The Molcajete project: where Arduino meets FreeRTOS

It’s ready the bundle in which you can program your Arduino apps using the FreeRTOS kernel. You can download it from

The Molcajete project

From the README.txt file:

The Molcajete project by Xavier R

OVERVIEW

The aim of this project is to program Arduino apps using the world’s most downloaded real time kernel, FreeRTOS, in a terminal through Arduino-mk makefile.

In fact, this project is kind of a FreeRTOS port for the ATMEGA328 chip so that it can be used in the Arduino platform. The client of this project MUST provide the several hooks that FreeRTOS needs to run. It’s assumed that the client has knowledged of what FreeRTOS is about, and in general, what a kernel is, and why and how it’s used.

Two examples are provided: The classical one that implements two tasks in a dynamic fashion in order to test the system. The other example does the same but with static tasks.

This port uses the standard malloc and free implementations for AVR when using dynamic tasks. For reliable embedded systems it’s recommended not to abuse in the creation and deletion of tasks in the heap. If the client chooses this way it’s recommended to create all tasks at once at the global space and to avoid any task deletion. Even better if static tasks are used as they are already enabled in the kernel (since FreeRTOS 9.0).

The system tick is fixed at 1 millisecond after the AVR’s T0 timer. Tickless and/or low-power modes have not been tested yet.

SKETCHES

I don’t provide any sketches. Everything I do I do it in a terminal. But you can contribute with some!

OPERATING SYSTEM

For now this project only works on 64 bits Linux boxes. Later I’ll try to create a patch file so you can apply it to your already Arduino installation.

CONFIGURATION

Two files need to be edited in order to configure each project:

  1. Makefile. There is one makefile per project. In this the client configures stuff related to Arduino.
  2. FreeRTOSconfig.h. There is one FreeRTOS configuration file for ALL projects. In this the client configures stuff related to FreeRTOS. Having one global FreeRTOS configuration file is a bad idea. However, due to my lack of knowledge about paths and makefiles, this undesired behavior is all I have by now. Hope to fix it ASAP.

STACK SIZE

The stack size assigned to each task depends on many factors. FreeRTOS includes some schemes so that the client can choose the number that fits the most each project’s tasks. Such a number can also be chosen by heart, provided a good understanding of the task’s job.

As mentioned earlier, the RAM should be asked for and given globally at the very start of the application. This is also true for static tasks. If dynamic tasks are used, then it’s recommended not to delete them; the heap might fragment and such situation isn’t good for reliable embedded systems.

PRIORITY LEVELS

Priority levels in FreeRTOS are implemented using a linked list. The more the priority levels, the more RAM it uses and more time is spent looking for the next task. Some projects use one priority level per task, while others share a priority level among several tasks. It depends totally on the project requirements.

I’ve written an article on how three priority levels fulfill the needs for a small systems using event-driven programming. The article can be reached at: https://bit.ly/2qoE2Kn

HOOKS

Some hook functions might be required for each project. The client MUST provide them. The number of hook functions, if some, depends on the FreeRTOS configuration.

MODIFIED FILES

I modified some files so that FreeRTOS can work along Arduino. If you are interested in study them, these are the files:

  • FreeRTOS side
    • port.c
  • Arduino side
    • wiring.c

Side effects

I’ve modified a little bit the optiboot bootloader, and this change is included in the bundle. If you don’t use it, then you have nothing to worry about. If you’re planning to burn the bootloader into your chips, you might want to check my modifications.

LICENSES

Each entity in this project might have its own licensing scheme. For details visit:

CONTRIBUTIONS

All contributions are welcome: examples, sketches, testing in boards other than UNO or Leonardo, testing in OSs other than Linux, etc. I’m NOT asking for money. I’ve done this little contribution due to my love for embedded systems and my beliefs on the open source and open hardware movement.

You can reach me at fjrg76 at hotmail dot com and ingenieria at fjrg76 dot com.

Non-blocking ADC converter class

The built-in analogRead() function blocks until the conversion is ready; for most scenarios this behavior is ok. However, for a high-responsive systems such behavior would be unacceptable.

Another reason I wrote this small non-blocking class is because I bought a lcd-keypad shield, and then I built some improved clones:

IMAG1112
Commercial LCD-keypad shield.
IMAG1110
My improved clon: it includes a relay, a buzzer and a screw terminal for the power supply. Besides, the distance among push-buttons is larger.

The keypad is implemented through an analog ladder, where each push-button is a rung. Tipically any keypad is decoded through a state machine driven inside the system-tick ISR for debounce purposes. That’s way I cannot use the analogRead() function. My class allows me to handle the conversion inside the system-tick ISR without blocking it. Weeee! You might discover other useful applications for a non-blocking conversions. I will glad to hear them!

Source code

My class is totally based upon the analogRead() source code. Moreover, I’ve left the original comments. What I did was to split it out in 3 stages: start, observe and read. Although my goal is to use a templetized class, this version works well  for now for most purposes.

// Analog.hpp

/*Copyright (C) 
 * 2018 - fjrg76 at hotmail dot com
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 * 
 */

#ifndef  ANALOG_INC
#define  ANALOG_INC

#include <Arduino.h>
#include "wiring_private.h"
#include "pins_arduino.h"

extern uint8_t analog_reference;

class Analog
{
public:
	explicit Analog( uint8_t _pin );
	void Start();
	bool IsReady() const;
	int Read() const;

private:
	uint8_t pin;
};


#endif   /* ----- #ifndef ANALOG_INC  ----- */

// Analog.cpp

#include "Analog.hpp"

Analog::Analog( uint8_t _pin ) : pin{ _pin }
{
	pinMode( this->pin, INPUT );
	digitalWrite( this->pin, LOW );

#if defined(analogPinToChannel)
#if defined(__AVR_ATmega32U4__)
	if (pin >= 18) pin -= 18; // allow for channel or pin numbers
#endif
	pin = analogPinToChannel(pin);
#elif defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
	if (pin >= 54) pin -= 54; // allow for channel or pin numbers
#elif defined(__AVR_ATmega32U4__)
	if (pin >= 18) pin -= 18; // allow for channel or pin numbers
#elif defined(__AVR_ATmega1284__) || defined(__AVR_ATmega1284P__) || defined(__AVR_ATmega644__) || defined(__AVR_ATmega644A__) || defined(__AVR_ATmega644P__) || defined(__AVR_ATmega644PA__)
	if (pin >= 24) pin -= 24; // allow for channel or pin numbers
#else
	if (pin >= 14) pin -= 14; // allow for channel or pin numbers
#endif

#if defined(ADCSRB) && defined(MUX5)
	// the MUX5 bit of ADCSRB selects whether we're reading from channels
	// 0 to 7 (MUX5 low) or 8 to 15 (MUX5 high).
	ADCSRB = (ADCSRB & ~(1 << MUX5)) | (((pin >> 3) & 0x01) << MUX5);
#endif

	// set the analog reference (high two bits of ADMUX) and select the
	// channel (low 4 bits).  this also sets ADLAR (left-adjust result)
	// to 0 (the default).
#if defined(ADMUX)
#if defined(__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
	ADMUX = (analog_reference << 4) | (pin & 0x07);
#else
	ADMUX = (analog_reference << 6) | (pin & 0x07);
#endif
#endif
}


void Analog::Start()
{
	// without a delay, we seem to read from the wrong channel
	//delay(1);

#if defined(ADCSRA) && defined(ADCL)
	// start the conversion
	sbi(ADCSRA, ADSC);
#endif
}

bool Analog::IsReady() const
{
#if defined(ADCSRA) && defined(ADCL)
	// ADSC is cleared when the conversion finishes
	return !bit_is_set( ADCSRA, ADSC );
#else
	// we don't want to get stuck
	return true;
#endif
}


int Analog::Read() const
{
	uint8_t low, high;

#if defined(ADCSRA) && defined(ADCL)
	// we have to read ADCL first; doing so locks both ADCL
	// and ADCH until ADCH is read.  reading ADCL second would
	// cause the results of each conversion to be discarded,
	// as ADCL and ADCH would be locked when it completed.
	low  = ADCL;
	high = ADCH;
#else
	// we dont have an ADC, return 0
	low  = 0;
	high = 0;
#endif

	// combine the two bytes
	return (high << 8) | low;
}

Example

Just a little example:

#include <Arduino.h>
#include "Analog.h"

int main(void)
{
	init();

	Analog a0( A0 );

	while( 1 )
	{
		a0.Start();
		while( not a0.IsReady() );
	        auto read = a0.Read();
                // do something with the reading
        }
}


Greetings!

System-tick for the Arduino platform

A real embedded system needs a system-tick, and Arduino doesn’t have one … No, the yield() function is not a real system-tick, so I’ve swimmed a little bit into the code so that I get a real one. This is my progress.

In the file hooks.c we need to add a place holder for our callback. Add this code at the end of such file:

static void __empty2() { 
 // Empty 
}
void tick_hook(void) __attribute__ ((weak, alias("__empty2")));

Then we need to declare our callback in Arduino.h. Look for the line

void yield(void);

and write after it:

void tick_hook( void );

It’s supposed that our callback is going to be called in every system tick, so look for timer 0 ISR function in wiring.c:

#if defined(__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__)
ISR(TIM0_OVF_vect)
#else
ISR(TIMER0_OVF_vect)
#endif
{

and write at its end the call to our callback:

tick_hook();

Finally, we need to write the body of our callback. My first attempt was to determine the tick period, so in the main source code I wrote:

void tick_hook()
{
  static bool ledState = false;
  // should exist a toggle function? Hope so!

  if( ledState ) {
    digitalWrite(LED_BUILTIN, HIGH);
    ledState = false;
  }
  else {
    digitalWrite(LED_BUILTIN, LOW);
    ledState = true;
  }
}

The result was that our callback is called every one millisecond, as seen in the next image:

IMAG1056

From here we can do whatever is required in our application using the system-tick. Just remember, any code inside an ISR should be as short and fast as possible.

Update: A complete example!

/*Copyright (C)
 * 2018 - fjrg76 at hotmail dot com
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 *
 */

//----------------------------------------------------------------------
//  Class: HeartBeat
//----------------------------------------------------------------------
template<const size_t Ticks, const uint8_t Pin>
class HeartBeat
{
public:
	HeartBeat();
	void Toggle();

private:
	bool pinState;
	size_t ticks;
};

template<const size_t Ticks, const uint8_t Pin>
HeartBeat<Ticks, Pin>::HeartBeat() : pinState{ false }, ticks{ Ticks }
{
	pinMode(Pin, OUTPUT);
}

template<const size_t Ticks, const uint8_t Pin>
void HeartBeat<Ticks, Pin>::Toggle()
{
	--ticks;
	if( ticks == 0 ) {
		ticks = Ticks;

		pinState = pinState ? false : true;
		digitalWrite( Pin, pinState );
	}
}

HeartBeat<500, LED_BUILTIN> heartBeat;

//----------------------------------------------------------------------
//  Our callback
//----------------------------------------------------------------------
void tick_hook()
{
	heartBeat.Toggle();
	// the built-in led is toggled every 500 milliseconds
}

void setup()
{
	// empty
}

void loop()
{
	// empty
}

In the console write:

make 
make upload

Greetings!

Building a solution from the command line

The Arduino’s IDE is just awful. Period. To write sketches is awful as well. Scketches make me sick. Hopefully we can build our solutions from the command line, directly from the main() function, even using our favorite code editor, VIM for me. A more-in-depth tutorial on this topic can be found here.

Installing Arduino

We need to download the latest Arduino‘s release, as for this writing, 1.8.5. Decompress it wherever you want. Then enter into the uncompressed folder and install it with:

./install.sh

Note: Uninstall any previous installation. Older releases will work,  but they won’t compile my tweakings.

Installing Arduino.mk

In order for me to work I’d rather install this tool from the Linux Mint repositories:

sudo apt install Arduino.mk

I guess that’s all for the needed tools. If any missing I’ll update this section.

Makefile for our solution

We need to write a Makefile for each of our projects:

ARDUINO_DIR = /your/home/dir/arduino-1.8.5
BOARD_TAG = uno
ARDUINO_PORT = /dev/ttyUSB*
ARDUINO_LIBS =
CFLAGS_STD = -std=c99 -Wall
CXXFLAGS_STD = -std=gnu++11 -Wall -DUNO
include /usr/share/arduino/Arduino.mk

The flag ‘-DUNO’ isn’t needed at all, but as I’m using the Leonardo and Uno boards, I need it to make the difference between them as easy as possible.

Test it!

Before we can upload our executable we must add our user to the dialout group so the serial port is available to us as humans:

$sudo adduser <your_user> dialout

Then log out and log in into your session again.


Last step is to write a test program. Let’s call it main.cpp:

#include <Arduino.h>

int main(void)
{
 init();
 // initialize the Arduino's environment

 pinMode(LED_BUILTIN, OUTPUT);

 while( 1 )
 {
   digitalWrite( LED_BUILTIN, LOW );
   delay( 100 );
   digitalWrite( LED_BUILTIN, HIGH );
   delay( 100 );
 }

 return 0;
}

For compiling write in the console:

make

and for uploading it:

make upload

And the serial monitor?

We have two options:

  • The arduino.mk includes one: make monitor (to exit ctrl-a + ctrl-k).
  • But if you don’t like it, then you might use one of the many serial monitors available in Linux. I like GtkTerm because you can close and open the USB port on demand (for when you’re going to upload the code, for example) without leaving it.

Greetings!

Building the Optiboot bootloader

The optiboot source code blundled along the Arduino package is out of date; it means, it doesn’t compile with the newer GCC releases. I got this similar situation, while trying to build my own release as of march 2018 (AVR-GCC 4.9). As it was suggested in the post I got the latest release on github. And it worked.

After that I wanted to customize the optiboot for my own boards, in which the reset line isn’t driven by my home-made USB-TTL adapter (and other commercial adapters). So I made two changes to the project: I made the led to blink 5 times, and let the board to wait longer than a second before the bootloader resets itself (one second wasn’t enough to sync the PC and the board). For this the optiboot.c and Makefile files must be edited.

Changes to optiboot.c

Look for this line (as for this writing it’s the line 509):

watchdogConfig(WATCHDOG_1S);

I changed it to:

watchdogConfig(WATCHDOG_8S);

It gives me enough time to sync. From lines 319 to 330 you can see other options.

Changes to Makefile

Look for the line 165 and set the number of flashes that you want. I chose 5:

LED_START_FLASHES_CMD = -DLED_START_FLASHES=5

Building the solution

Open a console in the same location as the files and type:

make clean
make atmega328

Depending on where you uncompressed or downloaded the Optiboot github project this can be the last step, but not for me.

Copying the optiboot_atmega329.hex file

In order not to break things, I decompressed the project in a new folder called Optiboot-master. So after the solution was build, I copied such a file into the original optiboot folder so the IDE is able to find it whenever I upload the bootloader to the chip. A softlink seems to be a better workaround.

Test

I tested all this mess using a USBAsp V2.0 programmer and a home-made chip place-holder, as shown:

IMAG1067

Open a terminal in ~/arduino-1.8.5/hardware/arduino/avr/bootloaders/optiboot and type:

$ avrdude -p m328p -P usb -c USBasp -e -U flash:w:optiboot_atmega328.hex -Ulock:w:0x3F:m -Uefuse:w:0xfd:m -Uhfuse:w:0xde:m -Ulfuse:w:0xff:m -C/usr/share/arduino/hardware/tools/avrdude.conf

and an output’s extract is shown as well:

Captura de pantalla de 2018-03-11 00-23-01

Greetings!