10. SPI Must-Know

1. Introduction to SPI

The Serial Peripheral Interface (SPI) is a widely-used protocol for communication between microcontrollers (MCUs) and various peripheral devices like sensors, memory chips, and other microcontrollers. Developed initially by Motorola, SPI is particularly popular in embedded systems due to its simplicity and efficiency.

Key Characteristics of SPI:

  • Serial Communication: Unlike parallel communication, SPI transfers data serially, one bit at a time, which significantly reduces the number of wires needed for communication.
  • Synchronous Protocol: It synchronizes data transmission using a clock signal, ensuring precise timing and data integrity.

Roles in MCU Communications:

  • Connecting Multiple Devices: SPI is ideal for setting up communication between a single master device (usually an MCU) and one or more slave devices. This master-slave setup is fundamental to SPI's operation.
  • Fast Data Transfer: Offering higher speeds compared to other protocols like I2C, SPI is suitable for applications requiring quick data exchange.
  • Flexibility and Scalability: While SPI itself is a simple protocol, it can be scaled to accommodate complex communication setups by adding more slave devices.

Typical Use Cases:

  • Device Control: MCUs use SPI to send control commands to peripheral devices.
  • Data Acquisition: SPI is often employed to gather data from sensors and other data-generating devices.
  • Inter-MCU Communication: It's also used for communication between multiple MCUs in complex systems.

In summary, SPI plays a crucial role in MCU communications by providing a reliable, fast, and efficient method for serial data transfer. Its widespread adoption is attributed to its simplicity and the ease with which it can be implemented in various electronic applications.

2. SPI Basics

Data Format in SPI Communication

  • Bit Transfer: SPI transmits data using bits. Each bit is sampled on the edges of a clock cycle.
  • Data Frame Size: Typically, the data frame size can range from 8 to 16 bits, though some systems may support different sizes.
  • Most Significant Bit (MSB) or Least Significant Bit (LSB) First: SPI can be configured to transmit either the MSB or LSB first.
  • Data Transfer Order: Data is sent and received simultaneously. While the master sends a bit on the Master Out, Slave In (MOSI) line, it receives a bit on the Master In, Slave Out (MISO) line.

Modes of Operation

SPI communication can operate in four different modes, determined by the clock polarity (CPOL) and clock phase (CPHA):

  • Mode 0 (CPOL = 0, CPHA = 0): The clock is idle low, and data are sampled on the rising edge of the clock.
  • Mode 1 (CPOL = 0, CPHA = 1): The clock is idle low, but data are sampled on the falling edge.
  • Mode 2 (CPOL = 1, CPHA = 0): The clock is idle high, and data are sampled on the falling edge.
  • Mode 3 (CPOL = 1, CPHA = 1): The clock is idle high, and data are sampled on the rising edge.

The choice of mode depends on the requirements of the MCU and the peripheral device. Compatibility between master and slave in terms of mode selection is crucial for successful communication.

Speed of SPI

  • Data Transfer Rates: SPI is known for its high-speed data transfer capabilities. The speed of SPI communication is primarily determined by the clock frequency.
  • Variable Clock Speed: The master device typically generates the clock signal, and it can vary the clock frequency depending on the application's requirements and the characteristics of the slave device. This flexibility allows SPI to work with a wide range of devices.
  • Typical Speed Ranges: SPI speeds can range from a few KHz to tens of MHz. High-speed SPI buses can operate at speeds of over 50 MHz, but the actual speed is often limited by the length and quality of the communication channel and the capabilities of the slave device.

3. SPI Components

Master and Slave Devices

  • Master Device: In SPI communication, the master device initiates and controls the data exchange. It generates the clock signal (SCK), decides when the communication starts, and controls the data flow. In most configurations, the master is a microcontroller.
  • Slave Device: A slave device is any component that communicates with the master as per the master's control. It receives the clock signal and other directives from the master. Slaves can be sensors, memory chips, or other microcontrollers. Unlike some communication protocols, slaves in an SPI setup do not communicate directly with each other.

Data Lines in SPI

SPI communication primarily involves four lines:

  1. MOSI (Master Out Slave In):

    • Function: This line carries data from the master to the slave.
    • Usage: During data transmission, the master places data on the MOSI line, which the slave then reads.

  2. MISO (Master In Slave Out):

    • Function: This line carries data from the slave back to the master.
    • Usage: When the slave needs to send data to the master, it uses the MISO line.

  3. SCK (Serial Clock):

    • Function: This is the clock line controlled by the master.
    • Usage: The SCK line synchronizes data transmission between the master and slave. The timing of data placement and data sampling on the MOSI and MISO lines is governed by this clock signal.

  4. SS (Slave Select):

    • Function: Also known as Chip Select (CS), this line is used by the master to select and manage individual slave devices. A slave device is selected when its SS line is pulled low to a logic 0. If it is not selected, the data lines are tri-stated, and the slave device ignores any data on the lines.
    • Usage: In a multi-slave configuration, each slave device has its own SS line connected to the master. By pulling the SS line of a specific slave device low, the master selects that device for communication, enabling a one-to-one connection.
    • Software Control: The SS line can be controlled by the master device's firmware, or it can be set internally by the SPI hardware. In the latter case, the SS line is automatically pulled low when the master initiates communication with a slave device and pulled high when the communication ends.

NSS_multiple_slaves

SPI Hardware

SPI_block_diagram

This SPI block diagram illustrates the key hardware components and their interaction in an SPI communication scenario. Here's how the elements work together:

  1. MOSI (Master Out Slave In) & MISO (Master In Slave Out):

    • MOSI is the line used by the master device to send data to the slave.
    • MISO is the line used by the slave to send data back to the master.
    • Both lines connect to a shift register, which is responsible for serializing and deserializing the data.

  2. Shift Register:

    • The shift register is a critical component in SPI communication. It shifts out data bit by bit on the MOSI line as the clock signals are received and shifts in data from the MISO line simultaneously.
    • For transmission (Tx), the register takes parallel data from the Tx buffer, converts it into a serial form, and sends it out via MOSI.
    • For reception (Rx), it collects the bits from MISO in sync with the clock and converts them back into a parallel form for the Rx buffer.

  3. Tx/Rx Buffer:

    • The Tx buffer holds the data that is ready to be sent out. When the shift register is ready to transmit new data, it retrieves the next byte from the Tx buffer.
    • Conversely, the Rx buffer collects the incoming data from the shift register, providing a place to store received data until the MCU or processor reads it.

  4. Baud Rate Generator:

    • This component generates the clock signal (SCK) that dictates the speed of communication. It is configured by the firmware to match the desired baud rate, which is typically a division factor of the MCU's clock.

  5. NSS (Negative Slave Select) or SS Logic:

    • This is the slave select line that the master uses to select which slave device it communicates with. It can be managed either by hardware (automatic control) or by firmware (software control), which is indicated as "Internal NSS" in the diagram.

  6. Address and Data Bus:

    • This bus is the communication pathway for address and data information between the SPI hardware and the MCU's core or other peripherals.
    • The firmware uses this bus to configure the SPI hardware, send data to be transmitted, and read received data.

  7. Communication Controller:

    • The communication controller includes control registers that the firmware configures to set up the SPI communication parameters, such as the data order (LSBFIRST for least significant bit first or MSBFIRST), clock polarity (CPOL), clock phase (CPHA), and data frame format (DFF).
    • It may also include advanced communication features like bidirectional data mode (BIDIMODE), receive-only mode (RXONLY), and CRC calculation for error checking (CRCEN, CRCNEXT).

  8. CRC (Cyclic Redundancy Check) Controller:

    • The CRC controller is an optional hardware module that can compute a checksum for transmitted and received data, providing an additional layer of error detection.

The firmware interacts with these hardware components to set up and control SPI communication. It writes to the control registers to specify the settings for the communication, fills the Tx buffer with data to send, and reads the Rx buffer to retrieve incoming data. The hardware handles the actual data transmission and reception processes, following the configuration and commands given by the firmware.

4. SPI Communication Process

Understanding the SPI (Serial Peripheral Interface) communication process is crucial for effectively managing data transfer between a microcontroller (MCU) and peripheral devices.

Data Transfer Mechanism in SPI

  • Simultaneous Transmission and Reception: In SPI, data transfer is a full-duplex process, meaning that data transmission and reception occur simultaneously. When the master sends a bit to the slave via the MOSI (Master Out Slave In) line, it concurrently receives a bit from the slave through the MISO (Master In Slave Out) line.
  • Data Frame Transfer: A typical SPI data transaction involves the master sending a data frame (which can be 8-bit, 16-bit, etc.) to the slave. The slave simultaneously sends back a data frame to the master. This exchange starts when the master selects a slave by activating its SS (Slave Select) line and lasts for the duration of the frame transmission.
  • Sequential Bit Processing: The master and slave shift out and in bits sequentially, usually with one bit per clock cycle. The data transfer typically starts with the most significant bit (MSB) or least significant bit (LSB), depending on the configuration.
  • End of Transaction: Once the data frame is completely transferred, the master deactivates the slave's SS line, marking the end of the transaction.

Clock Synchronization in SPI

  • Clock Signal Generation: The master device generates the clock signal (SCK) that dictates the timing of the SPI communication. This clock ensures that both the master and the slave devices are synchronized during data transfer.
  • Data Sampling and Setup: The SPI protocol defines two critical clock parameters: clock polarity (CPOL) and clock phase (CPHA). These parameters determine the idle state of the clock signal (high or low) and whether data should be sampled on the rising or falling edge of the clock signal.
  • Mode Configuration: The combination of CPOL and CPHA settings results in four possible clock configurations, known as SPI modes (Mode 0 to Mode 3). Both the master and slave must be configured to the same mode to ensure proper data sampling and setup.
  • Stable Data Signals: The clock signal helps maintain stable data signals between the master and the slave, preventing data corruption due to timing mismatches. This synchronization is crucial, especially at higher data transfer rates.

In summary, the SPI communication process revolves around a full-duplex data transfer mechanism, where data is simultaneously transmitted and received bit by bit. The synchronization of these data transfers is meticulously managed by the clock signal generated by the master device, with specific clock configurations ensuring accurate and reliable communication between the master and slave devices.

5. SPI Modes

While the full-duplex mode is the most common and well-known mode of operation for SPI (Serial Peripheral Interface), SPI also supports other modes that can be advantageous in certain applications. These modes determine how the SPI manages data transmission and reception, particularly in terms of the use of its data lines and clock synchronization.

Full-Duplex Mode

  • Description: This is the standard SPI mode where data transmission and reception occur simultaneously. The master sends data to the slave using the MOSI (Master Out Slave In) line and receives data from the slave via the MISO (Master In Slave Out) line, all within the same clock cycle.
  • Use Case: Ideal for high-speed, bi-directional data transfer where simultaneous send and receive operations are necessary.

Half-Duplex Mode

  • Description: In half-duplex SPI, data transmission and reception do not occur at the same time. Instead, the MOSI and MISO lines are shared or used alternately for sending and receiving data.
  • Use Case: Suitable for scenarios where the need to minimize pin usage outweighs the need for simultaneous bi-directional communication. This mode is commonly used in sensor applications where data flows predominantly in one direction at a time.

Simplex Mode

  • Description: Simplex SPI involves unidirectional data transfer. It uses only the MOSI line for sending data or only the MISO line for receiving data, but not both.
  • Use Case: Useful in situations where data flow is strictly unidirectional, such as in simple sensor readouts or display drivers where data only needs to move in one direction.

3-Wire Mode

  • Description: This is a variation of the half-duplex mode where the MOSI and MISO functions are combined into a single wire, reducing the total number of wires needed for communication.
  • Use Case: Chosen for applications where reducing the number of physical connections is crucial, such as in compact or highly integrated systems.

Multi-master SPI

  • Description: Unlike the standard SPI setup with one master and multiple slaves, multi-master SPI allows multiple master devices on the same bus.
  • Use Case: This mode is beneficial in complex systems where multiple controllers need to communicate with the same set of peripherals. It requires careful management of bus contention and arbitration mechanisms.

In summary, each SPI mode offers unique advantages and is chosen based on specific application requirements, such as the direction of data flow, pin availability, and system complexity. Understanding these modes and their appropriate use cases is vital for the efficient design and implementation of SPI communication in various electronic systems.

6. SPI Configuration

Configuring the SPI (Serial Peripheral Interface) on a microcontroller unit (MCU) involves setting up various registers that control the SPI operation. The specific steps can vary depending on the MCU family (like AVR, ARM, PIC, etc.), but the general process remains similar.

Register Configuration

  1. SPI Control Registers:

    • Enable SPI: Set the bit in the control register to enable the SPI module.
    • Master/Slave Selection: Configure the MCU as a master or a slave.
    • SPI Mode Selection: Choose the SPI mode by setting the clock polarity (CPOL) and phase (CPHA) bits.
    • Data Order: Select the data order (MSB-first or LSB-first).

  2. SPI Status Registers:

    • Read Status: Check for flags like SPI completion or collision errors.
    • Interrupts: Optionally enable SPI-related interrupts for data transfer completion.

  3. SPI Data Registers:

    • Writing/Reading Data: Use the data register for sending and receiving data.

  4. SPI Clock Configuration:

    • Clock Rate: Set the SPI clock frequency, usually as a division of the MCU's system clock.

  5. Slave Select (SS) Configuration:

    • SS Management: For master mode, configure GPIO pins connected to the SS line of slave devices.

Code Examples

Below are sample code snippets for setting up an SPI interface on an AVR MCU, such as the popular ATmega series. Please note that exact register names and bit definitions may vary across different MCU families.

#include <avr/io.h>

void SPI_MasterInit(void) {
    // Set MOSI, SCK, and SS as output, others as input
    DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK)|(1<<DD_SS);

    // Enable SPI, Master, set clock rate
    SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0);
}

void SPI_MasterTransmit(char data) {
    // Start transmission
    SPDR = data;

    // Wait for transmission complete
    while(!(SPSR & (1<<SPIF)))
        ;
}

int main(void) {
    SPI_MasterInit();
    SPI_MasterTransmit(0xAA); // Transmit example data
    // ...
}

This code sets up the MCU as an SPI master and demonstrates transmitting a byte of data. The exact register names (SPCR, SPSR, SPDR, etc.) and bit names (SPE, MSTR, SPR0, SPIF, etc.) are specific to AVR MCUs and should be adapted for other MCU types based on their datasheets or reference manuals.

Remember, SPI configurations can differ significantly between different MCU families. Always refer to the specific MCU's datasheet or reference manual for accurate register and bit definitions.

7. CPOL and CPHA

CPOL (Clock Polarity) and CPHA (Clock Phase) are two key parameters that define the four possible clock configurations used in SPI (Serial Peripheral Interface) communication. Together, they determine how data is clocked out and clocked in between the master and the slave devices.

CPOL (Clock Polarity):

CPOL specifies the level (high or low) that the clock line idles at when there is no data being transferred. There are two possible values for CPOL:

  • CPOL = 0: The clock line is low when idle, and the leading edge (the transition from low to high) is used to sample data.
  • CPOL = 1: The clock line is high when idle, and the leading edge (the transition from high to low) is used to sample data.

The idle state is important because it ensures that the clock's idle state is clear and distinct, preventing any misinterpretation when no communication is happening.

CPHA (Clock Phase):

CPHA determines whether data is captured on the clock's leading edge or trailing edge. Combined with CPOL, it tells the system exactly when to sample data and when to shift out the next bit of data. There are two possible values for CPHA:

  • CPHA = 0: Data is sampled on the first edge of the clock cycle (the leading edge). This means that if CPOL is 0, data is sampled on a rising edge. If CPOL is 1, data is sampled on a falling edge.
  • CPHA = 1: Data is sampled on the second edge of the clock cycle (the trailing edge). With CPOL = 0, this would be on the falling edge, and with CPOL = 1, it would be on the rising edge.

The combination of CPOL and CPHA results in four different SPI modes:

  • Mode 0: CPOL = 0, CPHA = 0

CPOL0_CPHA0

  • Mode 1: CPOL = 0, CPHA = 1

CPOL0_CPHA1

  • Mode 2: CPOL = 1, CPHA = 0

CPOL1_CPHA0

  • Mode 3: CPOL = 1, CPHA = 1

CPOL1_CPHA1

Each SPI slave device is designed to operate with a specific CPOL and CPHA setting, and the master device must be configured to match the slave's settings for proper communication. The choice of mode affects the stability and reliability of the SPI communication, as it dictates when the data should be read in relation to the clock signal.

8. Comparison of SPI with Other Protocols

Factor SPI I2C UART RS-232 RS-485
Speed High (up to several MHz) Moderate (up to 1 MHz in Fast Mode+) Low to Moderate (up to 115200 bps) Moderate (up to 115200 bps) Moderate to High (up to 10 Mbps)
Complexity Moderate (4+ wires) Low (2 wires) Low (2 wires) Moderate (3+ wires) Moderate (2 wires for half-duplex)
Data Transfer Full-duplex Half-duplex Full-duplex (asynchronous) Full-duplex (asynchronous) Half/Full-duplex
Topology Master-slave, multiple slaves Multi-master, multi-slave Point-to-point Point-to-point Multi-point
Range Short (a few meters) Short (a few meters) Medium (up to 50 feet) Medium (up to 50 feet) Long (up to 4000 feet)
Use Cases High-speed peripherals (SD cards) Low-speed peripherals (sensors) Serial communication over USB/RS-232 Legacy computer interfaces Industrial networks, long-distance comms
Scalability Limited by SS lines Good, with device addressing Low, typically point-to-point Low, typically point-to-point Good, supports many devices
Ease of Use Moderate Moderate Simple Simple Moderate

9. Advanced Topics in SPI

Daisy Chaining and Multiplexing

  • Daisy Chaining:

    • Description: In daisy-chaining, multiple SPI slave devices are connected in a series. The MISO of one slave is connected to the MOSI of the next.
    • Advantages: Reduces the number of Slave Select (SS) lines needed on the master, as a single SS line can control multiple devices.
    • Considerations: While it simplifies wiring, it can complicate the software side and may reduce the overall data transmission speed, as all devices share the same bus.

  • Multiplexing:

    • Description: Multiplexing involves using additional hardware like multiplexers to connect multiple slaves to a single SPI bus.
    • Advantages: Allows for a greater number of slaves on the bus without requiring more SS lines from the master.
    • Considerations: This approach may introduce additional latency and complexity in the hardware setup.

Interrupts and DMA in SPI

  • Interrupts in SPI:

    • Function: Interrupts are used in SPI to notify the CPU of events like the completion of data transmission, freeing up the CPU from constantly checking the status of the SPI.
    • Advantages: Improves efficiency by allowing the CPU to perform other tasks instead of polling the SPI status.
    • Implementation: Involves configuring SPI control registers to enable interrupts and writing an interrupt service routine (ISR) that handles SPI events.

  • Direct Memory Access (DMA) with SPI:

    • Function: DMA allows data to be transferred between the SPI and memory without CPU intervention.
    • Advantages: Significantly increases data transfer efficiency, especially for large data transfers, by freeing up the CPU.
    • Implementation: Requires configuring the DMA controller to handle data transfers. The DMA controller then automatically manages the data transfer directly between memory and the SPI data registers.