Table of Contents About GPIO Peripheral The pins which can be configured by the software at runtime to perform various functions are called GENERAL-PURPOSE INPUT/OUTPUT (GPIO) pins. With the help of software one can program the GPIO pins mainly as: Digital Output It compares the external voltage signal with a predefined threshold. Digital Input It controls the Voltage of the pin  Analog Function  It performs ADC (Analog to Digital Conversion) or DAC (Digital to Analog Conversion) Other Functions or Alternate Functions It makes the pin to perform other functions like PWM output, timer-based captures, external interrupts, and various other interfaces like SPI, I2C, UART communications. Before coming to the Schmitt Trigger Understanding, have a quick recap of Pull Up, Pull Down and Open Drain Configurations from MPU6050 Implementation blog Implementation of MPU6050 with STM32 – gettobyte GPIO Input: Schmitt Trigger GPIO Output Speed Slew Rate GPIO Input: Schmitt Trigger A SCHMITT TRIGGER is a device which uses a voltage comparator to convert a noisy or slow signal edge into a clean and desirable edge instantaneously.  For a real time system, the external signals do not change instantly, due to slower slew rate which depends on inheritance parasitic capacitance, resistance or an inductor at the  input side.  As the processor chip has a Schmitt Trigger, it increases the slew rate and increases the noise immunity for the signals which are captured. Let us understand the implementation of the Schmitt Trigger. It consists of a voltage comparator with positive feedback. The output Vout depends between two input voltage V+ and V–. If V+ > V–, Vout is quickly saturated to VSAT, otherwise Vout  = 0 For an ideal op-amp, the current flowing through resistor R3 is zero and thus we have Vref = V– The op-amp output Vout has two saturation values, as shown below Vout = VSAT if V– <V+       0 if V– < V+ However, V+ depends on Vout and Vin· Therefore, Vout depends on both the input Vin and the recent history of Vout· Such an effect is called hysteresis. Using KCL, assuming that the current flow in the non inverting input terminal of op-amp is zero, Vin – V+ /R2 = V+ – Vout / R1 On solving the above equation we will get, Vin = R2Vout + R1Vin / R1 + R2 At the time instant when Vout transits from one saturation value to the other saturation value, we have V+ = Vref Thus, Vref = R2Vout + R1Vin / R1 + R2 Solving further we get, Vin = (1 + R2/ R1)Vref – (R2/R1)Vout As discussed earlier, Vout has only two possible values. If Vout = 0 initially and Vin increases, we can obtain the trigger high threshold VTH at which Vout transits to VSAT: VTH = (1 + R2/R1) Vref – (R2/R1)*0 = (1 + R2/R1)Vref On the other hand, if Vout = VSAT initially and Vin decreases, we can obtain the trigger low threshold VTL at which Vout transits to 0: VTL = (1 + R2/R1) Vref – (R2/R1)VSAT Vout can be determined by comparing it with two thresholds VTH and VTL. When Vin climbs through VTH , Vout is rapidly switched to the upper limit VSAT· Conversely, once Vin falls below VTL, Vout makes a transition to the lower limit.  Note that VTH > VTL , i.e., the threshold for switching to high is greater than the threshold of switching to low. A Schmitt trigger when compared, Provide a better boise rejection. Larger threshold for switching high and low for switching. Immune to undesired noise. GPIO Output Speed Slew Rate The SLEW RATE of a GPIO pin is the speed of change of output voltage with respect to unit time. Slew Rate = ΔV/ Δt  In simple words, If the GPIO pin changes from LOGIC LEVEL 0 to LOGIC LEVEL 1, the voltage changes from 0V to 5V in just 5µs, then the slew rate is simply 1V/µs. The higher the slew rate, the shorter time the output voltage takes to rise or fall to desired values. Therefore, a higher slew rate allows faster speed at which the processor can toggle the logic level of a GPIO pin. A shorter rise and fall time allows a GPIO pin to change its logic value more rapidly. A high slew rate can result in significant electromagnetic interference (EMI), also known as radio frequency interference (RFI), to nearby electronic circuits. This is due to the large-amplitude and high-frequency harmonics produced by a fast-rising and falling signal, which can cause malfunctions in a victim circuit through radiation, conduction, or induction. To reduce EMI disturbance, a slower slew rate is generally preferred. GPIO IN STM32WB Each GPIO port has 4 32-bit Configuration Registers (GPIOx_MODER, GPIOx_OTYPER, GPIOx_OSPEEDR and GPIOx_PUPDR) 2 32-bit Data Register (GPIOx_IDR and GPIOx_ODR) A 32-bit Set/Reset Register (GPIOx_BSRR) A 32-bit locking Register (GPIOx_LCKR0 and 2 32-bit Alternate Function Select Register (GPIOX_AFHR and GPIOX_AFLR)   Main feature sog GPIO are Output states: push-pull or open drain + pull-up/down  Speed selection for each I/O Input states: floating, pull-up/down, analog  Fast toggle capable of changing every two clock cycles Highly flexible pin multiplexing allows the use of I/O pins as GPIOs or as one of several peripheral functions  GPIO Functional Description The port bit of GPIO can be configured by the software depending upon the hardware characteristics in various modes such as: Input floating  Input pull-up  Input-pull-down  Analog  Output open-drain with pull-up or pull-down capability  Output push-pull with pull-up or pull-down capability  Alternate function push-pull with pull-up or pull-down capability  Alternate function open-drain with pull-up or pull-down capability  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 access to any of the GPIOx_ODR registers. In this way, there is no risk of an IRQ occurring between the read and the modify access. The above diagram shows the basic structure of a