AT8xC51SND2C
Figure 128. Baud Rate Formula (Mode 2)
2SMOD1 ⋅ FPER
Baud_Rate=
32
Multiprocessor
Communication (Modes
2 and 3)
Modes 2 and 3 provide a ninth-bit mode to facilitate multiprocessor communication. To
enable this feature, set SM2 bit in SCON register. When the multiprocessor communica-
tion feature is enabled, the serial Port can differentiate between data frames (ninth bit
clear) and address frames (ninth bit set). This allows the AT8xC51SND2C to function as
a slave processor in an environment where multiple slave processors share a single
serial line.
When the multiprocessor communication feature is enabled, the receiver ignores frames
with the ninth bit clear. The receiver examines frames with the ninth bit set for an
address match. If the received address matches the slaves address, the receiver hard-
ware sets RB8 and RI bits in SCON register, generating an interrupt.
The addressed slave’s software then clears SM2 bit in SCON register and prepares to
receive the data Bytes. The other slaves are unaffected by these data Bytes because
they are waiting to respond to their own addresses.
Automatic Address
Recognition
The automatic address recognition feature is enabled when the multiprocessor commu-
nication feature is enabled (SM2 bit in SCON register is set).
Implemented in hardware, automatic address recognition enhances the multiprocessor
communication feature by allowing the Serial Port to examine the address of each
incoming command frame. Only when the Serial Port recognizes its own address, the
receiver sets RI bit in SCON register to generate an interrupt. This ensures that the CPU
is not interrupted by command frames addressed to other devices.
If desired, the automatic address recognition feature in mode 1 may be enabled. In this
configuration, the stop bit takes the place of the ninth data bit. Bit RI is set only when the
received command frame address matches the device’s address and is terminated by a
valid stop bit.
To support automatic address recognition, a device is identified by a given address and
a broadcast address.
Note:
The multiprocessor communication and automatic address recognition features cannot
be enabled in mode 0 (i.e, setting SM2 bit in SCON register in mode 0 has no effect).
Given Address
Each device has an individual address that is specified in SADDR register; the SADEN
register is a mask Byte that contains don’t care bits (defined by zeros) to form the
device’s given address. The don’t care bits provide the flexibility to address one or more
slaves at a time. The following example illustrates how a given address is formed.
To address a device by its individual address, the SADEN mask Byte must be
1111 1111b.
For example:
SADDR = 0101 0110b
SADEN = 1111 1100b
Given = 0101 01XXb
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4341D–MP3–04/05
ATMEL [ ATMEL ]
