Adafruit CH9328 UART to HID Keyboard Breakout

by Liz Clark

published June 12, 2024, last edited January 22, 2025

posted in Adafruit Products Arduino Compatibles CircuitPython Breakout Boards

Beginner

Product guide

Arduino

Using the CH9328 breakout with Arduino involves wiring up the breakout to your Arduino-compatible microcontroller, installing the Adafruit_CH9328 library and running the provided example code.

Wiring

Wire as shown when you are uploading code to the microcontroller. The CH9328 and the microcontroller will both be connected to your computer via USB but will share a ground connection. Make sure that the CH9328 onboard switches are set to Mode 3 (switch 2 off, switches 3 and 4 on) before powering it up.

Here is an Adafruit Metro wired up to the breakout using a JST SH cable.

Board GND to breakout JST SH GND (black wire)
Board pin 2 to breakout JST SH RX (white wire)

Here is an Adafruit Metro wired up using a solderless breadboard:

Board GND to breakout GND (black wire)
Board pin 2 to breakout RX (white wire)

Make sure that the CH9328 onboard switches are set to Mode 3 (switch 2 off, switches 3 and 4 on) before powering it up.

Library Installation

You can install the Adafruit CH9328 library for Arduino using the Library Manager in the Arduino IDE.

Click the Manage Libraries ... menu item, search for Adafruit CH9328, and select the Adafruit CH9328 library:

There are no additional dependencies for this library.

Example Code

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/* 
 *  This demo is for the "transparent transmission" mode #3 which you can enter by 
 *  setting the Adafruit CH9328 switches so that #2 is OFF, and #3 and #4 are ON
 *  In this mode you can send 'raw' HID key commands for more complex keypress combos!
 */
#include <Adafruit_CH9328.h>

Adafruit_CH9328 keyboard;

// Uncomment these lines if you need to use SoftwareSerial because there's no Serial1 device
#if defined(__AVR_ATmega328P__)
  #include <SoftwareSerial.h>
  SoftwareSerial Serial1(3, 2); // RX, TX pins
#endif

void setup() {
  // Start the debug serial port, wait till it opens
  Serial.begin(115200);
  while (!Serial) delay(100);

  // The default baud rate for the CH9328 is 9600
  Serial1.begin(9600);
  keyboard.begin(&Serial1);

  // Sending "Hello World!" as an ASCII character string
  keyboard.typeString("Hello World!");

  // Wait for 1 second
  delay(1000);

  // Send the backspace key 12 times to erase the string
  byte keys[6] = {KEY_BACKSPACE, 0, 0, 0, 0, 0}; // Keycode for backspace in US mapping
  byte noKeysPressed[6] = {0, 0, 0, 0, 0, 0};
  for (int i = 0; i < 12; i++) {
    keyboard.sendKeyPress(keys, 0);          // Press &
    keyboard.sendKeyPress(noKeysPressed, 0); // Release the key
  }
}

void loop() {
}

/* 
 *  This demo is for the "transparent transmission" mode #3 which you can enter by 
 *  setting the Adafruit CH9328 switches so that #2 is OFF, and #3 and #4 are ON
 *  In this mode you can send 'raw' HID key commands for more complex keypress combos!
 */
#include <Adafruit_CH9328.h>

Adafruit_CH9328 keyboard;

// Uncomment these lines if you need to use SoftwareSerial because there's no Serial1 device
#if defined(__AVR_ATmega328P__)
  #include <SoftwareSerial.h>
  SoftwareSerial Serial1(3, 2); // RX, TX pins
#endif

void setup() {
  // Start the debug serial port, wait till it opens
  Serial.begin(115200);
  while (!Serial) delay(100);

  // The default baud rate for the CH9328 is 9600
  Serial1.begin(9600);
  keyboard.begin(&Serial1);

  // Sending "Hello World!" as an ASCII character string
  keyboard.typeString("Hello World!");

  // Wait for 1 second
  delay(1000);

  // Send the backspace key 12 times to erase the string
  byte keys[6] = {KEY_BACKSPACE, 0, 0, 0, 0, 0}; // Keycode for backspace in US mapping
  byte noKeysPressed[6] = {0, 0, 0, 0, 0, 0};
  for (int i = 0; i < 12; i++) {
    keyboard.sendKeyPress(keys, 0);          // Press &
    keyboard.sendKeyPress(noKeysPressed, 0); // Release the key
  }
}

void loop() {
}

Upload the sketch to your board and open up the Serial Monitor (Tools -> Serial Monitor) at 115200 baud. The text "Hello World!" will be typed out and then deleted. This will only happen once, since the loop in the example is empty.

Page last edited January 21, 2025