Microchip Technology megaAVR 0 Series Manual page 295

the new clock divider and the six Least Significant bits of this value (the remainder) gives the new
fractional part. When the sync field has been received and all bits are found valid, the clock divider and
the fractional part are updated in the Baud Rate Generator register (USARTn.BAUD). After the break and
sync fields, n characters of data can be received.
When the USART receive mode is set to GENAUTO mode, a generic Auto-baud mode is enabled. In this
mode, there are no checks of the sync character to equal 0x55. After detection of a break field, the
USART expects the next character to be a sync field, counting eight low and high bit times. If the
measured sync field results in a valid BAUD value (0x0064-0xffff), the BAUD register is updated. Setting
the Wait for Break bit (WFB in USARTn.STATUS) before receiving the next break character, the next
negative plus positive edge of RxD line is detected as a break. This makes it possible to set an arbitrary
new baud rate without knowing the current baud rate.
22.3.2.9 Multiprocessor Communication Mode
The Multiprocessor Communication mode (MCPM) effectively reduces the number of incoming frames
that have to be handled by the receiver in a system with multiple microcontrollers communicating via the
same serial bus. This mode is enabled by writing a '1' to the MCPM bit in the Control B register
(USARTn.CTRLB). In this mode, a dedicated bit in the frames is used to indicate whether the frame is an
address or data frame type.
If the receiver is set up to receive frames that contain five to eight data bits, the first Stop bit is used to
indicate the frame type. If the receiver is set up for frames with nine data bits, the ninth bit is used to
indicate frame type. When the frame type bit is one, the frame contains an address. When the frame type
bit is zero, the frame is a data frame. If 5- to 8-bit character frames are used, the transmitter must be set
to use two Stop bits, since the first Stop bit is used for indicating the frame type.
If a particular slave MCU has been addressed, it will receive the following data frames as usual, while the
other slave MCUs will ignore the frames until another address frame is received.
Using Multiprocessor Communication Mode
The following procedure should be used to exchange data in Multiprocessor Communication mode
(MPCM):
1. All slave MCUs are in Multiprocessor Communication mode.
2. The master MCU sends an address frame, and all slaves receive and read this frame.
3. Each slave MCU determines if it has been selected.
4. The addressed MCU will disable MPCM and receive all data frames. The other slave MCUs will
ignore the data frames.
5. When the addressed MCU has received the last data frame, it must enable MPCM again and wait
for a new address frame from the master.
The process then repeats from step 2.
Using any of the 5- to 8-bit character frame formats is impractical, as the receiver must change between
using n and n+1 character frame formats. This makes full-duplex operation difficult since the transmitter
and receiver must use the same character size setting.
22.3.2.10 IRCOM Mode of Operation
The IRCOM mode enables IrDA
kbps. When IRCOM mode is enabled, Double-Speed mode cannot be used for the USART.
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2018 Microchip Technology Inc.
Universal Synchronous and Asynchronous Recei...
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1.4 compliant modulation and demodulation for baud rates up to 115.2
Datasheet Preliminary
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megaAVR 0-Series
DS40002015A-page 295