SPI (Serial Peripheral Interface) is a synchronous serial communication protocol used for short-distance communication between microcontrollers and peripheral devices, such as sensors and displays.
It typically requires four GPIOs (bidirectional communication), but this can sometimes be reduced to three GPIOs (in one-directional communication, i.e. microcontroller to display).
Displays (and similar peripherals) may on the other hand need more than four GPIOs to be connected: they often require an additional Reset Line (RST) (to reset the display to a known state), and a Data/Command (DC) Line, to distinguish between screen content (pixel information) and commands (control instructions) for the screen.
Both RST and DC are not really part of SPI though. More accurately, connecting displays often requires SPI plus display-specific lines such as RST and DC.
SPI is always used when large amounts of data need to be transferred fast. The much simpler I2C interface is often used when *much smaller amounts of data need to be transferred, i.e. a temperature reading, because I2C is much slower than SPI.
Overview
SPI uses three or four wires to transfer data with a maximum speed of up to 80MBit/s.
Master And Slave(s)
SPI uses a Master/Slave paradigm: one master (i.e. the microcontroller) communicates with a number of slaves (i.e. sensors, displays, and other peripherals conntected to the microcontroller).
The master is always generating the clock signal.
Dedicated Chip Select Lines
A dedicated chip select line from master to each client is used to select the slave that the master wants to contact. In SPI, slaves are selected via hardware (CS is pulled physically low to enable communication with a particular slave). In I2C, on the contrary, each device has its own unique address.
High GPIO Cost
SPI does not scale well with the number of slaves: each additional slave requires its own CS line, and each additional CS line requires an additional GPIO at the master.
The true hardware cost on the master side is three GPIO to support SPI plus one additional GPIO per slave.
This cost can be alleviated by using more efficient solutions such as a multiplexer (like the one below) that requires just one GPIO and can select one out of a large number of output pins.
Daisy-Chain Method
Daisy chaining the Chip Select (CS) lines is a proprietary method in certain use cases with a high number of slaves: it can share the CS line while still being individually addressable.
In essence, this approach implements device addresses (that normally do not exist in SPI) and makes SPI behave similar to device address-enabled interfaces such as I2C. Since SPI does not natively support this, it must be implemented by all devices as part of proprietary solutions.
In a nutshell, typical SPI devices require individual CS lines and do not have built-in addresses nor do they support address selection.
Flexible Speed
SPI is clock-based, so the actual data transfer speed can be adjusted and depends on the clock speed. ESP32 is capable of using clock speeds of up to 80MHz.
Slower microcontrollers or peripherals can negotiate and reduce the clock speed to any frequency that is needed or supported. This limits the data transfer rate while at the same time increasing the robustness of data transfer which can also be helpful in rough and noisy environments.
Short Distance
SPI connections are designed for short-distance communication within a circuit board or between closely located components, typically within one device. Connections are typically just a few centimeters long.
While SPI was not designed to be used over longer distances (and there are much better suited other interfaces and protocols like Ethernet), longer SPI connections can be used by lowering the data rate, using better wires, and adding shielding.
Pins
SPI requires a minimum of three or four wires.
Due to increased sensitivity in society, terms like master and slave have become controversial, even in microelectronics. That’s why pin labels that were introduced in the 1980s may be renamed in new devices: Master Out Slave In (MOSI) for example is now also known as Serial Data In (SDI). You’ll find all commonly used pin labels listed below.
Four-Wire
The four-wire connection is the default SPI setup and enables true full duplex communication:
| Pin | Common Labels | Description |
|---|---|---|
| Clock | CLK, SCL, SCLK | signal sent by master to synchronize data and set the data trasfer speed |
| Chip Select | CS, SS | signal used by master to select a slave |
| Master Out Slave In | MOSI, SDO, SDA | Data sent by master |
| Master In Slave Out | MISO, SDI | Data received by master |
If you come across a pin DC, then this is not part of SPI. A Data/Command (DC) pin is used by displays (low for command, high for data) to distinguish between screen content and device commands.
Three-Wire
Three-wire (aka Single-Wire SPI or Reduced Pin SPI) is used in scenarios where the number of wires and GPIOs are limited, or where data primarily travels in one direction only (like with displays).
In essence, the three wire setup combines the dedicated MOSI (Master Out Slave In) and MISO (Master In Slave Out) lines in one single data line (often called SDIO). In three-wire setups, you are loosing full duplex capabilities: data can no longer be transmitted in both directions at the same time.
Data Communication
Data communication over SPI follows these steps:
- Clock Signal: Master starts sending a clock signal, setting the data speed.
- Slave Select: Master selects the slave device it wants to communicate with by pulling this line low. The slave device with the low CS line starts listening.
- Sending Data: Master can now send data via MOSI to the slave. The slave can send back data at the same time via MISO.
- Closing Connection: Once the master sets back the CS signal to high, the slave stops listening, and the communication ends.
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(content created May 07, 2024 - last updated Oct 16, 2024)