The 40-pin header connector

On KNEO Pi board, you can find a 40-pin header. The pin headers have a 0.1in (2.54mm) pin pitch. The functions of the General Purpose I/O (GPIO) pins are predefined for KNEO Pi.

Info

In addition to functioning as GPIO input and output, these pins can be configured for alternate functions(SPI/I2C/UART/PWM/etc.) which vary depending on the specific pin. Configuration methods are detailed in the standard SDK. For advanced configuration options, refer to the full-function SDK page.

Electrical characteristics of The GPIO Pins

Input Properties

Symbol	Meaning	Description	Value
Vil	Input Low Voltage	Maximum voltage the pin considers as a "Low" signal.	0.7 V
Vih	Input High Voltage	Minimum voltage the pin considers as a "High" signal.	2.0 V
Iil	Input Leakage Current	Small current that may flow into or out of the pin when not actively driven (minimal power consumption).	836 nA

Output Properties

Symbol	Meaning	Description	Value
Vol	Output Low Voltage	Maximum voltage the pin outputs when driving a "Low" signal.	0.5 V
Voh	Output High Voltage	Minimum voltage the pin outputs when driving a "High" signal.	2.1 V
Iol	Output Low Current	Current the pin can source when pulling the signal low (0.5V).	13 mA
Ioh	Output High Current	Current the pin can sink when pulling the signal high (2.1V).	10.9 mA

Input/Output Voltage and Current Limitations

• These are 3.3 volt logic pins. A voltage near 3.3 V is interpreted as a logic one while a voltage near zero volts is a logic zero. A GPIO pin should never be connected to a voltage source greater than 3.3V or less than 0V, as prompt damage to the chip may occur as the input pin internal diodes conduct. Recommend that never source or sink more than 0.5 mA into an input pin.
• To prevent excessive power dissipation in the chip, you should not source/sink more current from the pin than its programmed limit. So, if you have set the current capability to 2 mA, do not draw more than 2 mA from the pin.
• Never demand that any output pin source or sink more than 12 mA.
• Do not drive capacitive loads. Do not place a capacitive load directly across the pin. Limit current into any capacitive load to a maximum transient current of 12 mA. For example, if you use a low pass filter on an output pin, you must provide a series resistance of at least 3.3V/12mA = 275 Ω.

Using GPIO

GPIO stands for General-Purpose Input/Output. These pins can be configured as either general-purpose input, general-purpose output, or as one of up to six special alternate settings, the functions of which are pin-dependent.

• GPIO Direction:

  A GPIO pin can be configured as either an input or an output.

  • When set as in, the pin reads the signal levels (high or low) applied externally.
  • When set as out, the pin drives the signal levels to connected devices.

• GPIO Value:

  A GPIO pin set as input will reflect the state of the connected signal. A GPIO pin set as output allows you to control its state, either high (1) or low (0).

Understanding the Global GPIO Calculation

In GPIO_2_IO_D[7], the pin is part of GPIO Controller-2 and represents the 7th line.

The global GPIO number is calculated as:
32 * Controller Number + Line Number = 32 * 2 + 7 = 71 (noted as gpio71).

Control via libgpiod-v2 Command-Line Tools

This section explains how to control GPIOs on the KNEO Pi using libgpiod version 2 command-line tools. Unlike the deprecated sysfs interface, libgpiod provides a modern and efficient way to interact with GPIOs.

Info

For advanced control, refer to the libgpiod documentation.

Understanding GPIO Numbering

Each GPIO pin is associated with a GPIO controller gpiochipX and a line number. The mapping can be found using gpioinfo. Ensure the correct gpiochipX is used when issuing commands.

Checking Available GPIOs

• To list available GPIO controllers:

  gpiodetect
  

  This will output a list of available GPIO chips (e.g., gpiochip0, gpiochip1, etc.).

• To see the GPIO lines available on a specific chip:

  gpioinfo -c 2
  

Configuring and Controlling GPIOs

Info

The following examples use pin 40, corresponding to gpiochip2, line 7.

• Set a GPIO as Output and Toggle Its State

  Set GPIO high (1)

  gpioset -c 2 7=1
  

  Set GPIO low (0)

  gpioset -c 2 7=0
  

  Ctrl+C to exit the command

• Set a GPIO as Input and Read Its Value

  gpioget -c 2 7
  

  The output will be 0 or 1, depending on the pin's state.

Control via Sysfs Interface

The sysfs GPIO interface is typically located at /sys/class/gpio/. Follow these steps to interact with GPIO pins:

Warning

Controlling GPIO via sysfs is deprecated and considered a legacy method. It is recommended to use libgpiod for modern GPIO control.

1. Export the GPIO Pin

   This makes the GPIO pin accessible in the sysfs interface. Replace with the number of your desired GPIO pin.

   $ echo <gpio_number> > /sys/class/gpio/export
   

2. Set GPIO Direction

   To set the pin as output:

   $ echo out > /sys/class/gpio/gpio<gpio_number>/direction
   

   To set the pin as input:

   $ echo in > /sys/class/gpio/gpio<gpio_number>/direction
   

3. Read or Write GPIO Value

   To write a value to the pin (when configured as output):

   $ echo 1 > /sys/class/gpio/gpio<gpio_number>/value   # Set pin high
   $ echo 0 > /sys/class/gpio/gpio<gpio_number>/value   # Set pin low
   

   To read the pin value (when configured as input):

   $ cat /sys/class/gpio/gpio<gpio_number>/value
   

   4. Unexport the GPIO Pin

   When you are done using the GPIO, unexport it to release resources:

   $ echo <gpio_number> > /sys/class/gpio/unexport
   

Two onboard GPIO-controlled LEDs

LED-green: gpio82
LED-blue: gpio83

Using PWM

Pulse Width Modulation (PWM) is a technique used to control the amount of power delivered to a device by varying the width of the pulses in a signal. It is commonly used for applications such as:

• Controlling the brightness of LEDs
• Adjusting the speed of motors
• Generating audio signals

In the KNEO Pi platform, GPIO pins(phyical pin-32 and pin-33) are configured to function as PWM outputs.

Basic Concepts of PWM

• Duty Cycle: The proportion of time the signal is "high" in a single period. A 50% duty cycle means the signal is high for half the time and low for the other half.
• Frequency: The number of times the signal completes a full cycle (high + low) per second, measured in Hertz (Hz).

Control via Sysfs Interface

The Linux kernel provides a sysfs interface for controlling PWM. Please use the following steps to control PWM.

1. Locate the PWM Chip and Channel

   Each PWM controller is represented as a PWM chip in the /sys/class/pwm directory.
   Channels are indexed under the chip.

   /sys/class/pwm/pwmchip0/
   

2. Export the PWM Channel

   To enable a specific PWM channel, export it:

   echo 0 > /sys/class/pwm/pwmchip0/export
   

   Here, 0 refers to the first PWM channel of pwmchip0.

   Info

   KNEO Pi provides 2 channels.

   If using AGPO_D[0](PWM)/physical pin 32, echo 0 > /sys/class/pwm/pwmchip0/export
   If using AGPO_D[1](PWM)/physical pin 33, echo 1 > /sys/class/pwm/pwmchip0/export

3. Configure PWM Parameters

   Navigate to the PWM channel directory:

   cd /sys/class/pwm/pwmchip0/pwm0
   

   Set the period (in nanoseconds):

   echo 1000000 > period    # 1,000,000 ns is equal to 1 KHz
   

   Set the duty cycle (in nanoseconds):

   echo 500000 > duty_cycle 
   

   Warning

   The period and duty cycle values must be consistent, with the duty cycle not exceeding the period.

   Enable the PWM signal:

   echo 1 > enable
   

4. Disable PWM

   To stop the PWM signal, disable it:

   echo 0 > enable
   

   If you no longer need the channel, unexport it:

   echo 0 > /sys/class/pwm/pwmchip0/unexport
   

Using UART

UART (Universal Asynchronous Receiver/Transmitter) is a hardware communication protocol used for serial communication. It enables two devices to exchange data over a serial connection, commonly used for debugging, connecting peripheral devices, or interfacing with external microcontrollers.

• Connect the USB-to-TTL serial cable to your computer and the GPIO pin header KNEO Pi.

  • GND at pin-6, TX at pin-8 and RX at pin-10

    

  Install Driver for CP2102N USB-to-UART Chip on Windows

  The USB-to-TTL serial cable included in the box features an embedded Silicon Labs CP2102N USB-to-UART chip. Follow these steps to install Virtual COM Port (VCP) driver for CP2102N on Windows:

  1. Download the Driver

     To enable the CP2102N to function as a standard COM port, you need to install the latest Virtual COM Port (VCP) drivers. You can download the most recent version from the official Silicon Labs website: CP210x USB-to-UART Bridge VCP Drivers

  2. Connect the Device via USB

     Connect the USB-to-TTL serial cable to your host PC.

  3. Detect the Device

     • If this is the first time connecting the CP2102N to your PC, it may appear as "CP2102N USB to UART Bridge Controller" in Device Manager.

     

     • If the driver is not yet installed, install it by executing the unzipped downloaded file, [unzipped folder]\CP210x_VCP_Windows\CP210xVCPInstaller_x64.exe

     

     • Once the driver installation is complete, the CP2102N will be recognized as a COM port in Device Manager. Note: The assigned COM port number may vary depending on your system.

     

• Use a console tool (e.g. Minicom and gtkterm on Linux OS or HyperTerminal and PuTTY on Windows OS) to connect to the KNEO Pi

• Configure the console tool with the appropriate baud rate and settings

  Option	Configuration
  Serial Port	Unix-like: ttyUSB# windows: COM#
  Bits per Second	115200
  Data Bits	8 bits
  Parity	None
  Stop Bits	1

• Once connected, you will have console access to control and manage the device, even without a network connection.

Using SPI

SPI (Serial Peripheral Interface) is a high-speed, full-duplex communication protocol commonly used for connecting displays, sensors, and other peripherals. It operates using four main signals:

• MOSI (Master Out Slave In)/pin-19 – Data sent from the master (KNEO Pi) to the peripheral.
• MISO (Master In Slave Out)/pin-21 – Data sent from the peripheral to the master.
• SCLK (Serial Clock)/pin-23 – Clock signal controlled by the master.
• CS (Chip Select)/pin-24 – Enables communication with a specific peripheral.

Using I2C

I2C (Inter-Integrated Circuit) is a commonly used, low-speed communication protocol designed for communication between integrated circuits on the same board. It supports multiple peripherals through addressable communication and is popular for interfacing sensors, displays, and other low-speed devices.

I2C uses two main signals:

• SCL (Serial Clock Line)/pin-5 – Provides the clock signal generated by the master (KNEO Pi).
• SDA (Serial Data Line)/pin-3 – Carries data between the master and the peripheral.

I2C enables multiple devices to share a common bus, with each device identified by an unique address. Communication is initiated by the master device, and data transfer is synchronized through a clock signal. The connection described here utilizes the I2C-0 bus.