R01UH0136EJ0210 Rev.2.10
Page 497 of 800
Jul 31, 2012
M16C/64A Group
23. Serial Interface UARTi (i = 0 to 2, 5 to 7)
Figure 23.25 Clock Synchronization
23.3.3.5
SCL Clock Frequency
The SCL clock duty generated in I
2
C mode is 50%. The low-level width of the SCL clock is 1.25
μ
s
when the I
2
C-bus setting is Fast-mode maximum SCL clock (400 kbps). This value does not satisfy the
Fast-mode I
2
C-bus specification (f
LOW
= minimum 1.3
μ
s). Set the SCL clock to 384.6 kbps or less to
satisfy the SCL clock low-level width of 1.3
μ
s or more.
When the clock synchronous function (Figure 23.25 “Clock Synchronization”) is enabled, there is a
sampling delay of the noise filter plus 1 to 1.5 cycles of UiBRG count source.
There is also a delay of the SCL clock when high is determined and the SCL clock high width is
extended. Therefore, the actual SCL clock becomes slower than SCL clock bit rate setting.
To calculate the effective value of SCL clock, take the SCL clock rise time (t
R
) into consideration.
The following is an example of an SCL clock calculation.
Example of an effective value of SCL clock calculation at 384.6 kbps
•
UiBRG count source: f1 = 20 MHz
•
UiBRG register setting value: n = 26 - 1
•
SCL clock rise time: t
R
= 100 ns
•
SCL clock fall time: t
F
= 0 ns
•
Noise filter width: t
NF
= 100 ns
(1)
•
Sampling delay: t
SD
= 1 cycle
Note:
1.
Maximum 200 ns.
Internal clock
SCLi
Change the internal clock
signal from high to low to
start counting low period
Stop counting
Resume
counting
(1) Clock synchronization
Clock output
of other device
1
(2) Synchronization period
2
3
4
5
6
7
8
9
SCLi
Internal clock
Write of transmit data
Synchronized period
i = 0 to 2, 5 to 7
~
~
f
SCL
(theoretical value) = f1 / (2(n + 1)) = 20 MHz / (2(25 + 1)) = 384.6 kbps
t
LOW
= 1 / (2f
SCL
(theoretical value)) = 1 / (2
×
384.6 kbps) = 1.3
μ
s
t
HIGH
= 1 / (2f
SCL
(theoretical value)) + t
NF
+ (t
SD
×
1 / f1)
= 1 / (2
×
384.6 kbps) + 100 ns + (1
×
1 / 20 MHz)
= 1.45
μ
s
f
SCL
(actual value) = 1 / (t
F
+ t
LOW
+ t
R
+ t
HIGH
) = 1 / (0 ns + 1.3
μ
s + 100 ns + 1.45
μ
s) 350.8 kbps
Содержание M16C/60 Series
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