591
I
2
C Bus (IIC)
Chapter 18
Preliminary User’s Manual U17566EE1V2UM00
Clock Stretching
Heavy capacitive load and the dimension of the external pull-up resistor on the
I
2
C bus pins may yield extended rise times of the rising edge of SCLn and
SDAn. Since the controller senses the level of the I
2
C bus signals it recognizes
such situation and takes countermeasures by stretching the clock SCLn in
order to ensure proper high level time t
SCLH
of SCLn.
After the microcontoller releases the (open-drain) SCLn pin it waits until the
SCLn level exceeds the valid high level threshold V
thH
. Then it does not pull
SCLn to low level before the nominal high level time t
SCLH_nom
has elapsed.
This mechanism is the same used, when a slow I
2
C slave device is pulling
down SCLn to low level to initiate a wait state.
Figure 18-3
shows an example.
Figure 18-3
Clock Stretching of SCLn
The effective clock frequency appearing at the SCLn pin calculates to
f
SCL_eff
= 1 / (T
SCL_nom
+ t
r
)
With a nominal frequency of f
SCL_nom
= 355 KHz (T
SCL_nom
= 2.817 µs and a
rise time of t
r
= 135 ns the effective frequency is f
eff
= 339 KHz.
t
r
t
SCLH
t
SCLL
t
r
T
SCL_nom
T
SCL_eff
SCL signal
effective SCL
clock
V
thH
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