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. input 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，我們就可以蠻確定在沒有外部訊號驅動時，pin是維持在高電位狀態。

Each I/O port bit is freely programmable, however the I/O port registers have to be accessed as 32-bit words, half-words or bytes. The purpose of the GPIOx_BSRR register is to allow atomic read/modify accesses to any of the GPIO registers. In this way, there is no risk of an IRQ occurring between the read and the modify access.

Figure below shows the basic structure of a 5 V tolerant I/O port bit:

.. image:: /embedded/GPIO_basic_src.PNG

Configuration

Input configuration

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
• 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 provides the I/O State

Output configuration

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

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

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>_