LPC2148 PWM Programming Tutorial

Introduction

This Tutorial is for learning Pulse Width Modulation (PWM) programming for ARM7 based LPC214x MCUs. For this Tutorial I’ll be covering Single Edge PWM and how to use to control Motors like Servos. Also for the scope of this tutorial I won’t be going into stuff like MCU controlled Voltage Regulation using PWM since I am planning a dedicated tutorial for that. I assume you are comfortable with Timers and its usage in LPC214x since PWM block is basically a Timer. I’ve posted a tutorial on Timers in LPC214x @ LPC2148 Timer Tutorial – just in case if you need it.

Pulse Width Modulation Basics :

I’ve Posted a Beginner PWM tutorial @ Basic PWM Tutorial. If you are new to PWM please have a look there.

Here is Diagram that sums it up. It shows Single Edge PWM with T-ON , T-OFF & Period. Duty Cycle is simply T-ON Divided by Period.

PWM Programming in ARM7 LPC2148

LPC2148 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 anywhere within the Period

A PWM block , like 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 Lpc214x Timer tutorial @ LPC2148 Timer Tutorial . Match Registers 1 to 6 (except 0) are pinned on LPC214x i.e. the corresponding outputs are diverted to actual Pins on LPC214x MCU. The PWM function must be selected for these Pins to get the PWM output. These pins are :

In LPC214x we have 7 match registers inside the PWM block . Generally the first Match register PWMMR0 is used to generate PWM period and hence we are left with 6 Match Registers PWMMR1 to PWMMR6 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. The way this works in case of Single Edge PWM is as follows (and similarly for Double Edge):

This can be shown in the diagram below:

For Double Edge Rules you can refer the User Manual Pg. 261.

Configuring and Initializing PWM:

Configuring PWM 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 output will be available. But first we need to do some basic Math Calculations for defining the Period time , the resolution using a prescaler value and then Pulse Widths. For this First we need to define the resolution of out PWM signal. Here the PWM resolution which I mean is the minimum increment that can use to increase or decrease the pulse width. More smaller the increment more fine will be the resolution. This resolution is defined using an appropriate Prescale Value. The calculation for Prescale is the same which I had shown in the Timer Tutorial .. but here it is once again :

PINSEL0 = ... ; // Select PWM Function for used Pins
PWMPCR = ... ; // Select PWM type - By default its Single Edged
PWMPR = ... ; //assign calculated PR value 
PWMMR0 = ... ; // Assign Period Duration
PWMMRx = ... ; // Assign pulse duration i.e widths for other Match Regs.. x=1 to 6
PWMMCR = (1<<1) | ... ; // Reset PWMTC on PWMMR0 match & Other conditions
PWMLER = (1<<1) | ... ; // update MR0 and other Match Registers
PWMPCR = ... ; // enable PWM outputs as required
PWMTCR = (1<<1) ; //Reset PWM TC & PR

//Now , the final moment - enable everything
PWMTCR = (1<<0) | (1<<3); // enable counters and PWM Mode
//Done!

Updating Match Registers i.e the Pulse Width

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

PWMMR1 = 50; // Update Pulse Width
PWMLER = (1<<1); // set the corresponding bit to 1

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

PWMMR1 = 50;
PWMMR2 = 68;
PWMMR3 = 20;

PWMLER = (1<<1) | (1<<2) | (1<<3); // Update Latch Enable bit for all MRs together

Some Real World Examples with sample code

I'll cover 2 examples using Single Edge PWM. These will be:
1) Controlling RC Servo
2) LED Dimming

To stay within the scope of this article I'm not including a bit involved stuff like "RC Servo Speed Control" and "DC Motor Speed Control". I guess I'll do it in a separate article.

Example #1) Simple RC Servo Control Using PWM :

Here we will use PWM1 output from Pin P0.0 to control a RC Servo Motor. We will also be using 4 tactile switches to bring the Servo at 4 different positions. P0.1 , P0.2 , P0.3 , P0.4 will be used to take the input. One end of the tactile switches will be connected to each of these Pins and other end will be connected to ground. Here we'll also require external 5V-6V source to drive the servo. Servos can work with 3.3v voltage levels for input PWM signal , hence we just need to connect the MCU PWM Output to Servo PWM Input wire. The setup is as shown in the schematic below :

When the user will press P0.1 the Pulse width will be 1ms , and similarly when the user presses P0.2 , P0.3 , P0.4 the Pulse Width will change to 1.25ms , 1.5ms and 1.75ms respectively and hence Servo will change its position correspondingly.

Source for Example 1:

/*
(C) Umang Gajera | Power_user_EX - www.ocfreaks.com 2011-13.
More Embedded tutorials @ www.ocfreaks.com/cat/embedded

LPC2148 PWM Tutorial Example 1 - RC Servo Control using PWM.
License : GPL.
*/
#include <lpc214x.h>

#define PLOCK 0x00000400
#define PWMPRESCALE 60   //60 PCLK cycles to increment TC by 1 i.e 1 Micro-second 

void initPWM(void);
void initClocks(void);

int main(void)
{
	initClocks(); //Initialize CPU and Peripheral Clocks @ 60Mhz
	initPWM(); //Initialize PWM

    //IO0DIR = 0x1; This is not needed!
    //Also by default all pins are configured as Inputs after MCU Reset.
   
    while(1)
    {
		if( !((IO0PIN) & (1<<1)) ) // Check P0.1
		{
			PWMMR1 = 1000;
			PWMLER = (1<<1); //Update Latch Enable bit for PWMMR1
		}
		else if( !((IO0PIN) & (1<<2)) ) // Check P0.2
		{
			PWMMR1 = 1250;
			PWMLER = (1<<1);
		}
		else if( !((IO0PIN) & (1<<3)) ) // Check P0.3
		{
			PWMMR1 = 1500;
			PWMLER = (1<<1);
		}
		else if( !((IO0PIN) & (1<<4)) ) // Check P0.4
		{
			PWMMR1 = 1750;
			PWMLER = (1<<1);
		}
    }
    //return 0; //normally this wont execute ever
}

void initPWM(void)
{
	/*Assuming that PLL0 has been setup with CCLK = 60Mhz and PCLK also = 60Mhz.*/
	/*This is a per the Setup & Init Sequence given in the tutorial*/

	PINSEL0 = (1<<1); // Select PWM1 output for Pin0.0
	PWMPCR = 0x0; //Select Single Edge PWM - by default its single Edged so this line can be removed
	PWMPR = PWMPRESCALE-1; // 1 micro-second resolution
	PWMMR0 = 20000; // 20ms = 20k us - period duration
	PWMMR1 = 1000; // 1ms - pulse duration i.e width
	PWMMCR = (1<<1); // Reset PWMTC on PWMMR0 match
	PWMLER = (1<<1) | (1<<0); // update MR0 and MR1
	PWMPCR = (1<<9); // enable PWM output
	PWMTCR = (1<<1) ; //Reset PWM TC & PR

	//Now , the final moment - enable everything
	PWMTCR = (1<<0) | (1<<3); // enable counters and PWM Mode

	//PWM Generation goes active now!!
	//Now you can get the PWM output at Pin P0.0!
}

Example #2) LED Dimming using PWM :

The basic Setup here is same as in Example 1 except that here we replace Servo by a LED and we dont need external 5V source along with slightly modified Program. Here we'll just need to connect the anode of LED to P0.0 and cathode to GND. In this example will use a period of 10ms. In this case the Pulse Widths Corresponding to the Switches connected to P0.1/2/3/4 will be 2.5ms/5ms/7.5ms/10ms respectively. As the Pulse Width Increases LED will keep on getting Brighter and the reverse will happen if Pulse Width is decreased.

Source for Example 2:

/*
(C) Umang Gajera | Power_user_EX - www.ocfreaks.com 2011-13.
More Embedded tutorials @ www.ocfreaks.com/cat/embedded

LPC2148 PWM Tutorial Example 2 - LED Dimming using PWM.
License : GPL.
*/
#include <lpc214x.h>

#define PLOCK 0x00000400
#define PWMPRESCALE 60   //60 PCLK cycles to increment TC by 1 i.e 1 Micro-second 

void initPWM(void);
void initClocks(void);

int main(void)
{
	initClocks(); //Initialize CPU and Peripheral Clocks @ 60Mhz
	initPWM(); //Initialize PWM

    //IO0DIR = 0x1; This is not needed!
    //Also by default all pins are configured as Inputs after MCU Reset.
   
    while(1)
    {
		if( !((IO0PIN) & (1<<1)) ) // Check P0.1
		{
			PWMMR1 = 2500; //T-ON=25% , Hence 25% Bright
			PWMLER = (1<<1); //Update Latch Enable bit for PWMMR1
		}
		else if( !((IO0PIN) & (1<<2)) ) // Check P0.2
		{
			PWMMR1 = 5000; //50% Bright
			PWMLER = (1<<1);
		}
		else if( !((IO0PIN) & (1<<3)) ) // Check P0.3
		{
			PWMMR1 = 7500; //75% Bright
			PWMLER = (1<<1);
		}
		else if( !((IO0PIN) & (1<<4)) ) // Check P0.4
		{
			PWMMR1 = 10000; //100% Bright
			PWMLER = (1<<1);
		}
    }
    //return 0; //normally this wont execute ever
}

void initPWM(void)
{
	/*Assuming that PLL0 has been setup with CCLK = 60Mhz and PCLK also = 60Mhz.*/
	/*This is a per the Setup & Init Sequence given in the tutorial*/

	PINSEL0 = (1<<1); // Select PWM1 output for Pin0.0
	PWMPCR = 0x0; //Select Single Edge PWM - by default its single Edged so this line can be removed
	PWMPR = PWMPRESCALE-1; // 1 micro-second resolution
	PWMMR0 = 10000; // 10ms period duration
	PWMMR1 = 2500; // 2.5ms - pulse duration i.e width (Brigtness level)
	PWMMCR = (1<<1); // Reset PWMTC on PWMMR0 match
	PWMLER = (1<<1) | (1<<0); // update MR0 and MR1
	PWMPCR = (1<<9); // enable PWM output
	PWMTCR = (1<<1) ; //Reset PWM TC & PR

	//Now , the final moment - enable everything
	PWMTCR = (1<<0) | (1<<3); // enable counters and PWM Mode

	//PWM Generation goes active now - LED must be 25% Bright after Reset!!
	//Now you can get the PWM output at Pin P0.0!
}

More stuff using PWM
I've had 2 requests for Sine Wave generation using PWM , which I'll be doing soon which you can find here : Sine Wave Generator using PWM with LPC2148 Microcontroller Tutorial. I might also cover MCU controlled voltage Regulation along with double egde PWM 🙂

Please do leave your Article Requests , Suggestions & Comments.