AT83SND2CMP3B_14 Datasheet, PDF (168/242 Pages) ATMEL Corporation – MPEG I/II-Layer 3 Hardwired Decoder

20. Serial I/O Port
The serial I/O port in the AT8xC51SND2C provides both synchronous and asynchronous com-
munication 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 automatic address recognition.
20.1 Mode Selection
SM0 and SM1 bits in SCON register (see Figure 20-3) are used to select a mode among the sin-
gle synchronous and the three asynchronous modes according to Table 20-1.
Table 20-1. Serial I/O Port Mode Selection
SM0
SM1
Mode
Description
0
0
0
Synchronous Shift Register
0
1
1
8-bit UART
1
0
2
9-bit UART
1
1
3
9-bit UART
Baud Rate
Fixed/Variable
Variable
Fixed
Variable
20.2 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 pur-
poses 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.
20.2.1 Timer 1
When using Timer 1, the Baud Rate is derived from the overflow of the timer. As shown in
Figure 20-1 Timer 1 is used in its 8-bit auto-reload mode (detailed in Section "Mode 2 (8-bit
Timer with Auto-Reload)", page 54). SMOD1 bit in PCON register allows doubling of the gener-
ated baud rate.
168 AT8xC51SND2C/MP3B
4341H–MP3–10/07