Datasheet

Table Of Contents
Section 13 Serial Communication Interface
Page 450 of 846 REJ09B0140-0900 Rev. 9.00
Sep 16, 2010
H8S/2215 Group
13.7.5 Receive Data Sampling Timing and Reception Margin
In Smart Card interface mode an internal clock generated by the on-chip baud rate generator can
only be used as a transmission/reception clock. In this mode, the SCI operates on a basic clock
with a frequency of 32, 64, 372, or 256 times the transfer rate (fixed to 16 times in normal
asynchronous mode) as determined by bits BCP1 and BCP0. In reception, the SCI samples the
falling edge of the start bit using the basic clock, and performs internal synchronization. As shown
in figure 13.29, by sampling receive data at the rising-edge of the 16th, 32nd, 186th, or 128th
pulse of the basic clock, data can be latched at the middle of the bit. The reception margin is given
by the following formula.
M =
|
(0.5 – ) – (L – 0.5) F – (1+ F)
|
× 100 [%]
2N
1
N
|
D – 0.5
|
Where M: Reception margin (%)
N: Ratio of bit rate to clock (N = 32, 64, 372, and 256)
D: Clock duty (D = 0 to 1.0)
L: Frame length (L = 10)
F: Absolute value of clock frequency deviation
Assuming values of F = 0, D = 0.5 and N = 372 in the above formula, the reception margin
formula is as follows.
M = (0.5 – 1/2 × 372) × 100%
= 49.866%
Internal
basic clock
372 clocks
186 clocks
Receive data
(RxD)
Synchronization
sampling timing
D0 D1
Data sampling
timing
185
371 0
371
185
0
0
Start bit
Figure 13.29 Receive Data Sampling Timing in Smart Card Mode
(Using Clock of 372 Times the Transfer Rate)