STM32 GPIO Configuration

• GPIOs are General Purpose Input/Outputs and utilize the STM32 (Cortex-M).

Objectives

• Identify GPIO pins.
• Configure GPIO pins for INPUT and OUTPUT.
• Manipulate Input/Output logic values through GPIO pins.

GPIO Identification

• The location of buttons, LEDs and connectors on the Nucleo-C031C6 are:
• Button B2 (Reset), LED LD3 (Power), LED LD4 (User), Arduino connectors CN5 and CN9, and ST morpho connector CN10.
• Micro-USB STLINK connector
• The location of the other buttons, LEDs, and connectors on the Nucleo-C031C6 are ST morpho connector CN7, ST Zio connectors CN6 and CN8, and button B1.

GPIO Terminals

• Determines when inputs or outputs need to be used.
• Outputs are connected to devices like seven-segment displays.
• Inputs are connected to sensors or switches.

Microcontroller Specs

• The microcontroller has 48 terminals, 45 of which are GPIOs for general purpose input/output.
• The Pxy notation indicates the general purpose pin Y in group X.
• PD3 is the third bit in Group D located at terminal 41.

GPIO Functions

• GPIO pins have various functions indicated by a letter followed by the number of pins:
• PORTA (16 pins)
• PORTB (16 pins)
• PORTC (5 pins)
• PC6-PC7, PC13-PC15
• PORTD (4 pins)
• PD0-PD3
• PORTF (4 pins)
• PF0-PF3
• Alternate Functions include:
• General-purpose Timers (TIM)
• External interrupts (EXTI)
• Analog-to-Digital Converter (ADC)
• Inter-Integrated Circuit Interface (I2C)
• Serial Peripheral Interface (SPI)
• Universal Synchronous Rx/Tx (USART)
• Independent Watchdog (IWDG)
• Real-time Clock (RTC)
• A table indicates pin names are related to alternate functions and additional functions, including ADC_IN13, ADC_IN12, ADC_IN11, ADC_IN8, RTC_REFIN

GPIO Registers

• GPIO registers have specific addresses outlined in thedatasheet.
• GPIOX_MODER: Configures the mode of the GPIO pin (input, output, alternate function, analog).
• GPIOX_OTYPER: Configures the output type (push-pull or open-drain).
• GPIOX_OSPEEDR: Configures the output speed.
• GPIOX_PUPDR: Configures pull-up or pull-down resistors.
• GPIOX_IDR: Input Data Register, reads the input value of the pin.
• GPIOX_ODR: Output Data Register, sets the output value of the pin.

GPIO Configurations

• GPIO pins are configured as inputs or outputs.
• Input configurations use floating, pull-up (PU), or pull-down (PD) settings.
• Output configurations use push-pull (PP) or open-drain (OD) settings.
• Alternate functions (AF) can also be configured and assigned.

Input Configuration

• The Output Buffer should be disabled.
• The Schmitt Trigger Input is activated.
• Pull-up and pull-down resistors can be activated through GPIOX_PUPDR.
• Data present on the I/O pin is sampled into the Input Data Register (GPIOx_IDR).
• Accessing the IDR reads the I/O State.

Output Configuration

• The Output Buffer is enabled.
• Open Drain Mode:
• 0 activates the N-MOS
• 1 leaves the port in Hi-Z
• Push-Pull Mode:
• 0 activates the N-MOS
• 1 activates the P-MOS
• The Schmitt Trigger Input is activated.
• Pull-up and pull-down resistors are activated through GPIOX_PUPDR.
• The Input Data Register (GPIOx_IDR) samples the I/O pin.
• Reading the IDR gets the I/O State.
• Reading the ODR gets the last written value.

Port Recommendations

• Check all configurations of each port by reviewing the reference manual of the STM32C031Cxx (Section 8).
• GPIOs share resources with other input-output peripherals like serial ports and analog inputs.
• Configure the GPIOs even if the desired configuration is the default after RESET.

GPIO Initialization

• PA5 and PA4 are outputs in push-pull mode and PA1 and PA0 are configured as input floating.
• It is important to modify specific bits without affecting others when initializing GPIOs.
• Use bitwise operators for proper configuration.

Bitwise Operators

• Used for manipulating individual bits within a variable.
• Key operators:
• &: AND (used for clearing bits)
• |: OR (used for setting bits)
• ^: XOR (used for toggling bits)
• ~: NOT (used for inverting all bits)

Recommended GPIO Initialization

• Utilize bitwise operators with masking to modify specific GPIO settings.
• Ensures that other pin configurations are not inadvertently altered during initialization.

Shift Operators

• Shift the bits to the LEFT ( << ) or to the RIGHT ( >> ) by 'n' positions
• Each LEFT shift multiplies a number by 2
• Each RIGHT shift divides a number by 2

Sign Extension

• Apply only to the RIGHT shift with SIGNED variables.

Simple C Program Structure

• A basic C program structure includes:
• Libraries, definitions, and global declarations.
• The superloop structure located in the int main( void )
• Followed by repetitive blocks
• Declarations and initializations should be divided from repetitive blocks

Header and C Files

• Declarations can be declared in a header file
• Header files are declared using #include directive
• Use "<>" to look to look in the compiler
• Use "" to look in the project directory

Data Types

• Data types include the min/max values and # of bits

Functions

• Should perform an specific well defined task
• Allows you to modulate your code
• Breaking in smaller pieces that may be used over and over again
• Can be reused in another programs
• Follows structure of return expression, name, header and optional params

Functions Definitions/Prototypes

• Declared before use
• Can prototype before the "int main(void)"
• Functions foo(void) returns void
• Functions bar(char a) returns char

Caling Functions

• The int main(void) function is where functions can be called
• x = bar(); sets x to whatever char bar(void)

Logical Operators

• Logical operators
• If x and y are both true, it will evaluate as a true
• If x or y are true, it will evaluate as a true
• If x is true, it will evaluate as false

Masking

• Used with bitwise operators such as &,|,^
• Can get port input data register (GPIOX_IDR)
• Can test low logic level on the pin 7 of PORTA

Input Voltages

• Chart illustrating: VIHmax,VIHmin,VILmax,VILmin

Interfacing with Buttons

• Pull-Down configuration: When the button is pressed +3.3V is read, otherwise GND is read
• Pull-Up configuration: When the button is pressed GND is read, otherwise +3.3V is read

Bouncing

• Bouncing can happen in switches
• Bouncing involves "bouncing" +3.3V on and off
• Bouncing can be resolved with debounce techniques such as digital filters

Debouncing Techinques

• There are many debouncing techniques:
  • Use a set-reset latch before the switch
  • Use a special integrated circuit to eliminate bounces
  • RC Filters
  • Firmware

Debounce Functions

• Can prevent bouncing signals
• Uses USER_Delay for double verification, as well as when pressed or while USER_Delay()

Multiple Inputs

Internal Pull-Up/Pulldown Resistors

• Can be toggled on with resistors
• These can be toggled in code,

Flowcharts

• You need to design an embedded system, define its specification, and begin with an organized plan
  • This is when you use flowcharts
• They can be used at the final documentation stage of a project, once the system is operational:
  • Can assist in its use or modification

Flowchart shapes

• Ovals define entry and exit points with each function having one of both
• Rectangles specify different process blocks
• Paralleograms define in input/ output function
• Diamonds specify a branch in the statement or function- these are conditional blocks
• Can be used as connectors for different diagrams

Coding Blocks with Flowcharts

• Can use sequence, conditional, and while-loops to create a program
• Allows a visual aid for others to understand

Debounce Flowcharts

• Can be used to illustrate debounce coding for switches
• Involves the use of input with push button, if pressed, is pressed, then debounce occurs and returns to the start