版本 6ef4cad315e1b7038afd083d1af75ce6f0515ce7
Changes from 6ef4cad315e1b7038afd083d1af75ce6f0515ce7 to 00df81ce02a71c96f6fdf8d7bc39ae905363c426
---
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 vs. 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跟是不一樣的類型，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)。
basic of structure (P.186, **Figure 17. Basic structure of a five-volt tolerant I/O port bit** in Reference manual)
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.. image:: /embedded/GPIO_basic_src.PNG
Vdd : 一個供電來源，電壓介在1.7 V - 3.6V之間
Vdd_ft : 5-volt tolerant用的供電來源。
Configuration
=============
Input configuration (P.193, **7.3.9 Input configuration** in Reference manual)
------------------------------------------------------------------------------
When the I/O port is programmed as Input:
- the output buffer is disabled
- the Schmitt trigger input is activated
- the pull-up and pull-down resistors are activated depending on the value in the GPIOx_PUPDR register
- 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.194, **7.3.10 Output configuration** in Reference manual)
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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.195, **7.3.11 Alternate function configuration** in Reference manual)
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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
Supplement：
GPIOx_AFRL[31:0] and GPIOx_ARHL[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 8. Alternate function mapping** from P.58-62 in STM32F407xx Datasheet.
.. image:: /embedded/af_mapping.png
Analog configuration (P.196, **7.3.12 Analog configuration** in Reference manual)
---------------------------------------------------------------------------------
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
====
Reference
=========
- `General Purpose Input/Output - Wikipedia, the free encyclopedia <http://en.wikipedia.org/wiki/General_Purpose_Input/Output>`_
- `STM32F407xx Datasheet<http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/DM00037051.pdf>`_
- `STM32F407xx Reference Manual<http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/REFERENCE_MANUAL/DM00031020.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>`_
