user-1: I'm using the RGBLCDShield library and a few analog input signals and since it seems the interrupts are being used by the SPI interface of the LCD Shield, I decided to try it with FreeRTOS instead.  I don't currently have my arduino with me (left if at parents') but I wanted to get some feedback on this code to see if it would run properly; I might not be getting my UNO back for a few days but I wanted to at least have a usable code ready to be tested when I do get it.
The Analog inputs are just 2 low level signals and the digital outputs just control some IC switches.  One PWM is fed through a lowpass filter and into an amplifier input.
Here is my code in it's entirety:
#include <Arduino_FreeRTOS.h>
#include <Adafruit_RGBLCDShield.h>
#include <Wire.h>
#include <math.h>



// define tasks
void TaskAnalogRead( void *pvParameters );  // Check voltage input
void TaskLCDShield( void *pvParameters );   // Control Display
void TaskUserInput( void *pvParameters );   // User Interface - System Process

Adafruit_RGBLCDShield lcd = Adafruit_RGBLCDShield();
unsigned int vSet = 0;  // Desired Voltage (potentiometer set)
unsigned int vOut = 0;  // Voltage on capacitor


// the setup function runs once when you press reset or power the board
void setup() {

  // Set up Tasks to run independently
  xTaskCreate(
    TaskAnalogRead
    ,  (const portCHAR *) "AnalogRead"   // A name just for humans
    ,  128  // This stack size can be checked & adjusted by reading the Stack Highwater
    ,  NULL
    ,  2  // Priority, with 1 being the highest, and 4 being the lowest.
    ,  NULL );

  xTaskCreate(
    TaskLCDShield
    ,  (const portCHAR *) "LCDShield"
    ,  128  // Stack size
    ,  NULL
    ,  3  // Priority
    ,  NULL );

  xTaskCreate(
    TaskUserInput
    , (const portCHAR *) "UserInput"
    , 128
    , NULL
    , 1
    , NULL );

  // Now the task scheduler, which takes over control of scheduling individual tasks, is automatically started.
  lcd.begin(16, 2);
}

void loop()
{
  // Empty. Things are done in Tasks.
}

/*--------------------------------------------------*/
/*---------------------- Tasks ---------------------*/
/*--------------------------------------------------*/

void TaskLCDShield(void *pvParameters)  // This is a task.
{
  (void) pvParameters;

  // Start Display
  lcd.setBacklight(0x7);
  lcd.setCursor(0,0);
  lcd.println(" PORTABLE SPARK ");
  delay(5000);
  lcd.clear();

  for (;;) // Continuously Update display
  {

    int volts = vSet * 100.0;
    int chg   = (vOut * 0.488)/volts;
    int vpos, cpos;
    lcd.setCursor(0,0);
    lcd.print("Voltage:       ");
    lcd.setCursor(15, 0);
    lcd.print("V");
    lcd.setCursor(0,1);
    lcd.print("Charge :       ");
    lcd.setCursor(15,1);
    lcd.print("%");

    lcd.setCursor(9,0);
    if(volts < 100)
      {
        lcd.print("    ");
        vpos = 13;
      }
    else if(volts < 1000)
      {
        lcd.print("    ");
        vpos = 11;
      }
     else if(volts < 10000)
     {
      lcd.print("     ");
      vpos = 10;
     }
     else vpos = 9;
    lcd.setCursor(vpos,0);
    lcd.print(volts, DEC);

    lcd.setCursor(9,1);
    if(chg < 10)
      {
        lcd.print("    ");
        cpos = 13;
      }
    else if(chg < 100)
      {
        lcd.print("   ");
        cpos = 12;
      }
    else cpos = 11;
    lcd.setCursor(cpos,1);
    lcd.print(chg, DEC);
    //digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
    vTaskDelay( 1000 / portTICK_PERIOD_MS ); // wait for one second
    //digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
    //vTaskDelay( 1000 / portTICK_PERIOD_MS ); // wait for one second
  }
}

void TaskAnalogRead(void *pvParameters)  // Read Analog Voltages (VSet, Emonitor)
{
  (void) pvParameters;

  for (;;)
  {
    int eMon = analogRead(A0); // read input pins
    vTaskDelay(1);
    int vA = analogRead(A1);

    vSet = (vA/1024) * 15000;  // calculate actual values
    vOut = (5.0/1024.0) * 10000 * eMon; 

    vTaskDelay(10);  // one tick delay (300ms) in between reads
  }
}  // End Analog Read


void TaskUserInput(void *pvParameters) // User Interface
{
  (void) pvParameters;

  // initialize digital pin 13 as an output.
  pinMode(8, OUTPUT);  // Fire signal
  pinMode(7, OUTPUT);  // Charge/Discharge signal
  pinMode(5, OUTPUT);  // Eprgm PWM output
  pinMode(4, OUTPUT);  // Cam/DAQ trigger

  bool charge = false;   // charge flag

  for (;;)
  {
    uint8_t button = lcd.readButtons();
    switch(charge)
    {
      case true:
        analogWrite(5, (vSet * 0.0204)/5);
        vTaskDelay(1);
        digitalWrite(7, HIGH);
        vTaskDelay(1);
        break;

       default:
        analogWrite(5, 0);
        vTaskDelay(1);
        digitalWrite(7, LOW);
        vTaskDelay(1);
        break;
    } // End Switch(charge)

    if(button & BUTTON_UP)  // Charge/Discharge button
    {
      charge = !charge;
    } // end if

    else if(button & BUTTON_DOWN) // Fire!
    {
      digitalWrite(8, HIGH);
      vTaskDelay(1);
      digitalWrite(4, HIGH);
      vTaskDelay(1);
      digitalWrite(7, LOW);
      charge = false;
      vTaskDelay(1);
      digitalWrite(4, LOW);
      vTaskDelay(1);
      digitalWrite(8, LOW);
    } // End else if

  }  // End for

} // End User Interface

My specific concern is with the 2wire interface of the LCD shield and whether it would be able to communicate with the Arduino - especially when it needs to read the button inputs.
Also, how do I determine the stack size for each task?  Is 128 for each enough to handle what I have?

user7508: This is not a direct answer to the question. Instead, I suggest that you
do not need an RTOS to do what you want. A lightweight alternative is to
use millis() to decide when it is time to perform such or such task:
void loop()
{
    uint32_t now = millis();

    // Update the LCD every second.
    static uint32_t last_lcd_update;
    if (now - last_lcd_update >= 1000) {
        last_lcd_update = now;
        update_lcd();
    }

    // Read the analog inputs every 300 ms.
    static uint32_t last_analog_read;
    if (now - last_analog_read >= 300) {
        last_analog_read = now;
        do_analog_read();
    }

    // User input is handled continuously.
    handle_user_input();
}

This will use significantly less resources (flash, RAM and CPU time)
than an RTOS. Another advantage is that you do not need to determine a
stack size for each task, which is definitely not a simple thing to
do.

-1: <p>I'm using the <strong>RGBLCDShield</strong> library and a few analog input signals and since it seems the interrupts are being used by the SPI interface of the LCD Shield, I decided to try it with <strong>FreeRTOS</strong> instead.  I don't currently have my arduino with me (left if at parents') but I wanted to get some feedback on this code to see if it would run properly; I might not be getting my UNO back for a few days but I wanted to at least have a usable code ready to be tested when I do get it.</p>

<p>The Analog inputs are just 2 low level signals and the digital outputs just control some IC switches.  One PWM is fed through a lowpass filter and into an amplifier input.</p>

<p><strong>Here is my code in it's entirety:</strong></p>

<pre><code>#include &lt;Arduino_FreeRTOS.h&gt;
#include &lt;Adafruit_RGBLCDShield.h&gt;
#include &lt;Wire.h&gt;
#include &lt;math.h&gt;



// define tasks
void TaskAnalogRead( void *pvParameters );  // Check voltage input
void TaskLCDShield( void *pvParameters );   // Control Display
void TaskUserInput( void *pvParameters );   // User Interface - System Process

Adafruit_RGBLCDShield lcd = Adafruit_RGBLCDShield();
unsigned int vSet = 0;  // Desired Voltage (potentiometer set)
unsigned int vOut = 0;  // Voltage on capacitor


// the setup function runs once when you press reset or power the board
void setup() {

  // Set up Tasks to run independently
  xTaskCreate(
    TaskAnalogRead
    ,  (const portCHAR *) "AnalogRead"   // A name just for humans
    ,  128  // This stack size can be checked &amp; adjusted by reading the Stack Highwater
    ,  NULL
    ,  2  // Priority, with 1 being the highest, and 4 being the lowest.
    ,  NULL );

  xTaskCreate(
    TaskLCDShield
    ,  (const portCHAR *) "LCDShield"
    ,  128  // Stack size
    ,  NULL
    ,  3  // Priority
    ,  NULL );

  xTaskCreate(
    TaskUserInput
    , (const portCHAR *) "UserInput"
    , 128
    , NULL
    , 1
    , NULL );

  // Now the task scheduler, which takes over control of scheduling individual tasks, is automatically started.
  lcd.begin(16, 2);
}

void loop()
{
  // Empty. Things are done in Tasks.
}

/*--------------------------------------------------*/
/*---------------------- Tasks ---------------------*/
/*--------------------------------------------------*/

void TaskLCDShield(void *pvParameters)  // This is a task.
{
  (void) pvParameters;

  // Start Display
  lcd.setBacklight(0x7);
  lcd.setCursor(0,0);
  lcd.println(" PORTABLE SPARK ");
  delay(5000);
  lcd.clear();

  for (;;) // Continuously Update display
  {

    int volts = vSet * 100.0;
    int chg   = (vOut * 0.488)/volts;
    int vpos, cpos;
    lcd.setCursor(0,0);
    lcd.print("Voltage:       ");
    lcd.setCursor(15, 0);
    lcd.print("V");
    lcd.setCursor(0,1);
    lcd.print("Charge :       ");
    lcd.setCursor(15,1);
    lcd.print("%");

    lcd.setCursor(9,0);
    if(volts &lt; 100)
      {
        lcd.print("    ");
        vpos = 13;
      }
    else if(volts &lt; 1000)
      {
        lcd.print("    ");
        vpos = 11;
      }
     else if(volts &lt; 10000)
     {
      lcd.print("     ");
      vpos = 10;
     }
     else vpos = 9;
    lcd.setCursor(vpos,0);
    lcd.print(volts, DEC);

    lcd.setCursor(9,1);
    if(chg &lt; 10)
      {
        lcd.print("    ");
        cpos = 13;
      }
    else if(chg &lt; 100)
      {
        lcd.print("   ");
        cpos = 12;
      }
    else cpos = 11;
    lcd.setCursor(cpos,1);
    lcd.print(chg, DEC);
    //digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
    vTaskDelay( 1000 / portTICK_PERIOD_MS ); // wait for one second
    //digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
    //vTaskDelay( 1000 / portTICK_PERIOD_MS ); // wait for one second
  }
}

void TaskAnalogRead(void *pvParameters)  // Read Analog Voltages (VSet, Emonitor)
{
  (void) pvParameters;

  for (;;)
  {
    int eMon = analogRead(A0); // read input pins
    vTaskDelay(1);
    int vA = analogRead(A1);

    vSet = (vA/1024) * 15000;  // calculate actual values
    vOut = (5.0/1024.0) * 10000 * eMon; 

    vTaskDelay(10);  // one tick delay (300ms) in between reads
  }
}  // End Analog Read


void TaskUserInput(void *pvParameters) // User Interface
{
  (void) pvParameters;

  // initialize digital pin 13 as an output.
  pinMode(8, OUTPUT);  // Fire signal
  pinMode(7, OUTPUT);  // Charge/Discharge signal
  pinMode(5, OUTPUT);  // Eprgm PWM output
  pinMode(4, OUTPUT);  // Cam/DAQ trigger

  bool charge = false;   // charge flag

  for (;;)
  {
    uint8_t button = lcd.readButtons();
    switch(charge)
    {
      case true:
        analogWrite(5, (vSet * 0.0204)/5);
        vTaskDelay(1);
        digitalWrite(7, HIGH);
        vTaskDelay(1);
        break;

       default:
        analogWrite(5, 0);
        vTaskDelay(1);
        digitalWrite(7, LOW);
        vTaskDelay(1);
        break;
    } // End Switch(charge)

    if(button &amp; BUTTON_UP)  // Charge/Discharge button
    {
      charge = !charge;
    } // end if

    else if(button &amp; BUTTON_DOWN) // Fire!
    {
      digitalWrite(8, HIGH);
      vTaskDelay(1);
      digitalWrite(4, HIGH);
      vTaskDelay(1);
      digitalWrite(7, LOW);
      charge = false;
      vTaskDelay(1);
      digitalWrite(4, LOW);
      vTaskDelay(1);
      digitalWrite(8, LOW);
    } // End else if

  }  // End for

} // End User Interface
</code></pre>

<p>My specific concern is with the 2wire interface of the LCD shield and whether it would be able to communicate with the Arduino - especially when it needs to read the button inputs.<br>
Also, how do I determine the stack size for each task?  Is 128 for each enough to handle what I have?</p>
 7508: <p>This is not a direct answer to the question. Instead, I suggest that you
do not need an RTOS to do what you want. A lightweight alternative is to
use <code>millis()</code> to decide when it is time to perform such or such task:</p>

<pre class="lang-c++ prettyprint-override"><code>void loop()
{
    uint32_t now = millis();

    // Update the LCD every second.
    static uint32_t last_lcd_update;
    if (now - last_lcd_update &gt;= 1000) {
        last_lcd_update = now;
        update_lcd();
    }

    // Read the analog inputs every 300 ms.
    static uint32_t last_analog_read;
    if (now - last_analog_read &gt;= 300) {
        last_analog_read = now;
        do_analog_read();
    }

    // User input is handled continuously.
    handle_user_input();
}
</code></pre>

<p>This will use significantly less resources (flash, RAM and CPU time)
than an RTOS. Another advantage is that you do not need to determine a
stack size for each task, which is definitely <em>not</em> a simple thing to
do.</p>
