Question
I have a customer using ADuM1251 for their I2C isolation application.
But currently, they meet the below picture status （Figure 1）:
Figure 1
This is tested from our ADuM1251 primary side. And the low level will have
about 900mV status.
I know from our DS, we can find below information （Figure 2）:
Figure 2
So it should be normal status for our part. But from customer side, they are
afraid this kind of status will cause some unexpected issues. Their current
controller is LPC2103, the Vil Max should be lower than 800mV.
Answer
The ADuM1251 uses a method for flow of control that depends on the 0.9V Vol
level. This was developed by Phillips and the level is consistent with the Vil
=1.0V level required by the I2C standard. Unfortunately many companies have
ignored the standard and produce I2C ports base on TTL I/O levels.
In most cases the actual switching level of the LPC2103 has margin and the
parts will work together. This is true if there are not extreme temperature
environments. Unfortunately we can not guarantee compatibility over all
operating conditions.
There are a few approaches that have been used.
1) The SCL line is unidirectional, so there will be no issues with false
clocking, that is good.
2) Size the SDA pull-up to draw less than 0.5mA and slow the data rate as much
as possible. This will allow the SDA line to settle to its lowest level.
3) Contact the manufacturer of the LPC2103 and find out if they can support an
850mV Vil over the customers operating temperature range.
4) Alternatively, if the device currently on the side 2 bus has Vil>1.0V,
consider switching to the ADuM1250 and flipping the part around so that side 2
faces the LPC2103.