SPI uses 4 wires to communicate

The SPI is a serial bus, which means that data is sent in series one bit at a time. SPI uses 4 wires to communicate between devices:

• MISO (Master In Slave Out) – The data line from the Master device to the Slave device.
• MOSI (Master Out Slave In) – The data line from the slave device to the master device.
• SCK (Serial Clock) – Clock signal that synchronizes data transmission between two devices.
• CS (Chip Select) – Identifies which of the slave devices to activate.

The chip select line (CS) is a select line that allows the master device to choose one of the many slaves connected to it. The clock line (CLK) is used by all devices connected to the SPI bus to synchronize their actions with respect to each other.

Master out slave in (MISO) is an output pin on which data is sent from the master and which switches to the input pin of the slave device when data is to be received by the master from the slave device. Similarly, Master in slave out (MOSI) enters the master’s output pin when data comes from the outside world through this line and the slave’s input side when it has new information to transmit on the SPI bus.

SPI is a synchronous protocol

By being able to use hardware synchronization, SPI can support full duplex communications. SPI has been widely adopted as a standard by many microcontrollers and embedded processors due to its simplicity, speed and low cost. It uses 4 wires for communication between devices: MISO, MOSI, CLK and CS (chip select). The first two are used for data transfer from master to slave and the last two for data transfer from slave to master.

SPI is a synchronous protocol, which means that the master and slave devices must share a common clock. The master device initiates data transfers by sending a start bit (usually a low logic value), followed by the slave address and one or more data bits to be transferred. If there are several slave devices on the bus, each of them has its own address and must be addressed individually in order to read or write their data.

SPI a single master and multiple slave nodes

There is a separate chip select signal on each SPI device which allows multiple devices on a single bus to share the same clock line and data lines, but to be individually selected from their respective CS signals at any time. This allows you to build an SPI system with many devices.

The clock signal determines when the transmitter and receiver can send data. Each device connected to the SPI bus must select at least one slave select signal for each master select signal used by the respective master.

The Master device initiates data transfers and controls their timing. It generates the clock signals for each transaction, which are sent to all slave devices via their respective MOSI lines. The master then loads the data onto the SPI bus and sends it to its MISO line as soon as it receives a valid ACK from each slave device which confirms the receipt of its address bits to establish communication.

If there are multiple slave nodes, each node must have different chip select lines. For example, CS0 can be used to select the first slave device connected to SPI0_MISO and SPI0_MOSI. The second slave device can be selected using CS1 (which connects to SPI1_MISO and SPI1_MOSI).

CS2 can be used to select the third slave device, connected to SPI2_MISO and SPI2_MOSI. If a fifth or sixth slave node is added, CS3 is used instead of CS2:

CS0 -> SPI0_MISO, SPI0_MOSI
CS1 -> SPI1_MISO, SPI1_MOSI
CS2 -> SPI2_MISO, SPI2_MOSI
CS3 -> SPI3_MISO, SPI3_MOSI

Conclusion

The SPI protocol is an efficient communication method that can be used in many applications. It has been implemented by many controllers and microcontrollers such as ATmega328P, which supports SPI communication. The SPI interface can be used in many applications such as ADC conversion, data transmission between microcontrollers and more. This article covered the basics of SPI communication and its implementation. If you are interested in learning more about this topic, stay tuned.