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The receive margin in asynchronous mode can therefore be expressed as shown in equation (1).
M = (0.5 –
1
2N
D – 0.5
N
) – (L – 0.5) F –
(1 + F)
×
100%
. . . . . . . . (1)
M:
Receive margin (%)
N:
Ratio of clock frequency to bit rate (N = 16)
D:
Clock duty cycle (D = 0 to 1.0)
L:
Frame length (L = 9 to 12)
F:
Absolute deviation of clock frequency
From equation (1), if F = 0 and D = 0.5, the receive margin is 46.875%, as given by equation (2).
When D = 0.5 and F = 0:
M = (0.5
–
2
×
16
)
×
100%
1
= 46.875%
. . . . . . . . (2)
This is a theoretical value. A reasonable margin to allow in system designs is 20% to 30%.
Restrictions on Use of an External Clock Source:
•
When an external clock source is used for the serial clock, after updates TDR, allow an
inversion of at least five system clock (
φ
) cycles before input of the serial clock to start
transmitting. If the serial clock is input within four states of the TDR update, a malfunction
may occur. (See figure 12.22)
SCK
D0
D1
D2
D3
D4
D5
D6
D7
TDRE
t
Note: In operation with an external clock source, be sure that t >4 states.
Figure 12.22 Example of Synchronous Transmission
Summary of Contents for H8/3008
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