What is SPI? Serial Peripheral Interface Explained

SPI can be used in a wide variety of applications, from sensors to data storage on SD cards. Learn how SPI presents an advantage in certain applications

What is SPI?

SPI stands for Serial Peripheral Interface—it’s a de facto synchronous communication bus standard. Developed by Motorola in the 1980s, SPI boasts both simple implementation and high-speed data transfer capability. For these reasons, we can see SPI at use in a wide variety of applications, from sensors to camera lens control, communications, and data storage on SD cards. SPI communication takes place in a master/slave configuration, where a single master can choose between one or more slave devices.

SPI Protocol and Bus Configuration

Sometimes known as a four-wire serial bus, SPI nominally requires that you use four data pins on a master controller. Here’s a quick guide to SPI’s configuration.

1. An SCLK, or Serial Clock signal, is output from the master device. The SCLK specifies when data bits will be transmitted.

2. MOSI—Master Output Slave Input—transmits data from the master to a slave, while MISO—Master Input Slave Output—transmits data from the slave to the master.

3. While the master generates a single clock, MOSI and MISO pins can transmit at the same time when necessary, allowing for full-duplex operation.

4. A fourth line, known as SS—Slave Select—specifies which slave is active on a bus. SS allows the bus to use multiple slaves, as needed.

When a user implements multiple slaves on a bus, each slave will share the SCLK, MOSI, and MISO lines. However, each of them will need a separate SS line. Having a separate SS line means two things:

- The only limit to the number of slaves is the output capacity of the master controller.

- No formal addressing scheme is necessary.

Practically speaking, SPI is intended for short-distance applications, and the number of SS pins will eventually become unwieldy, making this technology appropriate for a limited number of slave nodes.

SPI Speed and Hardware Options

When it comes to hardware, SPI is a straightforward interface method to implement, and with no specified clock limit, it’s possible to achieve very high speeds. Power requirements are also very low, and communication commonly takes place at 3.3 or 5V. As a de facto standard, users can adapt SPI to suit their application’s needs. These adaptations can include:

- Creating different word lengths

- Adding an extra flow-control pin

- Using other interfaces that combine the MOSI and MOSI lines into one

- Daisy-chaining hardware components together to pass data along with the MISO linked to one device and the MOSI linked to another, a common technique in addressable LED drivers

SPI’s flexibility presents an advantage in certain applications, but it could also lead to incompatibilities if users don’t carefully specify each component.

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