Figure 18-14. Baud Rate Generator Selection (mode 2)
PER
CLOCK
÷ 2
0
1
÷ 16
To Serial Port
SMOD1
PCON.7
Figure 18-15. Baud Rate Formula (Mode 2)
2SMOD1 ⋅ FPER
32
Baud_Rate =
18.5 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 communication feature is
enabled, the Serial Port can differentiate between data frames (ninth bit clear) and address
frames (ninth bit set). This allows the AT89C5132 to function as a slave processor in an environ-
ment 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 hardware 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.
18.6 Automatic Address Recognition
The automatic address recognition feature is enabled when the multiprocessor communication
feature is enabled (SM2 bit in SCON register is set).
Implemented in hardware, automatic address recognition enhances the multiprocessor commu-
nication 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 configu-
ration, 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 broad-
cast 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).
18.6.1
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
114
AT89C5132
4173E–USB–09/07
ATMEL [ ATMEL ]
