Multiprocessor Communication; Figure 235. Mute Mode Using Idle Line Detection - STMicroelectronics STM32F05 series Reference Manual

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25.5.7

Multiprocessor communication

It is possible to perform multiprocessor communication with the USART (with several
USARTs connected in a network). For instance one of the USARTs can be the master, its
TX output connected to the RX inputs of the other USARTs. 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 order to use the mute mode feature, the MME bit must be set in the USART_CR1 register.
In mute mode:
●
None of the reception status bits can be set.
●
All the receive interrupts are inhibited.
●
The RWU bit in USART_ISR register is set to 1. RWU can be controlled automatically
by hardware or by software, through the MMRQ bit in the USART_RQR register, 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 MMRQ bit is written to 1 and the RWU is
automatically set.
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_ISR register. An example of mute mode behavior using
Idle line detection is given in

Figure 235. Mute mode using Idle line detection

RX
RWU
MMRQ written to 1
Note: If the MMRQ is set while the IDLE character has already elapsed, mute mode will not be
entered (RWU is not set).
If the USART is activated while the line is IDLE, the idle state is detected after the duration
of one IDLE frame (not only after the reception of one character frame).
Universal synchronous asynchronous receiver transmitter (USART)
Figure 235.
Data 1 Data 2 Data 3
Mute Mode
Doc ID 018940 Rev 1
Data 4 IDLE
Idle frame detected
RXNE RXNE
Data 5 Data 6
Normal Mode
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