The SPI interface (Serial Peripheral Interface) is one of the most common interfaces used to connect peripherals to microcontrollers. It is a very fast interface that can transfer large amounts of data.
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. the sensors, displays, and other components conntected to it).
The master is always the party that generates 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).
High GPIO Cost
SPI does not scale well with an increasing 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 Chip Select (CS) lines in SPI (Serial Peripheral Interface) is a proprietary method in certain use cases with a high number of slaves to share the CS line while still being individually addressable.
In essence, this approach implements device addresses (that typically 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 conclusion, 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 are 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.
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 |
Occasionally, you may come across a pin labeled DC. If present, it differentiates the input between data and commands (low for command, high for data). Often, this pin is found with displays in order to help differentiate 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.
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.
Many devices do not require full duplex communications. For example, displays are pure receivers and never send back data to the microcontroller. In these scenarios, wasting resources on wires and GPIOs that are never used makes no sense, and a three-wire SPI is often the better choice.
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.
Comments
Please do leave comments below. I am using utteran.ce, an open-source and ad-free light-weight commenting system.
Here is how your comments are stored
Whenever you leave a comment, a new github issue is created on your behalf.
-
All comments become trackable issues in the Github Issues section, and I (and you) can follow up on them.
-
There is no third-party provider, no disrupting ads, and everything remains transparent inside github.
Github Users Yes, Spammers No
To keep spammers out and comments attributable, all you do is log in using your (free) github account and grant utteranc.es the permission to submit issues on your behalf.
If you don’t have a github account yet, go get yourself one - it’s free and simple.
If for any reason you do not feel comfortable with letting the commenting system submit issues for you, then visit Github Issues directly, i.e. by clicking the red button Submit Issue at the bottom of each page, and submit your issue manually. You control everything.
Discussions
For chit-chat and quick questions, feel free to visit and participate in Discussions. They work much like classic forums or bulletin boards. Just keep in mind: your valued input isn’t equally well trackable there.
(content created May 07, 2024)