James, Jr. Network Engineer
Hey Adam, I'm designing a data acquisition system with multiple sensors for our IoT device. I'm looking at SPI for interfacing since I need faster data rates than I²C, but I'm a bit overwhelmed by all the configuration options in the sensor datasheets. Any suggestions on where to start?
Adam, Sr. Network Engineer
SPI is a good choice for high-speed data transfer. It works as a master chef (the microcontroller) coordinating with multiple sous chefs (sensors) using a synchronized rhythm (clock signal). While I²C tops out around 5Mbps in Ultra-Fast-mode Plus, SPI can easily handle up to 50 Mbps (full-duplex) with modern MCUs. Now, what kind of sensors are you planning to interface with?
James, Jr. Network Engineer
I need to connect three components: a temperature sensor, an accelerometer, and a memory chip. I'm worried about managing multiple slave select lines and dealing with different clock speeds for each device. The datasheets mention different SPI modes from Mode 0 to Mode 3 and the CPOL and CPHA settings. Does this make connecting devices more complex?
Adam, Sr. Network Engineer
SPI is all about flexibility. There are different clock speeds for each slave device. The dedicated slave select lines make it easier to manage multiple devices without addressing conflicts. CPOL and CPHA determine how the data gets synchronized with the clock - when the sensor should "transmit" and "listen." For example, at Mode 0 (CPOL=0, CPHA=0), the data is sampled on the rising edge of the clock and shifted on the falling edge.
James, Jr. Network Engineer
That helps! But how can multiple sensors be managed on the same bus? Unlike I²C, I noticed each device needs its chip select line with the device address. Is this a limitation?
Adam, Sr. Network Engineer
The dedicated chip select lines are an advantage! It's like having a direct hotline to each sensor. Plus, since SPI is synchronous, all data transfers are precisely timed with the clock signal, eliminating the timing uncertainties you get with asynchronous protocols.
This conversation about SPI configurations might have sparked your curiosity about implementing efficient peripheral communications in your IoT designs. While choosing between various communication protocols like I²C, UART, or SPI can seem daunting, understanding the intricacies of SPI protocol is crucial for optimizing your IoT device's performance.
What is the SPI Protocol?
SPI or Serial Peripheral Interface protocol is a high-speed, synchronous serial communication protocol used primarily for short-distance communication, often between microcontrollers and peripheral devices such as sensors, memory, or display modules.
The SPI Protocol has two levels of operation:
| Parameter | SPI Interface | SPI Protocol |
|---|---|---|
| Physical Communication | Concerned with hardware pins and electrical signals. | Defines the timing, data structure, and operation logic. |
| Focus on IoT Deployment | Establishes physical connections between devices. | Synchronizes communication and ensures data integrity. |
| Dependency | Requires proper wiring and pin compatibility. | Requires correct configuration (e.g., CPOL, CPHA, data length). |
| Implementation | Involves hardware design, PCB layout, and wiring. | Involves software/firmware development to interpret data. |
| Example | Connecting an SPI flash memory chip to a microcontroller physically. | Writing firmware to read/write data from the flash memory. |
Evolution of SPI Protocol
The Serial Peripheral Interface Protocol was developed in the mid-1980s by Motorola. It facilitates communication between microcontrollers and peripheral devices within electronic systems. At that time, there was a growing need for a high-speed, efficient, and simple communication protocol to handle the increasing complexity of embedded systems.
Existing communication protocols like UART (Universal Asynchronous Receiver/Transmitter) and I²C (Inter-Integrated Circuit) were either too slow or possessed limitations in data transfer rates and complexity. Designers required a protocol that was easy to implement in hardware and software, with minimal overhead and maximum throughput.
It led to the introduction of a synchronous serial communication protocol, SPI. It achieves higher data rates by using a simple master-slave architecture and a minimal set of control lines. Due to its effectiveness, the SPI bus protocol quickly gained popularity and was adopted by various manufacturers for interfacing microcontrollers with peripherals like sensors, memory chips, and display modules.
SPI Protocol Signals
As mentioned, SPI protocol uses a master-slave architecture. The master device controls the communication, and one or more slave devices get connected to it. The SPI protocol typically involves the following four key signals:
- MOSI (Master Output Slave Input): Data line through which the master sends data to the slave.
- MISO (Master Input Slave Output): Data line in which the slave sends data back to the master.
- SCK (Serial Clock): This clock signal synchronizes the data transfer between the master and the slave.
- SS/CS (Slave Select/Chip Select): The signal from the master to select the active slave device.
Working of SPI Communication Protocol
The master generates the clock signal (SCLK) and controls the communication.
Data transmission occurs bit-by-bit over MOSI and MISO.
Slave device selection uses the SS/CS line: When SS is low, the corresponding slave is active; other slaves remain inactive.
The master and slave exchange data in full-duplex mode, synchronized with the clock edges.
SPI Modes
The SPI communication protocol has two settings:
Clock Polarity (CPOL)
This controls the idle state of the clock line (SCK). There are two possibilities:
- CPOL = 0: The clock is low when idle (0V).
- CPOL = 1: The clock is high when idle (Vcc).
Clock Phase (CPHA)
It determines if the data is read on the rising or falling edge of the clock.
- CPHA = 0: Data is sampled on the leading edge of the clock (when the clock transitions from low to high).
- CPHA = 1: Data is sampled on the trailing edge of the clock (when the clock transitions from high to low).
Together, these two settings determine the timing of when the data clocks in and out, and they ensure compatibility between the master and slave.
| Mode | CPOL | CPHA | Clock Idle State | Data Sampling Edge |
|---|---|---|---|---|
| 0 | 0 | 0 | Low | Rising Edge |
| 1 | 0 | 1 | Low | Falling Edge |
| 2 | 1 | 0 | High | Falling Edge |
| 3 | 1 | 1 | High | Rising Edge |
Key Advantages of SPI Communication Protocol
The key features of SPI communication protocol are:
- Synchronous Communication: It is a communication method in which data transfer between devices is synchronized with a clock signal. The sender and receiver agree to transfer data at specific intervals controlled by a shared clock.
Master-Slave Architecture: In a Master-Slave Architecture, one device, known as the master, controls the communication flow and timing, while one or more other devices, called slaves, follow the master's instructions.
In a daisy-chaining configuration, slaves are in a series connection, and data passes from one slave to the next in a chain. Instead of having individual SS lines for each slave, one SS line is shared across all devices.
- Full-Duplex Communication: SPI communication protocol allows simultaneous data transmission and reception. In full-duplex communication, data transmission and reception occur simultaneously, meaning the data is sent and received in real time.
- Simple Hardware Interface: Requires four minimal signal lines. Unlike the I²C protocol, SPI does not use a device address, simplifying the communication.
- No Handshaking or Acknowledgment: SPI bus communication does not include built-in error checking, handshaking, or acknowledgment mechanisms, reducing communication overhead.
Multiple Slave Configuration Options
- Independent Slave Select Lines: Each slave device can have its own SS line for individual selection.
- Daisy-Chaining: Some systems allow slaves to be connected in a chain, reducing the number of SS lines needed.
Daisy-Chaining vs. Independent Slave Select (SS) Lines in SPI Communication Protocol
When implementing SPI bus communication in systems with multiple devices (slaves), you have two primary methods for managing different devices on the same bus: using independent SS (Slave Select) lines or daisy-chaining.
Independent Slave Select (SS) Lines
In a system with independent SS lines, each slave device has its own dedicated SS line (sometimes called CS for Chip Select). The master device communicates with one slave at a time by toggling the relevant SS line between the slave it wants to interact with.
MOSI (Master Out Slave In), MISO (Master In Slave Out), and SCK (Serial Clock) are shared among all slaves.
Each slave gets activated by bringing its SS line low (active low), which tells the slave to listen to the SPI bus protocol.
Advantages
Simple and Direct
Each slave device is independently addressed via its SS line. It allows easy communication with individual devices without worrying about collisions or sharing data paths.
Flexible
Each device can operate at different SPI clock speeds, modes (CPOL/CPHA), and configurations.
No Need for Special Firmware
The SPI controller can select the slave by toggling the SS line, and there's no need for the slaves to be aware of each other.
Disadvantages
More Pins Required
A trade-off is the need for more GPIO pins on the master. For example, if you have five slave devices, you will need five independent SS pins on the master, which could limit pin availability.
Scalability
As the number of slaves increases, the number of SS lines required also increases, which may not be feasible in space-constrained designs.
When to Choose Independent SS Lines
- Best for systems where speed and low latency are critical, and each device needs to operate with different configurations or speeds.
- It is useful in systems with fewer slaves or systems where the master has enough GPIO pins to handle all the slaves.
- Ideal for real-time applications where data from each slave needs to be processed separately and with minimal delay.
Daisy-Chaining (SPI Bus with Serial-to-Parallel or Parallel-to-Serial Shifting)
How it works:
Each slave device receives data from the MOSI line and passes the data to the next slave in the chain.
Slaves transmit data back to the master over the MISO line, where each slave's data is forwarded along the chain until the master receives it.
The SS line activates the first device in the chain. Once the first slave is activated, the master sends data through the SPI bus communication to the first slave via MOSI.
The first slave receives data, processes it, and passes the remaining bits down the line to the next slave.
Each slave passes data to the next in line until the final slave sends its data back to the master via MISO.
The master may need to use the SS line to control the first device in the chain or communicate with individual devices, depending on the implementation.
Advantages:
Fewer Pins Required
There is only one SS line, and the number of GPIO pins required on the master does not increase with the number of slaves. Hence, it is scalable for large systems with many devices.
Reduces Wiring Complexity
With fewer lines needed, the physical complexity of the system is reduced, making it easier to route and manage.
Simplifies Large-Scale Systems
For IoT devices with many peripherals, a daisy-chaining approach helps keep the system clean and simple, with fewer constraints on pin resources.
Disadvantages:
Potential for Increased Latency
Each slave in the chain must receive and forward data, which introduces latency. If you have many slaves, the total data transfer time will increase because each slave must process and pass the data sequentially.
Fixed Communication Speed
All slaves must communicate at the same SPI speed and clock mode. Different communication speeds or configurations for each device in a daisy-chained setup are difficult.
Complexity in Data Handling
Depending on the specific implementation, the master requires special logic to manage data transmission and understand what data belongs to which slave.
When to Choose Daisy-Chaining:
- It is best for systems with many slaves and limited GPIO pins or when the master pin count is constrained (e.g., in small or cost-sensitive designs).
- Suitable for non-time-critical systems where communication speed and latency are not as important.
- It is common in sensor networks or sensor arrays where devices with similar data rates are controlled altogether.
Comparison of SPI protocol with I2C and UART
| Feature | SPI (Serial Peripheral Interface protocol) | I2C (Inter-Integrated Circuit) | UART (Universal Asynchronous Receiver-Transmitter) |
|---|---|---|---|
| Speed | High (1 Mbps - 50+ Mbps) | Moderate (100 kbps - 3.4 Mbps) | Low to Moderate (9600 bps - 1 Mbps) |
| Duplex Mode | Full-Duplex (Simultaneous send and receive) | Half-Duplex (Master sends, slave responds) | Full-Duplex (Simultaneous send and receive) |
| Number of Devices | Multiple slaves (each with a separate SS line) | Multiple devices (up to 127 devices with 7-bit addressing) | Point-to-point (typically two devices) |
| Pin Requirements | 4 (MISO, MOSI, SCK, SS) | 2 (SDA, SCL) | 2 (TX, RX) |
| Bus Topology | Master-Slave | Multi-Master or Master-Slave | Point-to-Point |
| Signal Integrity | Requires impedance matching at high speeds; 50Ω line impedance | Requires pull-up resistors for SDA and SCL; bus capacitance ≤ 400 pF | Voltage standards (RS-232, TTL); RS-485 for longer distances |
| Clocking | Synchronized via SCK (Clock) | Synchronized via SCL (Clock) | Asynchronous, no clock signal |
| Voltage Levels | Typically 3.3V or 5V CMOS | Typically 3.3V or 5V, uses pull-up resistors | RS-232 (±12V) or TTL (0V, 3.3V, 5V) |
| Error Detection | No automatic error detection | ACK/NACK response to each byte | Optional parity bits for error detection |
| Data Integrity | No built-in CRC or checksum (optional CRC via hardware) | ACK/NACK and address resolution ensure basic integrity | Parity bit or checksum (optional) |
| Bus Capacitance/Distance | <10 cm for high-speed operation (signal integrity) | Bus capacitance ≤ 400 pF; short to moderate distance (typically <1m) | Short distances (RS-232 up to 5m, RS-485 up to 15m) |
| Timing | Clock-controlled (uses CPOL/CPHA for synchronization) | Timing is controlled by a master; slaves respond asynchronously | Asynchronous, Baud Rate defines timing |
| Clock Modes/Phase (CPOL, CPHA) | 4 modes (CPOL=0/1, CPHA=0/1) must match between master and slave | No clock phase control, uses SDA and SCL | No clock; Start/Stop bits for framing |
| Data Framing | Full-byte or multi-byte frames (up to 32-bits) | 8-bit address and data frames, 7-bit or 10-bit addressing | 8 data bits, optional parity bit, and stop bits (1 or 2) |
| Clock Stretching | No clock stretching, must synchronize clock rates | Yes, slaves can stretch the clock (hold SCL low) if needed | No clock; no clock stretching |
| Bus Contention | No bus contention (master controls) | Arbitration if multiple masters are present | No contention; point-to-point |
| Max Devices per Bus | Limited by number of SS lines; typically 1-4 (with modern MCU) | Up to 127 devices (7-bit addressing) or 1023 (10-bit addressing) | Typically, only two devices (point-to-point) |
| Data Transfer | Full-duplex, can transfer data simultaneously in both directions | Half-duplex, data sent one byte at a time | Full-duplex, data transmitted and received in sequence |
| Pin Complexity | Each slave requires a separate Slave Select (SS) line | Only 2 lines (SDA, SCL) can support multiple devices | Only 2 lines (TX, RX); no extra control lines |
| Error Handling | No built-in error detection, error handling at the protocol layer | Basic ACK/NACK for detecting successful byte reception | Parity, Framing errors, optional error handling |
| Bus Utilization | Efficient for high-speed peripherals, SD cards, displays | Efficient for low-speed, multi-device peripherals | Simple, point-to-point communication (e.g., GPS, Bluetooth) |
| Power Consumption | Moderate (more pins, higher speed) | Low (especially at low speeds) | Moderate (depends on the transmission rate and distance) |
| Cable Length | Short to moderate (<1 meter), limited by speed and signal integrity | Short to moderate (<1 meter), influenced by capacitance and pull-ups | RS-232: up to 5m; RS-485: up to 15m |
| Use Cases | High-speed peripherals (e.g., SD cards, displays, ADC) | Low-speed, multi-device systems (e.g., sensors, EEPROMs) | Simple, point-to-point communication (e.g., GPS, Bluetooth) |
| Flow Control | No flow control (unless implemented at the protocol layer) | No flow control (uses ACK/NACK) | RTS/CTS (hardware) or XON/XOFF (software) for flow control |
| Max Baud Rate | 50 Mbps+ | 3.4 Mbps (High-Speed mode) | 115200 bps (standard, can go up to 1 Mbps) |
| Typical Applications | Sensors, EEPROM, LCD displays, SD cards, DACs | Sensors, RTC, EEPROM, I2C-controlled peripherals | GPS, RS-232 serial devices, Bluetooth, debugging |
Closing Notes
The Serial Peripheral Interface protocol is a robust and flexible solution for high-speed, full-duplex communication between a master device and one or more slave peripherals. Its ability to support high data rates makes it ideal for applications requiring fast data transfer, such as sensor interfacing, memory devices, displays, and SD cards. SPI bus protocol offers simplicity in its design and is highly efficient for communication between devices, especially when compared to protocols like I2C or UART.
In summary, SPI Protocol is a high-performing protocol, but IoT engineers should weigh its pin usage, signal integrity concerns, and complexity in multi-device systems to choose it for specific IoT applications. When implemented correctly, it remains a go-to protocol for embedded systems requiring fast, reliable data transfer with low overhead.
Amusing Tech Chronicles
Facts and Anecdotes Related to this Edition of Wireless By Design
The Traffic Light Symphony
Think of SPI bus communication as a sophisticated traffic light system at a busy intersection. The master device is like the main traffic control center, while slave devices are the individual traffic lights. The SCLK line is like the master clock that keeps all lights changing in perfect synchronization, while MOSI and MISO are like two-way streets, allowing traffic (data) to flow simultaneously in both directions.
The Radio Station
The radio station (master) broadcasts data (music or information) to listeners (slaves). The broadcast happens at a fixed frequency (SCLK), and each radio (slave) must tune into the station it is set to listen to, similar to SPI's ability to communicate with a specific slave at a time.
Go Beyond and Explore
What is LPWAN ?
- Sensors (e.g., temperature, pressure, IMUs)
- Memory devices (e.g., EEPROMs, Flash)
- Displays (e.g., OLED, TFT)
- SD cards for storage
- DACs/ADCs for analog-to-digital and digital-to-analog conversion
- Wireless modules for communication
The primary difference between MQTT-SN (Message Queuing Telemetry Transport for Sensor Networks) and Lightweight M2M lies in their purpose and functionality.
MQTT-SN is a lightweight version of MQTT designed specifically for resource-constrained devices in sensor networks. It focuses on publish-subscribe messaging, enabling devices to send or receive messages through a broker efficiently. MQTT-SN is ideal for applications requiring real-time data exchange, such as telemetry and sensor monitoring.
LwM2M, on the other hand, is a device management and communication protocol. It provides a structured object-based model for managing IoT devices and handling tasks like device configuration, monitoring, firmware updates, and security. LwM2M uses CoAP over UDP for its communication and includes built-in security via DTLS.
While MQTT-SN is geared toward lightweight messaging, LwM2M offers a broader range of features for IoT device management and monitoring.
Author
Drishya Manohar
Sr. Associate - Content Marketing
