16 clocks
8 clocks
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 1 2 3 4 5
Base clock
–7.5 clocks
+7.5 clocks
Receive data
(RxD)
Start bit
D0
D1
Synchronization
sampling timing
Data sampling
timing
Figure 15.24 Receive Data Sampling Timing in Asynchronous Mode
The receive margin in asynchronous mode can therefore be expressed as shown in equation (1).
1
2N
| D – 0.5 |
N
................. (1)
(1 + F) × 100%
M = (0.5 –
) – (L – 0.5) F –
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 – 1/(2 × 16)) × 100% = 46.875% ............................................ (2)
This is a theoretical value. A reasonable margin to allow in system designs is 20% to 30%.
Rev. 6.0, 07/02, page 654 of 986
RENESAS [ RENESAS TECHNOLOGY CORP ]
