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.
Previously, articles on SPI, UART, and I2C communication were shared separately. This article compares them.
Serial vs Parallel
Electronic devices communicate by sending data bits to each other. A bit is binary, which can only be 1 or 0. Through rapid changes in voltage, bits are transmitted from one device to another. In a 5V operating system, “0” is communicated through short pulses at 0V, while “1” is communicated through short pulses at 5V. Data bits can be transmitted in parallel or serial form. In parallel communication, data bits are transmitted simultaneously on multiple lines. The image below shows the parallel transmission of the letter “C” in binary (01000011):
In serial communication, bits are sent one by one over a single wire. The image below shows the serial transmission of the letter “C” in binary (01000011):
SPI Communication
SPI is a common general-purpose communication protocol. Its unique advantage is that it can transmit data without interruption, allowing for continuous sending or receiving of any number of bits. In contrast, I2C and UART send data in packets, which have a limited number of bits. In SPI devices, the devices are divided into master and slave systems. The master is the controlling device (usually a microcontroller), while the slave (usually a sensor, display, or memory chip) receives commands from the master. A complete SPI communication 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.
Characteristics of SPI Protocol In fact, the number of slaves is limited by the load capacitance of the system, 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 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 parties 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.Multiple CS/SS pins can exist on the master, allowing communication with multiple different slaves.
If the master has only one chip select pin available, the slaves can be connected as follows:
MOSI and MISO
The master sends data to the slave serially via MOSI, and the slave can also send data back to the master via MISO, allowing both to occur simultaneously. Therefore, theoretically, SPI is a full-duplex communication protocol.Transmission Steps1. 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 returns data to the master via MISO
Using SPI has its advantages and disadvantages, and when choosing between different communication protocols, careful consideration should be based on project requirements.Advantages and Disadvantages
Advantages
SPI communication has no start and stop bits, allowing for continuous data flow without interruption; it does not have a complex slave addressing system like I2C, and the data transmission rate is higher than I2C (almost twice as fast). Independent MISO and MOSI lines allow for simultaneous sending and receiving of data.
Disadvantages
SPI uses four lines (while I2C and UART use two), does not confirm successful signal reception (I2C has this feature), and has no 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 microcontrollers or an independent IC. The main purpose of UART is to send and receive serial data, its best advantage is that it only uses two lines to transmit data between devices. The principle of UART is easy to understand, but if you haven’t read about the SPI communication protocol, that might be a good starting point.
UART Communication
In UART communication, two UARTs communicate directly with each other. The transmitting UART converts parallel data from the controlling device (like a CPU) into serial form and sends it to the receiving UART. Only two lines are needed to transmit data between two UARTs, with data flowing from the Tx pin of the transmitting UART to the Rx pin of the receiving UART:
UART is 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, allowing the receiving UART to know when to read the data.
When the receiving UART detects the start bit, it will read the bits in the data frame at a specific baud rate. The baud rate is a measure of the speed of data transmission, expressed in bits per second (bps). Both UARTs must operate at approximately the same baud rate, with a maximum difference of about 10% between the transmitting and receiving UARTs.
Working Principle
After the transmitting UART obtains parallel data from the data bus, it adds a start bit, a parity bit, and a stop bit to form the data packet and serially outputs it from the Tx pin, while the receiving UART reads the data packet bit by bit on its Rx pin.
UART data includes one start bit, 5 to 9 data bits (depending on the UART), an optional parity bit, and one or two stop bits:
Start Bit:
UART data transmission lines typically remain at a high voltage level when not transmitting data. When starting transmission, the transmitting UART pulls the transmission line low within one clock cycle, and when the receiving UART detects the transition from high voltage to low voltage, it begins to read 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 reading the data frame, the receiving UART counts the number of bits with a value of 1 and checks whether the total is even or odd, matching it with the data.
Stop Bit: To signal the end of the data packet, the transmitting UART drives the data transmission line from low voltage to high voltage for at least the duration of two bits.
Transmission Steps The transmitting UART receives data in parallel from the data bus:
2. The transmitting UART adds the start bit, parity bit, and stop bit to the data frame:
3. The entire data packet is serially sent from the transmitting 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 No communication protocol is perfect, but UART excels at its job. Here are some pros and cons to help you determine if they fit your project needs:
Advantages
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Only uses two wires
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No clock signal required
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Has parity bits for error checking
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As long as both sides have the packet structure set up
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Well-documented and widely used method
Disadvantages
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Maximum data frame size is 9 bits
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Does not support multiple slave systems or multiple master systems
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The baud rate of each UART must be within 10% of each other
I2C Communication
The I2C bus is a simple, bidirectional, two-wire synchronous serial bus developed by Philips. It only requires two wires to transmit information. It combines the advantages of SPI and UART, allowing multiple slaves to connect to a single master (like SPI) while also enabling multiple masters to control one or more slaves. This is very useful when you want multiple microcontrollers to log data to a single storage card or display text on a single LCD.
SDA (Serial Data) – data line. SCL (Serial Clock) – clock line. I2C is a serial communication protocol, so data is transmitted bit by bit along the SDA. Like SPI, I2C also requires a clock synchronization signal, and the clock is always controlled by the master.
Working Principle I2C data transmission occurs in the form of multiple messages, each containing the binary address frame of the slave, as well as one or more data frames, including start and stop conditions, read/write bits, and ACK/NACK bits between the data frames:
Start Condition: When SCL is high, SDA switches from high to low.Stop Condition: When SCL is high, SDA switches from low to high.
Address Frame: A unique 7-bit or 10-bit sequence for each slave device used for address identification between master and slave devices.Read/Write Bit: A bit that is low if the master is sending data to the slave and high if requesting data.ACK/NACK: Each frame in the message is followed by an ACK/NACK bit. If the address frame or data frame is successfully received, the receiving device returns an ACK bit to indicate confirmation.
Addressing
Since I2C does not have chip select lines like SPI, it needs to use another way to confirm a specific slave device, which is addressing. The master sends the address of the slave it wants to communicate with to each slave, and each slave compares it with its own address. If the address matches, it sends a low-level ACK bit back to the master. If it does not match, no action is taken, and the SDA line remains high.
Read/Write Bit
The end of the address frame contains a read/write bit. If the master is sending data to the slave, it is low. If the master is requesting data from the slave, it is high.
Data Frame
Once the master detects the ACK bit from the slave, it can send the first data frame. The data frame is always 8 bits, followed by an ACK/NACK bit to verify the reception status. When all data frames are sent, the master can send a stop condition to terminate communication.Transmission Steps1. When the SCL line is high, the master starts bus communication by switching the SDA line from high to low.
2. The master sends the 7-bit or 10-bit address of the slave to communicate with, along with the read/write bit:
3. Each slave compares the address sent by the master with its own address. If the address matches, the slave returns an ACK bit by pulling the SDA line low. If the master’s address does not match the slave’s address, the slave pulls the SDA line high.
4. The master sends or receives data frames:
5. After transmitting each data frame, the receiving device returns another ACK bit to the sender to confirm successful receipt of that frame:
6. The master then switches SCL to high and then switches SDA to high, sending a stop condition to the slave.
Single Master vs Multiple Slaves
Because I2C uses addressing functionality, a single master can control multiple slaves. When using a 7-bit address, a maximum of 128 (2^7) unique addresses can be used. The use of a 10-bit address is uncommon but can provide 1,024 (2^10) unique addresses. When connecting multiple slaves to a single master, use a 4.7K ohm pull-up resistor to connect them, for example, connecting the SDA and SCL lines to Vcc:
Multiple Masters vs Multiple Slaves
I2C supports multiple masters connected to multiple slaves simultaneously. Problems can arise when two masters attempt to send or receive data simultaneously over the SDA line. Therefore, each master must check the SDA line before sending a message to see if it is low or high. If the SDA line is low, it means another master is controlling the bus. If the SDA line is high, it is safe to send data. When connecting multiple masters to multiple slaves, use a 4.7K ohm pull-up resistor to connect the SDA and SCL lines to Vcc:
Advantages and Disadvantages Compared to other protocols, I2C may sound complex. Here are some pros and cons to help you determine if they fit your project needs:Advantages Only uses two wires, supports multiple masters and multiple slaves. The hardware is simpler than UART. A well-known and widely used protocol.Disadvantages Data transmission rate is slower than SPI. Data frame size is limited to 8 bits.
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