Source: Automotive ECU Design

When you connect a microcontroller to sensors, displays, or other modules, have you considered how these two devices communicate with each other? What exactly are they saying?

In fact, communication between electronic devices is like communication between humans; both parties need to speak the same language. In electronic products, these languages are called communication protocols. First, we will start with some basic concepts and then explain the working principle of SPI in detail.

01

Serial vs Parallel

Electronic devices communicate by sending data bits back and forth. Bits are binary and can only be 1 or 0. Through rapid voltage changes, bits are transmitted from one device to another. In systems operating at 5V, “0” is communicated through a short pulse at 0V, while “1” is communicated through a short pulse at 5V.

Data bits can be transmitted in parallel or serial form. In parallel communication, data bits are transmitted simultaneously across multiple lines. The following diagram shows the parallel transmission of the letter “C” in binary (01000011):

Detailed Explanation of SPI, UART, and I2C Buses

In serial communication, bits are sent one by one through a single wire. The following diagram shows the serial transmission of the letter “C” in binary (01000011):

02

SPI Bus

SPI is a common general-purpose communication protocol for devices. It has a unique advantage of allowing uninterrupted data transmission, enabling the continuous sending or receiving of any number of bits. In contrast, I2C and UART send data in packets with a limited number of bits.

In SPI devices, there are master and slave systems. The master is the controlling device (usually a microcontroller), while the slave (typically a sensor, display, or memory chip) receives commands from the master. A complete SPI communication setup includes four signal lines:

MOSI (Master Output/Slave Input) – Signal line, master outputs, slave inputs.

MISO (Master Input/Slave Output) – Signal line, master inputs, slave outputs.

SCLK (Clock) – Clock signal.

SS/CS (Slave Select/Chip Select) – Chip select signal.

SPI Protocol Features

In practice, the number of slaves is limited by the system’s load capacitance, which reduces the master’s ability to switch accurately between voltage levels.

Working Principle

Clock Signal

One bit of data is transmitted per clock cycle, so the speed of data transmission depends on the frequency of the clock signal. The clock signal is generated by the master, so SPI communication is always initiated by the master.

Any communication protocol that shares a clock signal among devices is called synchronous. SPI is a synchronous communication protocol, while some asynchronous communications do not use a clock signal. For example, in UART communication, both sides are set to a pre-configured baud rate, which determines the speed and timing of data transmission.

Chip Select Signal

The master enables communication by pulling the slave’s CS/SS low. In idle/non-transmission state, the chip select line remains high. There can be multiple CS/SS pins on the master, allowing it to communicate with multiple different slaves.

If the master has only one chip select pin available, it can connect these slave devices in the following manner:

MOSI and MISO

The master sends data to the slave via MOSI, and the slave can also send data back to the master via MISO, allowing both to occur simultaneously. Thus, theoretically, SPI is a full-duplex communication protocol.

Transmission Steps

1. The master outputs the clock signal.

2. The master pulls the SS/CS pin low to activate the slave.

3. The master sends data to the slave via MOSI.

4. If a response is needed, the slave sends data back to the master via MISO.

There are some advantages and disadvantages to using SPI, and if choosing between different communication protocols, careful consideration should be given based on project requirements.

Advantages and Disadvantages

Advantages

SPI communication has no start or stop bits, allowing data to flow continuously without interruption; it does not have a complex slave addressing system like I2C, and the data transmission rate is higher than that of I2C (almost twice as fast). Independent MISO and MOSI lines allow for simultaneous data sending and receiving.

Disadvantages

SPI uses four lines (while I2C and UART use two), lacks confirmation of successful signal reception (I2C has this feature), and does not have any form of error checking (such as parity bits in UART).

UART stands for Universal Asynchronous Receiver/Transmitter, also known as serial communication. Unlike communication protocols like SPI and I2C, it is a physical circuit in the microcontroller or an independent IC. The main purpose of UART is to send and receive serial data, and its best advantage is that it only uses two wires to transmit data between devices. The principle of UART is easy to understand, but if you haven’t read about the SPI communication protocol, it might be a good starting point.

03

UART Communication

In UART communication, two UARTs communicate directly with each other. The sending UART converts the parallel data from the controlling device (like a CPU) into serial form and sends it to the receiving UART. Only two wires are needed to transmit data between the two UARTs, with data flowing from the Tx pin of the sending UART to the Rx pin of the receiving UART:

UART is an asynchronous communication, which means there is no clock signal; instead, start and stop bits are added to the data packet. These bits define the start and end of the data packet, so the receiving UART knows when to read this data.

When the receiving UART detects the start bit, it reads the bits in the data frame at a specific baud rate. Baud rate is a measure of the speed of data transmission, expressed in bits per second (bps). The two UARTs must operate at approximately the same baud rate, with the baud rate between the sending and receiving UARTs differing by no more than about 10%.

Working Principle

After the sending UART obtains the parallel data from the data bus, it adds a start bit, a parity bit, and a stop bit to form a data packet, which is then serially output bit by bit from the Tx pin. The receiving UART reads the data packet bit by bit on its Rx pin.

UART data consists of 1 start bit, 5 to 9 data bits (depending on UART), an optional parity bit, and 1 or 2 stop bits:

Start Bit

The UART data transmission line typically remains at a high voltage level when no data is being transmitted. When transmission begins, the sending UART pulls the transmission line from high to low within one clock cycle, and when the receiving UART detects the transition from high to low voltage, it begins reading the bits in the data frame at the baud rate.

Data Frame

The data frame contains the actual data being transmitted. If a parity bit is used, it can be 5 bits, up to 8 bits. If no parity bit is used, the length of the data frame can be 9 bits.

Parity Bit

The parity bit is a way for the receiving UART to determine if any data changes occurred during transmission. After the receiving UART reads the data frame, it counts the number of bits that are 1 and checks whether the total is even or odd, and whether it matches the data.

Stop Bit

To signal the end of the data packet, the sending UART drives the data transmission line from low voltage to high voltage for at least two bit times.

Transmission Steps

1. The sending UART receives parallel data from the data bus:

2. The sending UART adds the start bit, parity bit, and stop bit to the data frame:

3. The entire data packet is serially sent from the sending UART to the receiving UART. The receiving UART samples the data line at the pre-configured baud rate:

4. The receiving UART discards the start bit, parity bit, and stop bit from the data frame:

5. The receiving UART converts the serial data back to parallel data and transmits it to the data bus of the receiving end:

Advantages and Disadvantages