====== LamaPLC: CJMCU-3901/PMW-3901 compact optical flow sensor module/IC by PixArt with SPI communication ======
{{ :sensor:cjmcu3901_2.png?180|CJMCU-3901/PMW3901 compact optical flow sensor module/IC by PixArt with SPI communication}}
The **CJMCU-3901** is a compact optical flow sensor module that integrates the **PixArt PMW3901** chip. It is mainly designed for X-Y motion detection and precise positioning in drones and robotics, particularly in indoor or GPS-denied settings.

==== Key Technical Specifications ====

  * **Sensor:** PixArt PMW3901MB-TXQT.
  * **Operating Voltage:** **1.8V to 3.6V** (typically used at 3.3V).
  * **Operating Range:** 80mm to infinity (no lens adjustment required).
  * **Interface:** 4-wire [[com:basic_spi|SPI]] (up to 2MHz).
  * **Power Consumption:** Very low, typically less than 9mA in active mode.
  * **Field of View (FOV):** Approximately 40°.

{{page>:tarhal}}

==== CJMCU-3901 pinout ====
{{ :sensor:cjmcu3901_1.png?180|CJMCU-3901/PMW3901 compact optical flow sensor module/IC by PixArt with SPI communication}}
^Pin Name^Function^Description|
^3V3|Power Supply|Typically 3.3V (range: 1.8V – 3.6V)|
^GND|Ground|Connect to the common ground of your system|
^MOS|SPI Data In|Master Out Slave In; receives commands from the controller|
^CLK|SPI Clock|Serial clock signal generated by the master|
^MIS|SPI Data Out|Master In Slave Out; sends motion data to the controller|
^CS|Chip Select|Active-low signal to enable the sensor for communication|
^RST|Reset|Optional active-low hardware reset pin|
^MOT|Interrupt|Motion detection output; triggers when new motion is detected|
^VRE|reference voltage output|It provides access to the sensor's internal regulated voltage, which is used as a reference for its internal optoelectronic operations|

**Connection Requirements**

  * **Logic Levels:** This sensor uses **3.3V logic**. If you are using a 5V microcontroller, such as an Arduino Uno, you must use a level shifter to prevent damage to the chip.
  * **SPI Speed:** The module supports SPI clock speeds up to 2 MHz.
  * **Orientation:** Look for a small notch or arrow on the board; this usually indicates the "back" or "front" to ensure correct X-Y coordinate tracking in your firmware.

==== Arduino wiring ====
Since the CJMCU-3901 operates on 3.3V logic, you must use a 3.3V Arduino (e.g., Nano 33 IoT) or a level shifter if using a 5V Arduino (e.g., Uno).
 
^CJMCU-3901 Pin^Arduino Pin (Uno/Nano)^Function|
^VCC|3.3V|Power Supply|
^GND|GND|Ground|
^CS|D10 (or any Digital Pin)|Chip Select|
^MOSI|D11|SPI Data In|
^MISO|D12|SPI Data Out|
^SCLK|D13|SPI Clock|

==== Arduino code ====
Need to install the **Bitcraze_PMW3901 library** via the Arduino Library Manager.
<code c>
#include "Bitcraze_PMW3901.h"

// Using digital pin 10 for chip select
Bitcraze_PMW3901 flow(10);

void setup() {
  Serial.begin(9600);

  // Initialize the sensor
  if (!flow.begin()) {
    Serial.println("Initialization of the flow sensor failed");
    while(1); // Halt if sensor not found
  }
}

int16_t deltaX, deltaY;

void loop() {
  // Get motion count since the last call
  flow.readMotionCount(&deltaX, &deltaY);

  Serial.print("X: ");
  Serial.print(deltaX);
  Serial.print(" | Y: ");
  Serial.println(deltaY);

  delay(100); // Small delay for readability
}
</code>
**Operational Notes**

  * **Measurement Units:** The sensor outputs unitless //"ticks"// representing the amount of motion.
  * **Surface Texture:** The sensor requires a textured surface (not solid black or mirrored) to track movement effectively.
  * **Focus:** The built-in lens is typically pre-focused from 80mm to infinity.

{{tag>CJMCU-3901 CJMCU PMW3901 PMW-3901 optical_flow sensor PixArt SPI communication arduino code PMW3901MB-TXQT}}

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