AT8xC51SND2C
Serial I/O Port
The serial I/O port in the AT8xC51SND2C provides both synchronous and asynchro-
nous communication modes. It operates as a Synchronous Receiver and Transmitter in
one single mode (Mode 0) and operates as an Universal Asynchronous Receiver and
Transmitter (UART) in three full-duplex modes (Modes 1, 2 and 3). Asynchronous
modes support framing error detection and multiprocessor communication with auto-
matic address recognition.
Mode Selection
SM0 and SM1 bits in SCON register (see Figure 160) are used to select a mode among
the single synchronous and the three asynchronous modes according to Table 158.
Table 158. Serial I/O Port Mode Selection
SM0
SM1
Mode
Description
Baud Rate
Fixed/Variable
Variable
0
0
1
1
0
1
0
1
0
1
2
3
Synchronous Shift Register
8-bit UART
9-bit UART
Fixed
9-bit UART
Variable
Baud Rate Generator
Depending on the mode and the source selection, the baud rate can be generated from
either the Timer 1 or the Internal Baud Rate Generator. The Timer 1 can be used in
Modes 1 and 3 while the Internal Baud Rate Generator can be used in Modes 0, 1
and 3.
The addition of the Internal Baud Rate Generator allows freeing of the Timer 1 for other
purposes in the application. It is highly recommended to use the Internal Baud Rate
Generator as it allows higher and more accurate baud rates than Timer 1.
Baud rate formulas depend on the modes selected and are given in the following mode
sections.
Timer 1
When using Timer 1, the Baud Rate is derived from the overflow of the timer. As shown
in Figure 114 Timer 1 is used in its 8-bit auto-reload mode (detailed in Section "Mode 2
(8-bit Timer with Auto-Reload)", page 53). SMOD1 bit in PCON register allows doubling
of the generated baud rate.
Figure 114. Timer 1 Baud Rate Generator Block Diagram
PER
CLOCK
÷ 6
0
1
Overflow
TL1
(8 bits)
÷ 2
0
1
To serial
Port
T1
C/T1#
TMOD.6
SMOD1
PCON.7
INT1
TH1
(8 bits)
GATE1
TMOD.7
T1
CLOCK
TR1
TCON.6
169
4341D–MP3–04/05
ATMEL [ ATMEL ]
