/**
  @page NVIC_IRQ_Priority NVIC IRQ Priority example
  
  @verbatim
  ******************** (C) COPYRIGHT 2012 STMicroelectronics *******************
  * @file    NVIC/IRQ_Priority/readme.txt 
  * @author  MCD Application Team
  * @version V1.1.1
  * @date    13-April-2012
  * @brief   Description of the NVIC IRQ Priority example.
  ******************************************************************************
  *
  * Licensed under MCD-ST Liberty SW License Agreement V2, (the "License");
  * You may not use this file except in compliance with the License.
  * You may obtain a copy of the License at:
  *
  *        http://www.st.com/software_license_agreement_liberty_v2
  *
  * Unless required by applicable law or agreed to in writing, software 
  * distributed under the License is distributed on an "AS IS" BASIS, 
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
  ******************************************************************************
   @endverbatim

@par Example Description 

This example demonstrates the use of the Nested Vectored Interrupt Controller (NVIC): 

- Configuration of 2 EXTI Lines (Joystick_sel button EXTI Line & Key button EXTI Line)
  to generate an interrupt on each falling edge and use the SysTick interrupt.
- These interrupts are configured with the following parameters:
    - Joystick_sel button EXTI Line:  
        - PreemptionPriority = PreemptionPriorityValue
        - SubPriority = 0
    - Key button EXTI Line:    
        - PreemptionPriority = 0
        - SubPriority = 1           
    - SysTick Handler:  
        - PreemptionPriority = !PreemptionPriorityValue
        - SubPriority = 0             
First, the PreemptionPriorityValue is equal to 0, the Joystick_sel button EXTI Line 
has higher preemption priority than the SysTick handler. 

In the key button EXTI Line interrupt routine the Joystick_sel button EXTI Line and 
SysTick preemption priorities are inverted. 
In the Joystick_sel button EXTI Line interrupt routine the pending bit of the SysTick 
interrupt is set this will cause SysTick ISR to preempt the Joystick_sel button EXTI 
Line ISR only if it has higher preemption priority.

The system behaves as following:
 
1) The first time Key button EXTI Line interrupt occurs the SysTick preemption 
become higher than Joystick_sel button EXTI Line one. So when the Joystick_sel button EXTI 
Line interrupt occurs, the SysTick ISR is executed and the PreemptionOccured 
variable become TRUE and the four leds (LED1, LED2, LED3, LED4) start toggling.

2) When the next Key button EXTI Line interrupt occurs the SysTick preemption
become lower than Joystick_sel button EXTI Line one. So when the Joystick_sel button EXTI Line
interrupt occurs, the PreemptionOccured variable became FALSE and the four leds
(LED1, LED2, LED3, LED4) stop toggling.

Then this behavior is repeated from point 1) in an infinite loop.



@par Directory contents 

  - NVIC/IRQ_Priority/stm32l1xx_conf.h    Library Configuration file
  - NVIC/IRQ_Priority/stm32l1xx_it.c      Interrupt handlers
  - NVIC/IRQ_Priority/stm32l1xx_it.h      Interrupt handlers header file
  - NVIC/IRQ_Priority/main.c              Main program
  - NVIC/IRQ_Priority/system_stm32l1xx.c  STM32L1xx system source file
  
@note The "system_stm32l1xx.c" is generated by an automatic clock configuration 
      system and can be easily customized to your own configuration. 
      To select different clock setup, use the "STM32L1xx_Clock_Configuration_V1.1.0.xls" 
      provided with the AN3309 package available on <a href="http://www.st.com/internet/mcu/family/141.jsp">  ST Microcontrollers </a>
         
@par Hardware and Software environment

  - This example runs on STM32L1xx Ultra Low Power High-, Medium-Density and Medium-Density Plus Devices.
  
  - This example has been tested with STMicroelectronics STM32L152D-EVAL (STM32L1xx 
    Ultra Low Power High-Density) and STM32L152-EVAL (STM32L1xx Ultra Low 
    Power Medium-Density) evaluation board and can be easily tailored to any 
    other supported device and development board.

  - STM32L152-EVAL Set-up
    - Use LED1, LED2, LED3 and LED4 connected respectively to PD.00, PD.01, PD.04
      and PD.05 pins
    - Use the Key push-button connected to pin PA.00 (EXTI Line0).
    - Use the Joystick_sel push-button connected to pin PE.08 (EXTI Line8).
    - Make sure that The JP18 and JP19 are closed.

  - STM32L152D-EVAL Set-up
    - Use LED1, LED2, LED3 and LED4 connected respectively to PD.03, PD.07, PG.14
      and PG.15 pins
    - Use the Key push-button connected to pin PA.00 (EXTI Line0).
    - Use the Joystick_sel push-button connected to pin PG.13 (EXTI Line13).
    
      
@par How to use it ? 

In order to make the program work, you must do the following :
 - Copy all source files from this example folder to the template folder under
   Project\STM32L1xx_StdPeriph_Templates
 - Open your preferred toolchain 
 - Rebuild all files and load your image into target memory
 - Run the example

@note
- Ultra Low Power Medium-density devices are STM32L151xx and STM32L152xx 
  microcontrollers where the Flash memory density ranges between 64 and 128 Kbytes.
- Ultra Low Power Medium-density Plus devices are STM32L151xx, STM32L152xx and 
  STM32L162xx microcontrollers where the Flash memory density is 256 Kbytes.
- Ultra Low Power High-density devices are STM32L151xx, STM32L152xx and STM32L162xx 
  microcontrollers where the Flash memory density is 384 Kbytes.
    
 * <h3><center>&copy; COPYRIGHT STMicroelectronics</center></h3>
 */


