Scc Uart Mode - Freescale Semiconductor MPC850 User Manual

Chapter 22

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 22-1.
UART TCLK
8x, 16x, or 32x
UART TXD
Start 5, 6, 7, or 8 Data Bits with the Addr Parity 9/16 to 2
Bit Least Significant Bit First Bit Bit Stop Bits
(Optional)
NOTE:
1. Clock is not to scale.
Figure 22-1. 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.
Chapter 22. SCC UART Mode