EM78P809N
8-Bit Microcontroller
34 • Product Specification (V1.0) 07.26.2005
(This specification is subject to change without further notice)
In Universal Asynchronous Receiver Transmitter (UART), each transmitted or received
character is individually synchronized by framing it with a start bit and stop bit.
Full duplex data transfer is possible since the UART has independent transmit and
receive sections. Double buffering for both sections allows the UART to be
programmed for continuous data tran sfer.
The figure below shows the general format of one character sent or received. The
communication channel is normall y held in the marked state (high). Ch aracter
transmission or reception starts with a transition to the space state (low).
The first bit transmitted or rece ived is the start bit (low). It is f ollowed by the data bits, in
which the least significant bit (LSB) comes first. The data bits are followed by the parity
bit. If present, then the stop bit or bits (high) confirm the end of the frame.
In receiving, the UART synchronizes on the falling edge of the start bit. When two or
three “0” are detected during three samples, it is recognized as normal start bit and the
receiving operation is started.
Fig. 12. DATA Format in UART
4.7.1 UART MODE: There are three UART modes. Mode 1 (7 bits dat a) and Mode 2 (8 bits data) allow the
addition of a parity bit. The parity bit addition is not available in Mode 3. The Figure
below shows the data format in each mode.
UMODE PRE
0 0 0
0 0 1
0 1 0
0 1 1
1 0 X
12345 11109876
START STOP
7 bits DATA
START STOP
7 bits DATA Parity
START STOP
8 bits DATA
START STOP
8 bits DATA Parity
START STOP
9 bits DATA
Mode 1
Mode 3
Mode 2
Fig. 13. UART Mode
START
bit D0 D1 D2 Dn Parity
bit STOP
bit
1 bit 7 or 8 bits
One character or frame
1 bit 1 bits