LPC1768 PWM Programming Tutorial

This discussion will guide you through ARM Cortex-M3 LPC1768 PWM programming Tutorial. It is also applicable for LPC1769. I’ll be covering Single Edge PWM along with examples like how to use to control Motors like Servos and dimming an LED. I assume you are comfortable with Timers and its usage in ARM Cortex-M3 based LPC176x micro-controllers since PWM block is basically a Timer.

Pulse Width Modulation Basics:

I’ve have posted a Beginner PWM tutorial. If you are new to PWM please have a look there. A Diagram that sums it up is given below. It shows Single Edge PWM with T-ON, T-OFF & Period. Duty Cycle is simply T-ON Divided by Period.

ARM Cortex-M3 LPC1768 PWM Module

These MCUs Contains a single PWM Module called PWM1 and supports 2 types of PWM:
1) Single Edge PWM – Pulse starts with new Period i.e Pulse is always at the beginning
2) Double Edge PWM – Pulse can be present anywhere within the Period

A PWM block, similar to a Timer block, has a Timer Counter and an associated Prescale Register along with Match Registers. These work exactly the same way as in the case of Timers. I’ve explained them in the introduction part of the LPC1768 Timer tutorial . Match Registers 1 to 6 (except 0) are pinned on LPC176x i.e. the corresponding outputs are given to actual Pins on LPC176x MCU. The PWM function must be selected for these Pins, using PINSELx Register, to get the PWM output. These pins are:

There are 7 match registers inside the PWM1 block. The first Match register PWM1MR0 is used to generate PWM period and hence we are left with 6 Match Registers PWM1MR1 to PWM1MR6 to generate 6 Single Edge PWM signals or 3 Double Edge PWM signals. Double edge PWM uses 2 match registers hence we can get only 3 double edge outputs.

Consider that our PWM Period duration is 5 milliseconds and TC increments every 1 millisecond using appropriate prescale value. We set PWM1MR0 to 5 i.e. 5 ticks of PWM TC. We have PWM1MR1 = 2 and PWM1MR2 = 4. Whenever the value in TC matches the value in any of the match register, its corresponding output it set to low until the start of next PWM cycle as shown in PWM Timing Diagram below:

For Double Edge Rules you can refer the User Manual Pg. 524.

Registers used in LPC176x PWM Programming

Now lets have a look at some important Registers:

Configuring and Initializing PWM Block

Now, lets see how to use PWM block. Configuring PWM module is very much similar to Configuring Timer except, additionally, we need to enable the outputs and select PWM functions for the corresponding PIN on which PWM output will be available. But first we need to do some basic Math Calculations for defining the Period time, the resolution using a prescale value and then Pulse Widths. For this First we need to define the resolution of our PWM signal. PWM resolution is the minimum time delay that can used to increase or decrease the pulse width. More smaller the increment more fine will be the resolution. PWM Resolution is set using an appropriate Prescale Value. The calculation for PWM Prescaler is the same which I had shown in the Timer Tutorial, here it is once again:

Example :

//Select PWM Function Using appropriate PINSEL register
LPC_PWM1->PCR = ... ; //Select PWM type - By default its Single Edged
LPC_PWM1->PR = ... ; //assign calculated PR value 
LPC_PWM1->MR0 = ... ; //Assign Period Duration
LPC_PWM1->MRx = ... ; //Assign pulse duration i.e widths for other Match Regs.. x=1 to 6
LPC_PWM1->MCR = (1<<1) | ... ; //Reset PWM1TC on PWMMR0 match & Other conditions
LPC_PWM1->LER = (1<<1) | ... ; //update MR0 and other Match Registers
LPC_PWM1->PCR = ... ; //enable PWM outputs as required
LPC_PWM1->TCR = (1<<1) ; //Reset PWM TC & PR

//Now , the final moment - enable PWM TC
LPC_PWM1->PWM1TCR = (1<<0) | (1<<3); //enable counters and PWM Mode
//Done!

Updating Match Registers i.e. the Pulse Width

Once PWM is initialized, you can update any of the Match Registers at anytime using PWMLER. This can be done as follows:

LPC_PWM1->MR1 = 50; //Update Pulse Width
LPC_PWM1->LER = (1<<1); //set the corresponding bit to 1

When you want to update Multiple Match Registers then this can be done as follows:

LPC_PWM1->MR1 = 50;
LPC_PWM1->MR2 = 68;
LPC_PWM1->MR3 = 20;

LPC_PWM1->LER = (1<<1) | (1<<2) | (1<<3); //Update Load Enable bit for all MRs together

Some Real World PWM Examples with sample code

Below I have given LPC1768 PWM Examples. For LCP1769 just change the PWM Prescale from 24 to 29 and Timer Prescale from 24999 to 29999.

LPC1768 PWM Example 1: Servo Control

In this LPC1768 PWM Example we will sweep a servo from 1000us to 2000us pulse widths, back and forth in steps. The period is set to 20ms(20000us) which is common for servos. PWM1.1 output is selected and will be available on Pin P1.18. In the C++ code given below, I have also used Timer0 which is used by delayMS(..) function to generate delay before we update PWM1MR1 register with new value. Warning: Double check all connections when connecting the motor or it will damage your board.

Source Code for Example 1:

/*(C) Umang Gajera - www.ocfreaks.com
More Embedded tutorials @ www.ocfreaks.com/cat/embedded
LPC1768 PWM Tutorial Example 1 - Simple RC Servo Control - KEIL uV5 ARM.
License: GPL */

#include <LPC17xx.h>
#define PWMPRESCALE (25-1) //25 PCLK cycles to increment TC by 1 i.e. 1 Micro-second

void initPWM(void);
void updatePulseWidth(unsigned int pulseWidth);
void delayMS(unsigned int milliseconds);
void initTimer0(void);

int main(void)
{
	//SystemInit(); //gets called by Startup code before main()
	initTimer0(); //Initialize Timer
	initPWM(); //Initialize PWM
	
	int pulseWidths[] = {1000,1250,1500,1750,2000};
	const int numPulseWidths = 5;
	int count=1;
	int dir=0; //direction, 0 = Increasing, 1 = Decreasing 

	while(1)
	{
		updatePulseWidth(pulseWidths[count]); //Update Servo Pulse Width
		delayMS(1000); //Wait for servo to reach the new position

		if(count == (numPulseWidths-1) || count == 0)
		{
			dir = !dir; //Toggle direction if we have reached count limit
		}
		
		if(dir) count--;
		else count++;
	}
  //return 0; //normally this won't execute ever
}

void initPWM(void)
{
	/*Assuming that PLL0 has been setup with CCLK = 100Mhz and PCLK = 25Mhz.*/

	LPC_PINCON->PINSEL3 |= (1<<5); //Select PWM1.1 output for Pin1.18
	LPC_PWM1->PCR = 0x0; //Select Single Edge PWM - by default its single Edged so this line can be removed
	LPC_PWM1->PR = PWMPRESCALE; //1 micro-second resolution
	LPC_PWM1->MR0 = 20000; //20000us = 20ms period duration
	LPC_PWM1->MR1 = 1250; //1ms - default pulse duration i.e. width
	LPC_PWM1->MCR = (1<<1); //Reset PWM TC on PWM1MR0 match
	LPC_PWM1->LER = (1<<1) | (1<<0); //update values in MR0 and MR1
	LPC_PWM1->PCR = (1<<9); //enable PWM output
	LPC_PWM1->TCR = (1<<1); //Reset PWM TC & PR

	LPC_PWM1->TCR = (1<<0) | (1<<3); //enable counters and PWM Mode

	//PWM Generation goes active now!
	//Now you can get the PWM output on Pin P1.18
}

void updatePulseWidth(unsigned int pulseWidth)
{
	LPC_PWM1->MR1 = pulseWidth; //Update MR1 with new value
	LPC_PWM1->LER = (1<<1); //Load the MR1 new value at start of next cycle
}


void delayMS(unsigned int milliseconds) //Using Timer0
{
	LPC_TIM0->TCR = 0x02; //Reset Timer
	LPC_TIM0->TCR = 0x01; //Enable timer

	while(LPC_TIM0->TC < milliseconds); //wait until timer counter reaches the desired delay

	LPC_TIM0->TCR = 0x00; //Disable timer
}

void initTimer0(void) //To setup Timer0 used delayMS() function
{
	/*Assuming that PLL0 has been setup with CCLK = 100Mhz and PCLK = 25Mhz.*/

	LPC_TIM0->CTCR = 0x0;
	LPC_TIM0->PR = 25000-1; //Increment TC at every 24999+1 clock cycles
	//25000 clock cycles @25Mhz = 1 mS

	LPC_TIM0->TCR = 0x02; //Reset Timer
}

LPC1768 PWM Example 2: LED Dimming

This example is similar to Example 1, except here we reduce the PWM Period to 1ms and Duty-cycle will vary from 0%(LED OFF) to 100%(Brightest). Connect the LED anode to P1.18 and cathode to GND using a suitable resistor (like >330 Ohms).

Source code for Example 2

/*(C) Umang Gajera - www.ocfreaks.com
More Embedded tutorials @ www.ocfreaks.com/cat/embedded
LPC1768 PWM Tutorial Example 2 - LED Dimming using PWM - KEIL uV5 ARM.
License: GPL.
*/

#include <LPC17xx.h>
#define PWMPRESCALE (25-1) //25 PCLK cycles to increment TC by 1 i.e. 1 Micro-second

void initPWM(void);
void updatePulseWidth(unsigned int pulseWidth);
void delayMS(unsigned int milliseconds);
void initTimer0(void);

int main(void)
{
	//SystemInit(); //gets called by Startup code before main()
	initTimer0(); //Initialize Timer
	initPWM(); //Initialize PWM
	
	int pulseWidths[] = {0,250,500,750,1000}; //Pulse Widths for varying LED Brightness
	const int numPulseWidths = 5;
	int count=1;
	int dir=0; //direction, 0 = Increasing, 1 = Decreasing 

	while(1)
	{
		updatePulseWidth(pulseWidths[count]); //Update LED Pulse Width
		delayMS(1000); 

		if(count == (numPulseWidths-1) || count == 0)
		{
			dir = !dir; //Toggle direction if we have reached count limit
		}
		
		if(dir) count--;
		else count++;
	}
  //return 0; //normally this won't execute ever
}

void initPWM(void)
{
	/*Assuming that PLL0 has been setup with CCLK = 100Mhz and PCLK = 25Mhz.*/

	LPC_PINCON->PINSEL3 |= (1<<5); //Select PWM1.1 output for Pin1.18
	LPC_PWM1->PCR = 0x0; //Select Single Edge PWM - by default its single Edged so this line can be removed
	LPC_PWM1->PR = PWMPRESCALE; //1 micro-second resolution
	LPC_PWM1->MR0 = 1000; //1000us = 1ms period duration
	LPC_PWM1->MR1 = 250; //250us - default pulse duration i.e. width
	LPC_PWM1->MCR = (1<<1); //Reset PWM TC on PWM1MR0 match
	LPC_PWM1->LER = (1<<1) | (1<<0); //update values in MR0 and MR1
	LPC_PWM1->PCR = (1<<9); //enable PWM output
	LPC_PWM1->TCR = (1<<1); //Reset PWM TC & PR

	LPC_PWM1->TCR = (1<<0) | (1<<3); //enable counters and PWM Mode

	//PWM Generation goes active now!
	//Now you can get the PWM output on Pin P1.18
}

void updatePulseWidth(unsigned int pulseWidth)
{
	LPC_PWM1->MR1 = pulseWidth; //Update MR1 with new value
	LPC_PWM1->LER = (1<<1); //Load the MR1 new value at start of next cycle
}


void delayMS(unsigned int milliseconds) //Using Timer0
{
	LPC_TIM0->TCR = 0x02; //Reset Timer
	LPC_TIM0->TCR = 0x01; //Enable timer

	while(LPC_TIM0->TC < milliseconds); //wait until timer counter reaches the desired delay

	LPC_TIM0->TCR = 0x00; //Disable timer
}

void initTimer0(void) //To setup Timer0 used delayMS() function
{
	/*Assuming that PLL0 has been setup with CCLK = 100Mhz and PCLK = 25Mhz.*/

	LPC_TIM0->CTCR = 0x0;
	LPC_TIM0->PR = 25000-1; //Increment TC at every 24999+1 clock cycles
	//25000 clock cycles @25Mhz = 1 mS

	LPC_TIM0->TCR = 0x02; //Reset Timer
}