RM0008 Universal synchronous asynchronous receiver transmitter (USART)
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24.3.5 Multiprocessor communication
There is a possibility of performing multiprocessor communication with the USART (several
USARTs connected in a network). For instance one of the USARTs can be the master, its
TX output is connected to the RX input of the other USART. The others are slaves, their
respective TX outputs are logically ANDed together and connected to the RX input of the
master.
In multiprocessor configurations it is often desirable that only the intended message
recipient should actively receive the full message contents, thus reducing redundant USART
service overhead for all non addressed receivers.
The non addressed devices may be placed in mute mode by means of the muting function.
In mute mode:
● None of the reception status bits can be set.
● All the receive interrupts are inhibited.
● The RWU bit in USART_CR1 register is set to 1. RWU can be controlled automatically
by hardware or written by the software under certain conditions.
The USART can enter or exit from mute mode using one of two methods, depending on the
WAKE bit in the USART_CR1 register:
● Idle Line detection if the WAKE bit is reset,
● Address Mark detection if the WAKE bit is set.
Idle line detection (WAKE=0)
The USART enters mute mode when the RWU bit is written to 1.
It wakes up when an Idle frame is detected. Then the RWU bit is cleared by hardware but
the IDLE bit is not set in the USART_SR register. RWU can also be written to 0 by software.
An example of mute mode behavior using idle line detection is given in Figure 242.
Figure 242. Mute mode using Idle line detection
Address mark detection (WAKE=1)
In this mode, bytes are recognized as addresses if their MSB is a ‘1’ else they are
considered as data. In an address byte, the address of the targeted receiver is put on the 4
LSB. This 4-bit word is compared by the receiver with its own address which is programmed
in the ADD bits in the USART_CR2 register.
RWU written to 1
Data 1 IDLE
RX
Data 2 Data 3 Data 4 Data 6Data 5
RWU
Mute Mode Normal Mode
Idle frame detected
RXNE RXNE
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