---
title: General-purpose Input/Output (GPIO)
categories: GPIO, STM32F4
...

Introduction
============
**General Purpose Input/Output (GPIO)** is a generic pin on a chip whose behavior (including whether it is an input or output pin) can be controlled (programmed) by the user at run time.

通常我們program使用的data都是放在memory比較多,而GPIO也提供類似的操作方法給programmer(讓我們去更改記憶體內容就可以去控制pin,並影響周遭設備的運行),但真正的設備的位置並非真正落在記憶體上(如:LED、Button、...等),故GPIO的核心是記憶體操作與設備之間的一些電路特性。

Main Feature
============
GPIO雖然建立起記憶體與設備之間的橋梁,但也並非我們就可以隨意使用,我們必須要經過設定之後才能讓我們想要的設備正常工作。

一個pin一次只能被設定成是input或output,而input會有三種狀態表現(floating, pull-up/down, analog),而output只有兩種狀態表現(push-pull or open drain + pull-up/down)。

- input/output方向解說 : input是指記憶體方接收來自設備的訊號源,output是指記憶體傳送訊號給設備。

當pin被設定成input時,非analog的設定下,我們可以利用GPIO的input data register(GPIOx_IDR) 或是memory中提供給目標設備的data register (當設成alternate function的時候)去接收data。

當pin被設定成output時,非analog的設定下,GPIO本身有提供output data register (GPIOx_ODR)來對目標設備做控制,但要是pin不是使用原本預先定義好的功能時(非預先定義的功能都算是alternate function的類別),此時要用memory中,另外規劃給目標設備用的register。

如果pin被設成analog的話,無論input or output都會由adc那邊做處理。

Functional Description
======================
input
-------------------

- floating vs. pull-up/pull-down

當input pin被處在高阻抗的模式下,若沒有外部訊號源進來的話,此時是無法確定pin的狀態(不能確定現在處在高電位或低電位),除非有外部訊號來驅動電路。換句話說,input floating,這個input電位狀態完全是由外部訊號來決定,沒有訊號驅動的話,就會呈現高阻抗狀態。

剛剛提到floating在沒有外部訊號驅動的情況下是呈現高阻抗狀態(無法確定電位狀態=>不能明確表示現在值是0或1),如果我們需要這個pin有一個明確的預設狀態時,必須借助pull-up(pull-down)resistor來做調整,在pull-up resistor(pull-up外接高電壓,pull-down通常會接地)加入之下,讓pin的維持在明確的高電壓狀態(pull-down則是讓pin維持在低電壓狀態)。舉例來說,如果我們定電壓在3-4 V之間是1的狀態,0-1之間是0的狀態,高阻抗的時候,電壓是不明確的,有可能電壓值會落在1-3之間的不明確地帶,甚至是沒有在任何一個狀態維持一段時間,此時的狀態是未定的,但如果我們加入pull-up resistor的話,這個pin接受來自pull-up另一端的電壓供應,讓pin至少維持在3v以上時,我們就可以確定在沒有外部訊號驅動時,pin是維持在高電位狀態。

output
-------------------
- push-pull with pull-up/pull-down
- open-drain with pull-up/pull-down


analog
-------------------

前面所述的input/output跟現在要談的類比模式是不一樣的類型,前者的資料型態主要是高低電位的數位型態(0/1的分別),而類比訊號是普遍自然界的訊號型態,故當我們設定成類比輸入的模式時,進來GPIO pin的原始訊號源在還沒經過施密特觸發器(Schmitt trigger)會有另一個線路將訊號做導向(通常是要導到ADC去),另一方面,當我們用了類比輸出模式後,GPIO的內部將會有一條線路接收DAC處理完的類比訊號,在經過此pin傳遞到外部去。

- 施密特觸發器(Schmitt trigger) : 將類比訊號的波形整成數位電路所能處理的方波波形(處理完只能分辨出高低電位的差別)。

.. image:: /embedded/Schmitt_trigger.jpg
(施密特觸發器具有整流效果)

basic of structure (P.137, **Figure 13. Basic structure of a five-volt tolerant I/O port bit** in Reference manual)
-------------------------------------------------------------------------------------------------------------------

.. image:: /embedded/GPIO_basic_src.PNG
- VDD:芯片內部的工作電壓
- VSS:接地點

Configuration
=============
Input configuration (P.143, **6.3.9 Input configuration** in Reference manual)
------------------------------------------------------------------------------
.. image:: /GPIO_ Input_configurations.png
When the I/O port is programmed as Input:

- the output buffer is disabled
- the Schmitt trigger input is activated
  - 這種模式處理的數位訊號只在意高低電位的差別(開關控制)。

- the pull-up and pull-down resistors are activated depending on the value in the GPIOx_PUPDR register 
  - 想要讓pin的state變成一個確定的狀態,可以設定pull-up/pull-down的使用。

- The data present on the I/O pin are sampled into the input data register every AHB1 clock cycle
  - input data的更新主要就是以AHB1本身的更新週期做決定,每一個cycle抵達時,data register就會根據當時Schmitt trigger整流完的狀態做更新。

- A read access to the input data register provides the I/O State
  - 對data register的理解,我覺得用'狀態'比'數值'的敘述來的更好(大部分都是開關,就是外部訊號源是否有狀態改變,如 : button的按下與放開),而此處寫I/O state的意思是,即使我們現在是設成output(如 : LED控制),但我們仍然可用input data register來檢查LED現在的狀態。

Output configuration (P.144, **6.3.10 Output configuration** in Reference manual)
---------------------------------------------------------------------------------
.. image:: /GPIO_ Output_configurations.png
When the I/O port is programmed as output:

- The output buffer is enabled:
   - Open drain mode: A “0” in the Output register activates the N-MOS whereas a “1” in the Output register leaves the port in Hi-Z (the P-MOS is never activated)
   - Push-pull mode: A “0” in the Output register activates the N-MOS whereas a “1” in the Output register activates the P-MOS
- The Schmitt trigger input is activated
- The weak pull-up and pull-down resistors are activated or not depending on the value in the GPIOx_PUPDR register
- The data present on the I/O pin are sampled into the input data register every AHB1 clock cycle
- A read access to the input data register gets the I/O state
- A read access to the output data register gets the last written value

Alternate function configuration (P.144, **6.3.11 Alternate function configuration** in Reference manual)
---------------------------------------------------------------------------------------------------------
.. image:: /GPIO_Alternate_function_configuration.PNG
When the I/O port is programmed as alternate function:

- The output buffer can be configured as open-drain or push-pull
- The output buffer is driven by the signal coming from the peripheral (transmitter enable and data)
- The Schmitt trigger input is activated
- The weak pull-up and pull-down resistors are activated or not depending on the value in the GPIOx_PUPDR register
- The data present on the I/O pin are sampled into the input data register every AHB1 clock cycle
- A read access to the input data register gets the I/O state

Remark:

GPIOx_AFRL[31:0] and GPIOx_AFRH[31:0] provide ways to select alternation functions. However, different alternate functions maps to different bits of ports. Table below is part of alternate function mapping, which is mainly about USART2/3.

For more information, please refer to **Table 9. Alternate function mapping** from P.60-68 in STM32F407xx Datasheet.

.. image:: /embedded/af_mapping.png

Analog configuration (P.145, **6.3.12 Analog configuration** in Reference manual)
---------------------------------------------------------------------------------
.. image:: /GPIO_Analog_configurations.PNG
When the I/O port is programmed as analog configuration:

- The output buffer is disabled
- The Schmitt trigger input is deactivated, providing zero consumption for every analog value of the I/O pin. The output of the Schmitt trigger is forced to a constant value (0).
- The weak pull-up and pull-down resistors are disabled
- Read access to the input data register gets the value “0”

Note: In the analog configuration, the I/O pins cannot be 5 Volt tolerant.

Demo
====
工具介紹
-------
.. image:: /embedded/BNC line.JPG

- BNC對鱷魚夾線

.. image:: /embedded/oscilloscope.JPG

- 示波器(oscilloscope) 畫面很差...

示波器驗證
---------
.. image:: /embedded/stable.JPG

- 示波器我們調整的scale是1 volt為單位。
- 我們接線是使用BNC對鱷魚夾線,黑接Groud,紅接要觀察的Pin。此圖為我們還沒做任何操作呈現的狀態。

.. image:: /embedded/button_pressed.JPG

- 以button做為觀察GPIO input的部份
- USER button對應的腳位為PA0
- 按下button馬上放開後會產生一個小方波,由此可知,按下button是產生高電位訊號。(看'按下'多久高電位就維持多久)

.. image:: /embedded/LEDOn_2.8v.JPG

- 用LED4來觀察GPIO output的部份
- LED4對應的腳位是PD12,當LED4亮的時候,我們可以看到波形產生變化(從0v左右的位置升到高電位2.8v左右)

.. image:: /embedded/PD12_PD14CNT.JPG

- 我們在接Pin的過程的時候,發現有些燈會亮起來,圖中紅色鱷魚夾線把PD12跟PD14同時夾起來,LED5在LED4亮起來的時候也會亮,對照PD14的位置是LED5。
- 我們觀察示波器用的Code只有對button跟LED4做設定,我們推測原因是LED4亮起來的時候,PD12處於高電位狀態,而鱷魚夾線連通PD14讓此腳位也處於高電位使LED5也亮起來了。

Code Section
-------------

sample code download : 

.. code-block:: c

    git clone https://github.com/shengwen1997/stm32_GPIO_demo.git
    cd stm32_GPIO_demo/

test for code :

.. code-block:: c

    make
    make flash

測試方法 : 

- input使用button作為範例,output是LED控制。

stm32f4_discovery library的預先定義(in libstm/Utilities/STM32F4-Discovery/stm32f4_discovery.h) :

.. code-block:: c

    #define LEDn                             4

    #define LED4_PIN                         GPIO_Pin_12
    #define LED4_GPIO_PORT                   GPIOD
    #define LED4_GPIO_CLK                    RCC_AHB1Periph_GPIOD  
  
    #define LED3_PIN                         GPIO_Pin_13
    #define LED3_GPIO_PORT                   GPIOD
    #define LED3_GPIO_CLK                    RCC_AHB1Periph_GPIOD  
  
    #define LED5_PIN                         GPIO_Pin_14
    #define LED5_GPIO_PORT                   GPIOD
    #define LED5_GPIO_CLK                    RCC_AHB1Periph_GPIOD  
  
    #define LED6_PIN                         GPIO_Pin_15
    #define LED6_GPIO_PORT                   GPIOD
    #define LED6_GPIO_CLK                    RCC_AHB1Periph_GPIOD

    #define BUTTONn                          1  

    #define USER_BUTTON_PIN                GPIO_Pin_0
    #define USER_BUTTON_GPIO_PORT          GPIOA
    #define USER_BUTTON_GPIO_CLK           RCC_AHB1Periph_GPIOA
    #define USER_BUTTON_EXTI_LINE          EXTI_Line0
    #define USER_BUTTON_EXTI_PORT_SOURCE   EXTI_PortSourceGPIOA
    #define USER_BUTTON_EXTI_PIN_SOURCE    EXTI_PinSource0
    #define USER_BUTTON_EXTI_IRQn          EXTI0_IRQn 

- LED是屬於GPIO port D,第12-15個pin分別是控制,LED4、LED3、LED5、LED6。
- Button是屬於GPIO port A,第0個pin是可以用來檢查button狀態的腳位。在文件中,有時會看到Wake-up button這一名稱,其實就是在說User button,兩者是指同一個東西。


input 設定 :

- button initialization(in libstm/Utilities/STM32F4-Discovery/stm32f4_discovery.c) :

.. code-block:: c

    void STM_EVAL_PBInit(Button_TypeDef Button, ButtonMode_TypeDef Button_Mode)
    {
       GPIO_InitTypeDef GPIO_InitStructure;
       EXTI_InitTypeDef EXTI_InitStructure;
       NVIC_InitTypeDef NVIC_InitStructure;

       /* 要設定好Clock才能正確更新Button的State,BUTTON_CLK[Button]是指RCC_AHB1Periph_GPIOA */
       RCC_AHB1PeriphClockCmd(BUTTON_CLK[Button], ENABLE);
       RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG, ENABLE);

       /* 設定為input mode,我們不給pin任何預設狀態,所PuPd的部份設成NOPULL */
       GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN;
       GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
       GPIO_InitStructure.GPIO_Pin = BUTTON_PIN[Button];
       GPIO_Init(BUTTON_PORT[Button], &GPIO_InitStructure);

       if (Button_Mode == BUTTON_MODE_EXTI)
       {
         /* 啟用interrupt並建立好與button腳位的連結 */
         SYSCFG_EXTILineConfig(BUTTON_PORT_SOURCE[Button], BUTTON_PIN_SOURCE[Button]);

         /* Configure Button EXTI line */
         EXTI_InitStructure.EXTI_Line = BUTTON_EXTI_LINE[Button];
         EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
         EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising;  
         EXTI_InitStructure.EXTI_LineCmd = ENABLE;
         EXTI_Init(&EXTI_InitStructure);

         /* Enable and set Button EXTI Interrupt to the lowest priority */
         NVIC_InitStructure.NVIC_IRQChannel = BUTTON_IRQn[Button];
         NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0x0F;
         NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0x0F;
         NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;

         NVIC_Init(&NVIC_InitStructure); 
       }
     }

- Button interrupt handler(in main.c)

.. code-block:: c

    void EXTI0_IRQHandler(void) 
    {
      /* 當Button按下後觸發interrupt,用自定義變數紀錄Button state的改變。 */
      UserButtonPressed = 0x01;
  
      /* Clear the EXTI line pending bit */
      EXTI_ClearITPendingBit(USER_BUTTON_EXTI_LINE);
    }

output 設定 :

- LED initialization (in libstm/Utilities/STM32F4-Discovery/stm32f4_discovery.c) :

.. code-block:: c

    void STM_EVAL_LEDInit(Led_TypeDef Led)
    {
      GPIO_InitTypeDef  GPIO_InitStructure;
  
      /* LED是屬於Port D,要設定好Clk讓Port D可以正確被更新 */
      RCC_AHB1PeriphClockCmd(GPIO_CLK[Led], ENABLE);

      /* 設定各LED對應的腳位,output type設成push-pull,並使用pull-up電阻,LED初始化完成時,會呈現高電位狀態(亮燈)。 */
      GPIO_InitStructure.GPIO_Pin = GPIO_PIN[Led];
      GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
      GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
      GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
      GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
      GPIO_Init(GPIO_PORT[Led], &GPIO_InitStructure);
    }

main.c : 

- main function :

.. code-block:: c

    int main(void)
    {
      RCC_ClocksTypeDef RCC_Clocks;
  
      /* button初始化,並使用interrupt。 */
      STM_EVAL_PBInit(BUTTON_USER, BUTTON_MODE_EXTI); 

      /* 初始化需要用到的LED。 */
      STM_EVAL_LEDInit(LED4);
      STM_EVAL_LEDInit(LED3);
      STM_EVAL_LEDInit(LED5);
      STM_EVAL_LEDInit(LED6);

      /* 初始化有使用到pull-up電阻,預設會亮燈,在這邊把燈關掉。 */
      STM_EVAL_LEDOff(LED4);
      STM_EVAL_LEDOff(LED3);
      STM_EVAL_LEDOff(LED5);
      STM_EVAL_LEDOff(LED6);
    
      /* 初始化自定義變數 */
      UserButtonPressed = 0x00;

      /* Create a task to flash the LED. */
      xTaskCreate(LED_task,
             (signed portCHAR *) "LED Flash",
             512 /* stack size */, NULL,
             tskIDLE_PRIORITY + 5, NULL);

      /* Create a task to button check. */
      xTaskCreate(button_task,
             (signed portCHAR *) "User Button",
             512 /* stack size */, NULL,
             tskIDLE_PRIORITY + 5, NULL);

      /* Start running the tasks. */
      vTaskStartScheduler(); 

      return 0;
    }

- LED control task

.. code-block:: c

    static void LED_task(void *pvParameters)
    {
      RCC_ClocksTypeDef RCC_Clocks;
      uint8_t togglecounter = 0x00;

      while(1)
      {    
          /* Toggle LED5 */
          STM_EVAL_LEDToggle(LED5);
          vTaskDelay(100);
          /* Toggle LED6 */
          STM_EVAL_LEDToggle(LED6);
          vTaskDelay(100);
      }
    }

- Button check task :

.. code-block:: c

    static void button_task(void *pvParameters)
    {
       	    while (1)
            {
                /* Waiting User Button is pressed */
    	        if (UserButtonPressed == 0x01)
    	        {
      	            /* Toggle LED4 */
      	            STM_EVAL_LEDToggle(LED4);
      	            vTaskDelay(100);
      	            /* Toggle LED3 */
      	            STM_EVAL_LEDToggle(LED3);
      	            vTaskDelay(100);
      	        }
      	        /* Waiting User Button is Released */
      	        while (STM_EVAL_PBGetState(BUTTON_USER) == Bit_SET);
      	        UserButtonPressed = 0x00;
      	   }
      }


Supplement
==========
- 一些有預設控制元件的腳位(詳見- STM32F4-Discovery 中文使用手冊P.16)
  - LD3:橙色,連接到STM32F407VGT6的PD13
  - LD4:綠色,連接到STM32F407VGT6的PD12
  - LD5:红色,連接到STM32F407VGT6的PD14
  - LD6:藍色,連接到STM32F407VGT6的PD15
  - B1(USER):連接到STM32F407VGT6的PA0
  - B2(RESET):連接到NRST,用於重置STM32F407VGT6

Reference
=========
- `General Purpose Input/Output - Wikipedia, the free encyclopedia <http://en.wikipedia.org/wiki/General_Purpose_Input/Output>`_
- `STM32F407xx Datasheet<http://www.st.com/web/en/resource/technical/document/datasheet/DM00037051.pdf>`_
- `STM32F407xx Reference Manual<https://www.google.com.tw/url?sa=t&rct=j&q&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=http%3A%2F%2Fmeasure.feld.cvut.cz%2Fsystem%2Ffiles%2Ffiles%2Fcs%2Fvyuka%2Fpredmety%2FA4M38KRP%2FDM00031020.pdf&ei=Yd4tUrCbFIyZlQWy5IDAAw&usg=AFQjCNG98i1xkmhMAT5oY4owZfumzgq-3Q&sig2=BW3iz-5aJmyGhiu6HR32tA>`_
- `STM32F4-Discovery 中文使用手冊</embedded/STM32-Discovery-Manual-Chinese.pdf>`_
- `稀里糊塗學 STM32 第二講:源源不絕</embedded/learn-stm32-part-2.pdf>`_
- `GPIO PPT</embedded/GPIO_v3.ppt>`_