04 PWM example

Something about PWM:

We all know that microcontrollers, well processors do everything with ones and zeros. Which means that when we would like to make simple sine wave we couldn’t. Because if microcontroller works with 3.3V and 0V as logical 1 and 0, it can not produce for example 1V or 2.2V or any other value different than 0V and 3.3V. But if it’s possible to generate square wave signal than it’s possible to generate any value between minimum and maximum value that that microcontroller produce. Now I will explain how’s that possible… How does PWM works:

Just for better explanation let’s solve simple task:

Using PWM wih reference voltages of (min) 0V and (max) 3.3V generate 2V voltage.

1. If we have reference voltages of 0V and 3.3V then we have to calculate coefficient that somehow connects values 0, 3.3 and 2. Since the biggest value our chip can give is 3.3V you can assume that signal period is consisted from bigger part of ‘high’ output and smaller part of ‘low’ output.

2. Calculation:

3.3V  -> 2V
3.3V * x = 2V
x = 0.6061

Practically, this means that if period is 1ms then 60.61% (named as duty cycle) of that time signal should be driven ‘high’, and rest ‘low’.

3. Now programming preamble…


I decided to use PWM3 output signal. That microcontroller pin is multiplexed with GPIO PB1 output. From picture is seen that PWM3 is generated by PWM Genrator 1 (just like PWM2 output signal).

Most important part of the code:

/* Enables PWM and GPIO peripheral.
*  By default PWM isn't enabled.
*  It's because that PWM block uses clock for its work,
*  since one of our biggest tasks is to make low power
*  devices, all unnecessary blocks (that are using
*  clock) are being disabled by default.

/* Pin PB1 is also PWM3. We're using this pin to turn on
*  LED with different brightness. It is necessary to
*  define that pin as GPIO with PWM output. (This is why
*  is written before
*  "SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOB);"
*  even though it's used PWM output.)

/* Remember that block diagram from above? Well, it's
*  important to notice that generator 1 (not 0 or 2)
*  generates raw PWM signal.

/* In here you made most basic part of PWM (the one I
*  explained before with calculation). First of all, it's
*  important to set period, and later on duty cycle. With
*  this parameters, below, Period is 1000, and duty cycle
*  is 20%.
 PWMGenPeriodSet(PWM_BASE, PWM_GEN_1, 1000);
 PWMPulseWidthSet(PWM_BASE, PWM_OUT_3, 200);

/* When all is set enable generator. If you use 2
*  generators (2nd and 1st) and you want to turn
*  them on in synchronously then this function should
*  look like:
*  PWMGenEnable(PWM_BASE, PWM_GEN_2 | PWM_GEN_1);"

 /* When generator is enabled, the output isn't!
 PWMOutputState(PWM_BASE, PWM_OUT_3_BIT, 1);


How does this module actually work?

For start, it uses clock that is connected to the PWM generators (0, 1, 2). Every generator gives 2 raw PWM output signals. Usable (the that is on output pin of microcontroller) signals are those that are shown in Figure 17.-1., named as: PWM 1, PWM 2, …, PWM 5. I’m saying usable because that ones you’re using when you connect LEDs on your ports.
Generators generate wanted signal. Inside generator, comparators (A and B) compares written values (in registers PWMnCMPA and PWMnCMPB) with current timer value. Written values are the ones that causes high pulse (only 1 PWM clock tick) on comparators outputs. Those outputs are triggers for Signal generator that uses these pulses and direction signal for generating raw PWM signal. Since I haven’t explained direction signal probably the best is to take a look at this picture:


The triangle lines are representation of PWM counter. There are 2 types of counters; up-down mode and down mode. In this picture it’s used up-down mode. CompA and CompB dashed lines represent triggers for PWM comparators, Load represents value at which counters start counting in different direction (for this mode). If it’s used down counter mode then load value would be the one which resets counter back to zero (the counting direction doesn’t changes). Zero is just trigger for counter that it should start new cycle (in this case change direction of counting).

If we take a look at down signals, especially A, B and Dir, we would see that A and B are high only for one clock tick and only after the counter value reaches the PWMnCMPA value. Dir signal is the one that indicates if the counter is counting from Zero to the Load value (Dir = high value) or from Load value to the Zero (Dir = down value).

Complete project file you can download here.

P.S. All pictures are the ones you can find in: “Stellaris LM3S6965 Microcontroller DATA SHEET


03 PIR motion sensor

Hy guys!

Today I finally finished example code for PIR and decided to do little post about it. PIR is Passive infrared sensor usually used for outdoor lights (I really can’t remember any other usage except as part of the alarm system for your room :P). I wouldn’t like to explain how does it work because this page made perfect job.

Something that’s really important to you as programmers is kind generated signal when PIR senses a motion. First of all this is how it looks like (down here). On the left side is lens and on the right you see green board with sensor. Be careful, when you connect sensor to the Stellaris, you don’t exactly connect sensor to the Stellaris, but you connect one pin from chip that is bellow that green board to the Stellaris.


Well when that is clear, now I can “comment” program I made. The task was to make simple code that will turn on on-board LED when the sensor senses the motion, else LED will be off.

On picture above you can see how was it all connected. Little tip, always connect GND wires. When you use 2 different power supplies also connect GND together! When I was younger it made me a lot of problems (I was inexperienced). Since the PIR sensor has to have its power supply, I decided to bring it from Stellaris board. On green PIR sensor board was written +5V, OUT, GND, so it has to be connected with respect to that marks. Only OUT pin is up to you. I decided to connect mine at PA7 pin at Stellaris board (or written at the board SDA1).

Little explanation why we have to connect power supply for PIR sensor board:

As I have mention earlier, below sensor is chip which is used for decoding signal (that sensor sends to it), every chip needs to have its own power supply so this +5V, GND is for that particular chip. I think I mention everything that could cause problems while trying to make something with PIR sensor. Now it’s time for code explanation…

Full example code you can download HERE.

As you can see on pictures bellow before little guy left LED on Stellaris board is OFF, and when little guy passes by it goes ON.


Everything important for code is explained in comments. You’ll see in there that I made few .c files for PIR and LED, it’s mostly because I didn’t want have troubles with shifting pins in GPIO functions, and I wanted to use my own declarations, not the ones in gpio.h file.


02 UART communication

Hello, here’s second tutorial, also some basic stuff. I’m going to explain how to accomplish communication between computer and lm3s6965. For this task I decided to use serial communication through USB port (COM3-this is something  you can easily find out by using program Terminal Bray, I will explain that later).

For this project you’ll need:

– lm3s6965 (Stellaris board, or some other, the principle is the same)
– computer with USB port
– program Terminal Bray
– 3 female connector wires
– USB-TTL / STC-ISP (about few few dollars on ebay, pictures bellow)

Something about our task. We would like to send one character from our computer to the Stellaris and than display that character on OLED.  Here is block scheme, for better understanding and code writing.

I had a little problem in beginning because I had no idea how to write a code for something like this (this was my first project ever whit communication between computer and a chip). And  TI example code wasn’t a big help. When I remember all that “problems” it makes me laugh, because it’s so simple that I’m almost ashamed of talking “I know how to make successful serial communication between computer and stellaris”.

Now let’s get down to business. On my GoogleDrive you can download whole project ith comments.


If you haven’t configure your onboard debugger already, here is little tip how to do that (so that you can download program to the stellaris).

Configuration of OnBoard Stellaris Debugger in Keil uVision 4:

Open this window by clicking on Flash-> Configure Flash tools-> settings should look like there on picture.

After that’s completed, Left click on project-> Options for target ‘UARTDisplay’… or F7-> Choose Debug and set properties like on picture down here

Now you have to connect Stellaris and USB-TTL to the laptop. Here is picture how to do that. You have to be careful how you connect 3 of that wires. Black one is 3.3V, red one is Tx (because computer transmits data to stellaris) and white one is GND. You probably ask yourselves “why I have to connect 3.3V and GND?”. Well it’s because when you’re not in debug mode, and your stellaris is not directly connected to the computer it has to have some sort of battery.

After that you can Build (Project-> Build target) your project and start downloading (Flash-> Download) it to your Stellaris. When it’s done, open Terminal Bray (remember that program from the beginning of this post?! On picture bellow is what you have to set:

COM3 is set by Terminal Bray. It depends on where did you put USB-TTL. Speed depends on your code, where you define it, and when it’s ready click connect (on mine picture is disconnect). After all this you can type a letter or a number and it will be shown on you display.

=) Hope it helps!

01 How to manage everything (for nice start) in Keil uVision4

This tutorial is for complete beginners in Keil.

You can download Keil here. Be careful, if you have uController from Texas Instruments (ARM, cortex-M) download file “MDK-ARM”.

1. Find out which uprocessor you have. I have this little powerful boad: Stellaris lm3s6965, so I will make empty project for it =) (processor: lm3s6965)

2. Open Keil and pick Project->New uVision Project -> Select empty folder and choose its name (mine is Example)-> Save-> Find your uController (mine is in group Texas Instruments, lm3s6965)-> OK-> now there is question: “Copy “Startup.s” to Project folder and add file to Project?”, click NO. This is because you’ll have to add one later (which is litter different than this one).

3. Rename Source group in src. Right click on “Source group1” and “Manage components…”. Simply rename “Source group1” into “src”, and click “OK”. Also add another group “library”.

4. Now go to folder where you created folder Example. Folder “drivers and inc” you can download from mine Google drive. I didn’t have enough time to make all includes in drivers workable. So everything you’ll need to to is to delete this kind of parts “driverlib/”, “inc/”, “drivers/”, and leave only .h file (it’ll have to look like this:

#include “gpio.h”

5. Now it’s almost done! Create new file (ctrl+N), and save in src folder (file name: “exampleFile.c”). Also DO NOT FORGET ADD .c EXTENSION! Add that file to the src group in Keil (Right click on Target1-> Manage components-> Add “exampleFile.c”-> OK). “1.” and “2.” means that first you have to add file and then click OK, after that it will be shown in src group. While you’re in Manage components add “startup_rvmdk.S” (this file is MUST HAVE), and for example “gpio.c”, these 3 files you have to add in src group, and “driverlib.lib” file in library group. Then click OK.

6. As I have already mentioned in step 4., open “gpio.h”, and where you see in include files path like driverlib/, or inc/ or something like this, delete it and then try to compile. Since you  probably didn’t write main function, compiler wouldn’t want to compile it.

7. So…Just write something like

int main ( ){
return 0;


00 Hello

Hi guys, it’s my first post on this blog. For start I would like to tell you something about me and future posts.   Currently I’m a student at Faculty of electrical engineering and computing in Zagreb, last 3 years I’ve been programming microcontrollers. PIC was mine first one, and now I’m interested in ARM. There hasn’t been much big projects, but there has been some smaller ones. Some of them I would like to share on this blog.

Some ideas are:

– how to make completely new blinky project in Keil uVision 4 (I had quite big problems with making project from scratch, mostly because I’m a only beginner with ARM microcontrollers) – how to send character from computer to lm3s6965 by serial port
– well that’s it for now =)  

P.S. There’s another blog for those who like geek fashion Geeke Fatal