SPI Protocol

1. Introduction 

SPI (Serial Peripheral Interface) is a synchronous communication protocol that enables data transfer between microcontrollers and peripheral devices. It is a full-duplex, master-slave protocol which means data can be sent and received simultaneously. When the master device sends data to the slave device, the slave device can send data back to the master device without waiting for a separate read operation. It is commonly used in embedded systems, especially in applications requiring high-speed data transfer. 

2. Pin Configuration: MOSI, MISO, SCLK, and CS 

The SPI protocol requires four pins: 

• • MOSI (Master Output Slave Input): The MOSI pin is used by the master device to transmit data to the slave device. 
  • MISO (Master Input Slave Output): The MISO pin is used by the slave device to transmit data back to the master device. 
  • SCLK (Serial Clock): The SCLK pin provides the clock signal used to synchronize the data transfer between the master and slave devices. 
  • CS (Slave Select): The CS pin is used by the master device to select the slave device with which it wants to communicate. 

3. Hardware Configuration

SPI typically uses a push-pull output configuration for the SCLK, MOSI, and CS pins. This means that the output pin can drive both high and low voltages, providing a strong signal to the slave device. The MISO pin, on the other hand, is typically configured as an open-drain output.

This means that it can only drive the signal low, and relies on an external pull-up resistor to bring the signal back to a high voltage level. This configuration allows multiple slave devices to share the same MISO line, with each device driving the line low when it has data to send.

The open-drain configuration also provides a level of safety in case two devices try to drive the line high at the same time, preventing any damage to the devices.

4. Speed: Clock Frequency and Data Rate 

The clock frequency of SPI is determined by the speed of the microcontroller or device and the desired data rate. The clock frequency is typically measured in hertz (Hz) and is the number of clock cycles per second. The clock frequency of SPI can range from a few kilohertz (kHz) to several megahertz (MHz). Several factors affect the SPI clock frequency, including the following:

• • Microcontroller or Device Speed: The clock frequency of SPI is limited by the speed of the microcontroller or device. Faster microcontrollers or devices can generate higher clock frequencies.

  • Cable Length: The length of the cable between the master and slave devices can affect the maximum clock frequency. Longer cables can introduce capacitance and inductance, which can limit the maximum clock frequency.

  • Noise: Noise in the system can also affect the maximum clock frequency. Noise can cause errors in the data transfer, which can reduce the maximum clock frequency.

  • Data Rate: The maximum clock frequency is also dependent on the desired data rate. Higher data rates require higher clock frequencies.

5. Data Transfer

The data transfer can be initiated by either the master or the slave device. The SPI data transfer process consists of several steps, which are as follows:

• • Selecting the Slave Device: The master device selects the slave device by asserting the CS line low (as shown below). This tells the slave device that it is being addressed and should prepare to receive or transmit data.

  • Configuring the Data Transfer Parameters: The master device sets up the data transfer parameters, such as the clock frequency, data format, and the number of bits to be transferred per transaction.

  • Sending Data: The master device sends data to the slave device through the MOSI line. The data is sent in a serial format, one bit at a time, starting from the most significant bit.

  • Receiving Data: While sending data, the master device also receives data from the slave device through the MISO line. The received data is also in a serial format, one bit at a time, starting from the most significant bit.

  • Clocking Data: The data transfer is synchronized by the clock signal on the SCK line. The clock signal is generated by the master device, and the slave device synchronizes its data transfer with this clock signal.

  • Completing the Data Transfer: After the data transfer is complete, the master device de-asserts the CS line (as shown below), indicating that the slave device is no longer being addressed. The slave device can then release any resources that were used during the data transfer.

6. Data Transfer at Register Level

At the register level, data is transferred between the master and slave devices one byte at a time. The data transfer occurs between specific registers in the devices. Each register contains a specific function or data that needs to be transferred. As shown below:

• • The master device sends a clock signal on the SCLK line to synchronize the data transfer.
  • The master device sets the MOSI line to the appropriate logic level to send the first bit of the byte to the slave device.
  • The slave device receives the bit on the MISO  line and stores it in its shift register.
  • The clock signal is toggled, and the next bit of the byte is sent by the master device on the MOSI line.
  • The slave device receives the bit on the MISO line and shifts it into its shift register.
  • The process continues until all 8 bits of the byte have been transferred.

7. Daisy Chaining: Connecting Multiple Devices 

SPI supports daisy-chaining of multiple devices, which allows multiple devices to share a single SPI bus. In a daisy chain configuration, the SS pin of each slave device is connected to the MOSI pin of the previous slave device, forming a chain. The master device sends a single SS signal to select the first slave device in the chain, and then it can communicate with all the slave devices in the chain by sending data and clock signals. 

8. Other Aspects: Error Detection, Clock Polarity and Phase, and More 

SPI communication also includes other aspects like error detection, clock polarity, and clock phase. Error detection mechanisms like CRC (Cyclic Redundancy Check) can be used to ensure data integrity. The clock polarity and clock phase settings determine when data is sampled and when it is transmitted, allowing for flexibility in the protocol. 

9. Advantages of SPI Communication over Other Protocols 

Compared to other communication protocols like I2C and UART, SPI offers several advantages: 

• • High-Speed Data Transfer: SPI can achieve high data transfer rates compared to other protocols, making it an excellent choice for applications that require rapid data transfer. 
  • Simple Protocol: SPI is a simple protocol with fewer overheads, making it a more efficient choice for some applications. 
  • Full Duplex Communication: SPI allows for simultaneous data transfer in both directions, making it a more efficient choice than other protocols like I2C that require separate read and write operations. 
  • Daisy Chain Support: As discussed above, SPI supports daisy-chaining of multiple devices, making it a more flexible choice for some applications. 

10. Conclusion 

SPI communication offers several advantages over other communication protocols, including high-speed data transfer, simple protocol, full-duplex communication, and daisy chain support. It is a popular choice in embedded systems and is commonly used in applications that require high-speed data transfer. With its flexibility and versatility, SPI communication will likely continue to be an important communication protocol in the future.