13.10 SCI Communication Format
The SCI asynchronous communication format uses either single line (one way) or two line (two way)
communications. In this mode, the frame consists of a start bit, one to eight data bits, an optional even/odd
parity bit, and one or two stop bits (shown in
). There are eight SCICLK periods per data bit.
The receiver begins operation on receipt of a valid start bit. A valid start bit is identified by four consecutive
internal SCICLK periods of zero bits as shown in
. If any bit is not zero, then the processor starts over
and begins looking for another start bit.
For the bits following the start bit, the processor determines the bit value by making three samples in the middle
of the bits. These samples occur on the fourth, fifth, and sixth SCICLK periods, and bit-value determination is on
a majority (two out of three) basis.
illustrates the asynchronous communication format for this with a
start bit showing where a majority vote is taken.
Since the receiver synchronizes itself to frames, the external transmitting and receiving devices do not have to
use a synchronized serial clock. The clock can be generated locally.
Majority
vote
Start bit
LSB of data
SC ICLK
(internal)
SCIRXD
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
8 SCICLK periods per data bit
8 SCICLK periods per data bit
Figure 13-7. SCI Asynchronous Communications Format
13.10.1 Receiver Signals in Communication Modes
illustrates an example of receiver signal timing that assumes the following conditions:
• Address-bit wake-up mode (address bit does not appear in idle-line mode)
• Six bits per character
RXENA
RXRDY
1
6
3
4
5
2
0
1
2
3
4
5
0
1
2
Start
Stop
Start
Ad
Pa
SCIRXD pin
Frame
Figure 13-8. SCI RX Signals in Communication Modes
Notes:
1. Flag bit RXENA (SCICTL1, bit 0) goes high to enable the receiver.
2. Data arrives on the SCIRXD pin, start bit detected.
3. Data is shifted from RXSHF to the receiver buffer register (SCIRXBUF); an interrupt is requested. Flag bit
RXRDY (SCIRXST, bit 6) goes high to signal that a new character has been received.
4. The program reads SCIRXBUF; flag RXRDY is automatically cleared.
5. The next byte of data arrives on the SCIRXD pin; the start bit is detected, then cleared.
6. Bit RXENA is brought low to disable the receiver. Data continues to be assembled in RXSHF but is not
transferred to the receiver buffer register.
Serial Communications Interface (SCI)
808
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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