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

basic of structure (P.186, Figure 17. Basic structure of a five-volt tolerant I/O port bit in Reference manual)

.. 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
• 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 (P.194, 7.3.10 Output configuration in Reference manual)

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)

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>_