Serial Communication Interface (SCI) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 452

Actual start bit

RxD pin

1 0

1 1 0 0 0

Samples
RT clock
(16 × baud
rate)

[1]

[2]

RT clock state

1. A 0 following three 1s.
2. Majority of samples 3, 5, and 7 are 0s.

Figure 9.5 ■ Detection of start bit (ideal case)

The stop bit is high. Using this format, it is possible to transfer data character by character
without any gap.
The term baud rate is defined as the number of signal changes per second. Since the RS-232
standard uses a non-return-to-zero (NRZ) encoding method, baud rate is identical to bit rate. In
an NRZ code, a logic 1 bit is sent as a high value and a logic 0 bit is sent as a low value. When
a logic 1 follows another logic 1 during a data transfer, the voltage does not drop to zero before
it goes high.

Example 9.1
▼ Sketch the output of the letter g when it is transmitted using the format of 1 start bit, 8 data
bits, and 1 stop bit.
Solution: Letters are represented in ASCII code. The ASCII code of letter g is $67 (5 01100111).
Since the least significant bit goes out first in the TIA-232 protocol, the format of the output of
letter g is as shown in Figure 9.6.

0 1 1 1 0 0 1 1 0 1

(a) Output waveform on microcontroller interface

1 1

0 1 0 0 0 1 0 1

(b) Output waveform on TIA-232E interface

Figure 9.6 ■ Data format for letter g

▲