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Internal
basic clock
Receive data
(RXD3x)
Synchronization
sampling timing
Data sampling
timing
Figure 10-23 Receive Data Sampling Timing in Asynchronous Mode
Consequently, the receive margin in asynchronous mode can be expressed as shown in equation
(1).
1
D – 0.5
M ={(0.5 –
) –
2N
where
M: Receive 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 frequency deviation
Substituting 0 for F (absolute value of clock frequency deviation) and 0.5 for D (clock duty) in
equation (1), a receive margin of 46.875% is given by equation (2).
When D = 0.5 and F = 0,
M = {0.5 — 1/(2 × 16)} × 100 [%]
= 46.875%
However, this is only a computed value, and a margin of 20% to 30% should be allowed when
carrying out system design.
16 clock pulses
8 clock pulses
7
Start bit
– (L – 0.5) F} 5 100 [%]
N
..... Equation (2)
15 0
7
D0
..... Equation (1)
15 0
D1
311
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