The document discusses the STM32 USART (Universal Synchronous/Asynchronous Receiver/Transmitter). It can operate synchronously or asynchronously and in full-duplex mode. Synchronous operation involves transmitting data in frames with character boundaries, while asynchronous operation uses start and stop bits to encode timing properties. The USART can be used for serial communication and interfaces like I2C, RS-232, and Ethernet. It involves transmitting one bit at a time compared to parallel communication which transmits multiple bits simultaneously. The USART includes features like interrupt requests and a data frame format involving start, data, parity, and stop bits.

1. STM32 USART
(Universal Synchronous/Asynchronous
Receiver/Transmitter)

2. What is USART?
A universal synchronous and asynchronous receiver-
transmitter (USART) is a type of a serial interface device
that can be programmed to communicate asynchronously
or synchronously.

3. Synchronous Operation
The operation of a USART is intimately related to the various protocols like
• USARTs in synchronous mode transmits data in frames. In synchronous
operation, characters must be provided on time until a frame is complete; if the
controlling processor does not do so, this is an "underrun error" and
transmission of the frame is aborted.
• USARTs operating as synchronous devices used either character-oriented or
bit-oriented mode. In character (STR and BSC) modes, the device relied on
particular characters to define frame boundaries; in bit (HDLC and SDLC) modes
earlier devices relied on physical-layer signals, while later devices took over the
physical-layer recognition of bit patterns.
• A synchronous line is never silent; when the modem is transmitting, data is
flowing.

5. Asynchronous Operation
Asynchronous serial communication in its most primitive form is implemented
over a symmetric pair of wires connecting two devices e.g. host and target.
Whenever the host has data to send to the target, it does so by sending an
encoded bit stream over its transmit (TX) wire; this data is received by the
target over its receive (RX) wire. Similarly, when the target has data to send to
the host it transmits the encoded bit stream over its TX wire and this data is
received by the host over its RX wire.
This arrangement is illustrated in Figure 5.1. This mode of communications is
called “asynchronous” because the host and target share no time reference.
Instead, temporal properties are encoded in the bit stream by the transmitter
and must be decoded by the receiver.

6. STM32 USART Introduction
 STM32 USART Can realize
1. Synchronous or asynchronous Full-duplex
communication
2. Synchronous one-way communication
3. Half-duplex single line communication.

7. Communication Interface
 For exchanging information with another IC or
external devices, e.g. I2C,RS-232,USB,Ethernet…
 Serial communication：
A single bit is transferred at a time
 Synchronization: A clock signal is sent along with the data; the device
that generates the clock is called the master and other devices are
slaves.
 Asynchronous: No clock is transmitted, fewer wires.
 Parallel communication:
Multiple bits are transferred at a time

8. I2C is a multi-master bus, which means that multiple chips can be
connected to the same bus and each one can act as a master by
initiating a data transfer.
RS-232
I2C

9. How Parallel Communication Works
In parallel communication an entire byte of data is transmitted at a
time. That is each bit has dedicated line. Thus for 8-bit data transfer
we need 8 dedicated lines as shown above.

10. How Serial Communication works
 Simplex communication
Can only have one direction of
communication without the reverse
the direction of the interaction.
 Half-duplex Communication
Both sides of the communication
can send information, but Both can not
be sent at the same time (and of
course cannot be received at the same
time).
 Full-duplex Communication
Both sides of the communication
can send and receive information at
the same time.

11. STM32 System structure

12. What is UART?
A universal asynchronous receiver-transmitter (UART) is a computer
hardware device for asynchronous serial communication in which the
data format and transmission speeds are configurable.
The electric signaling levels and methods are handled by a driver circuit
external to the UART.
A UART is usually an individual (or part of an) integrated circuit (IC)
used for serial communications over a computer or peripheral device
serial port. UARTs are now commonly included in microcontrollers. A
related device, the universal synchronous and asynchronous receiver-
transmitter (USART) also supports synchronous operation.

13.  RX： Receiving Data Pins
 TX： Send Data Pins
 SCLK： Send clock output
 Equipment for controlling shift registers
 Clock polarity and phase programmable
 nCTS：
 Hardware flow control pins
 High power, indicating that data is sent after the current data
transfer ends.
 nRTS：
 Hardware flow control pins
 Low electricity peacetime, said USART ready to receive data
USART Module Pins
The following pins are required in IrDA mode:
IrDA_RDI: Data entry in IrDA mode.
IrDA_TDO: Data output in IrDA mode.

14. Asynchronous communication applications
1 Close-range communication between
chips
2 Communication with PC
3 Long-distance communication between modules

15. Parity Bit
A parity bit or check bit, is a bit added to a string of
binary code to ensure that the total number of 1-bits in
the string is even or odd. Parity bits are used as the
simplest form of error detecting code.
There are two variants of parity bits: even parity bit and odd parity bit.

16. USART Communication process
 Data receiving process schematic diagram
 Data transmission process diagram

17.  USART Data Frame composition
 Starting bit (1-bit)
 Data Words (8/9 bits，M@USART_CR1）
 It contains the parity bit (0/1-bit，PCE@USART_CR1）
 Stop bit (0.5/1/1.5/2 bit，STOP@USART_CR2）
8 Bit Data
9 Bit Data
Data Frame Format

18. USART Interrupt Request
In a computer, an interrupt request (or IRQ) is
hardware signal sent to the processor that
temporarily stops a running program and allows
a special program, an interrupt handler, to run
instead.
Advantages of using interrupts-
 No polling or waiting for data
 Background tasks easier
 Fast response
 Reduced chance of overflow errors
 More common for receiving data

19. 2023/1/24 19
In computing, interrupt latency is the time that
passes from when an interrupt is generated to when
the source of the interrupt is serviced.
For many operating systems, devices are serviced/operated
as soon as the device's interrupt handler/ISR is executed.
Interrupt latency