Freescale Semiconductor MPC8260 - Manual (Page 705)

MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 21-1
Chapter 21
SCC UART Mode
The universal asynchronous receiver transmitter (UART) protocol is commonly used to send low-speed
data between devices. The term asynchronous is used because it is not necessary to send clocking
information along with the data being sent. UART links are typically 38400 baud or less and are
character-based. Asynchronous links are used to connect terminals with other devices. Even where
synchronous communications are required, the UART is often used as a local port to run board debugger
software. The character format of the UART protocol is shown in
Figure 21-1 .
Figure 21-1. UART Character Format
Because the transmitter and receiver operate asynchronously, there is no need to connect the transmit and
receive clocks. Instead, the receiver oversamples the incoming data stream (usually by a factor of 16) and
uses some of these samples to determine the bit value. Traditionally, the middle 3 of the 16 samples are
used. Two UARTs can communicate using this system if the transmitter and receiver use the same
parameters, such as the parity scheme and character length.
When data is not sent, a continuous stream of ones is sent (idle condition). Because the start bit is always
a zero, the receiver can detect when real data is once again on the line. UART specifies an all-zeros break
character, which ends a character transfer sequence.
The most popular protocol that uses asynchronous characters is the RS-232 standard, which specifies baud
rates, handshaking protocols, and mechanical/electrical details. Another popular format is RS-485, which
defines a balanced line system allowing longer cables than RS-232 links. Even synchronous protocols like
HDLC are sometimes defined to run over asynchronous links. The Profibus standard extends UART
protocol to include LAN-oriented features such as token passing.
All standards provide handshaking signals, but some systems require only three physical lines—Tx data,
Rx data, and ground. Many proprietary standards have been built around the UART’s asynchronous
character frame, some of which implement a multidrop configuration where multiple stations, each with a
specific address, can be present on a network. In multidrop mode, frames of characters are broadcast with
the first character acting as a destination address. To accommodate this, the UART frame is extended one
bit to distinguish address characters from normal data characters.
UART TCLK
UART TXD
8x, 16x, or 32x
Start
Bit
Addr
Bit
Parity
Bit
(Optional)
5, 6, 7, or 8 data bits with the
least significant bit first
9/16 to 2
stop bits
(Not to scale)