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Rev. 1.00, 05/04, page 251 of 544
12.4.2 Receive Data Sampling Timing and Reception Margin in Asynchronous Mode
In asynchronous mode, the SCI operates on a basic clock with a frequency of 16 times the bit rate.
In reception, the SCI samples the falling edge of the start bit using the basic clock, and performs
internal synchronization. Since receive data is latched internally at the rising edge of the 8th pulse
of the basic clock, data is latched at the middle of each bit, as shown in figure 12.3. Thus the
reception margin in asynchronous mode is determined by formula (1) below.
M = (0.5 – ) – (1 + F) – (L – 0.5) F } × 100 [%]
...
Formula (1)
2N
1
N
D – 0.5
[Legend]
M: Reception margin (%)
N : Ratio of bit rate to clock (N = 16)
D : Clock duty (D = 0.5 to 1.0)
L : Frame length (L = 9 to 12)
F : Absolute value of clock rate deviation
}
Assuming values of F = 0 and D = 0.5 in formula (1), the reception margin is determined by the
formula below.
M = {0.5 – 1/(2 × 16)} × 100 [%] = 46.875 %
However, this is only the computed value, and a margin of 20% to 30% should be allowed in
system design.
Internal
basic clock
16 clocks
8 clocks
Receive data
(RxD)
Synchronization
sampling timing
Start bit D0 D1
Data sampling
timing
15 0 7 15 00 7
Figure 12.3 Receive Data Sampling Timing in Asynchronous Mode