 MHz
{
		long i,j;
		for (i = 0; i < ms; i++ )
		for (j = 0; j < 26659; j++ );
}