[PXCT-294] Added SPI info and wiring guide

content/micropython/06.communication/spi/spi.md

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+---
+title: Serial Peripheral Interface (SPI)
+description: Learn how to use SPI with Micropython
+author: Pedro Sousa Lima
+---
+
+Serial Peripheral Interface, or **SPI**, is a widely used communication protocol for connecting devices. It enables high-speed, full-duplex communication over four primary wires, making it an excellent choice for applications requiring fast data exchange between microcontrollers and peripherals.
+
+## How SPI Works
+
+SPI communication relies on:
+
+- **MOSI** (Controller Out, Peripheral In): Sends data from the controller to the peripheral.
+
+- **MISO** (Peripheral Out, Controller In): Receives data from the peripheral to the controller.
+
+- **SCLK** (Serial Clock): Synchronizes data transmission.
+
+- **CS** (Chip Select): Selects which peripheral device to communicate with.
+
+![SPI Wiring](assets/spi.png)
+
+Unlike UART, SPI is synchronous, meaning it uses a clock signal for precise data timing. This enables faster and more reliable data exchange. SPI operates in a master-peripheral architecture, where a single controller device can communicate with one or multiple peripheral devices. Each peripheral is selected using the Chip Select (CS) line, ensuring that only the intended device responds to communication.
+
+## Key Features of SPI
+
+- **High-Speed Communication:** Supports much higher data rates than UART, making it suitable for applications requiring rapid data transfer.
+
+- **Multi-Peripheral Support:** Multiple devices can share the SPI bus, each being activated by a separate Chip Select line.
+
+- **Full-Duplex Data Transfer:** Unlike I2C, SPI allows data to be transmitted and received simultaneously, improving efficiency.
+
+- **Flexible Configuration:** SPI supports various clock phase and polarity settings, allowing it to be tailored to different peripheral requirements.
+
+## Common Use Cases
+
+SPI is commonly used in a variety of applications, including:
+
+- **Interfacing with Sensors:** Many modern sensors, such as temperature and accelerometers, communicate using SPI.
+
+- **Display Modules:** OLED, LCD, and LED matrix displays often rely on SPI for data transfer.
+
+- **Memory Storage:** Flash memory and SD card modules utilize SPI for high-speed data access.
+
+Embedded Communication: SPI is often used in microcontroller-based projects to enable communication between different modules.
+
+## Testing SPI Communication
+
+You can easily test SPI communication without an external device by performing a loopback test. This involves connecting the MOSI and MISO pins together so that data sent out is received back, verifying that the SPI interface is working correctly.
+
+### SPI Loopback Test Example
+
+```python
+from machine import SPI, Pin
+import time
+
+# Configure the SPI bus.
+# Adjust these parameters as needed for your setup.
+spi = SPI(1,
+          baudrate=500000,
+          polarity=0,
+          phase=0,
+          sck=Pin(7),
+          mosi=Pin(8),
+          miso=Pin(9))
+
+# Set up the chip-select pin (active low)
+cs = Pin(10, Pin.OUT)
+cs.value(1)  # start with CS inactive
+
+def loopback_test(command):
+    """
+    Sends a command (as bytes) over SPI.
+    IF MOSI and MISO are connected together (loopback), the response
+    will be identical to the command.
+    """
+    # Prepare a response buffer of the same length as the command.
+    response = bytearray(len(command))
+
+    cs.value(0)              # Activate communication (CS low).
+    spi.write_readinto(command, response)
+    cs.value(1)              # End communication (CS high).
+
+    return response
+
+while True:
+    command = b'ping'
+    response = loopback_test(command)
+    print("Sent:", command, "Received:", response)
+    time.sleep(1)
+```
+
+This method allows you to verify that SPI data transmission and reception are functioning correctly. Unfortunately, MicroPython does not currently support configuring SPI as a peripheral device, meaning only controller mode is available for communication.
+
+## Conclusion
+
+SPI is a powerful and efficient protocol for high-speed data communication between microcontrollers and peripherals. Its flexibility, full-duplex operation, and support for multiple devices make it a preferred choice for many embedded applications. Performing a loopback test is an effective way to validate SPI communication, especially when external peripherals are unavailable.