INTEGRATED CIRCUITS DIVISION
CPC5902G/CPC5903G Evaluation Board
User’s Guide
2PRELIMINARY R00A
same as that used at devices on their buses, the
performance of the actual system at power up will not
be observable. VDDA does not need to be the same
value as VDDB, but, as mentioned above, should be
the same voltage as used on the rest of the sideA bus
segment.
The posts at JP1 on the SCLA line can be used with a
0.1-inch header jumper to add 500(nominal) of
pull-up resistance to the SCLA line. Similarly JP3 adds
500of pull-up resistance to the SDAA line. These
jumpers are generally not used if there are already
minimum value pull-up resistors elsewhere on the bus.
For operation at 3.3V at VDDA and a bus driver
delivering 0.4V active low, the IOL current will be
2.9V/500= 5.8mA. This is close to the minimum
guaranteed value of 6mA for I2C fast mode. For
operation at 5V, this resistance should be increased:
5.0-0.4=4.6V and 4.6V/6mA = 766. The 500has
been implemented by using R1=R2=1kin parallel. A
quick way to evaluate a 5V system would be to use a
soldering iron to remove either R1 or R2 to increase
the resistance to 1k; however, slightly faster
operation would result from using a pull-up resistor of
perhaps 820.
In systems with significant cable length, it is often
preferred to split the pull-up resistance by paralleling a
physical resistor at both ends of the non-isolated bus
segment. To evaluate the performance of a bisected
pull-up, a soldering iron can be used to remove either
R1 or R2, and a 1kresistor (for a 3.3V system) can
be added, if necessary, near the bus master on the
sideA bus segment.
The 2 posts at JP2 and JP4 can be jumpered to add
390pF of capacitive load to the SCLA and SDAA lines.
These jumpers can be used to load the I2C lines to I2C
fast mode’s worst case: 400pF in standalone mode.
The jumpers can also be used to add capacitance to
the sideA bus segment if it is extremely lightly loaded,
but loading beyond 400pF total does not guarantee
operation at 400kHz by the I2C fast mode
specification.
On the right side, the posts at JP5 and JP7 can be
used to add 964pull-ups to the SCLB and SDAB
lines. For VDDB = 3.3V and VOL = 0.23VDDB=0.76V,
this would yield 3.3 - 0.76 = 2.54V across 964, or
2.635mA of output current sunk by the sideB drivers.
Other devices on the sideB bus might drive to VOL =
0.4V and sink 3.01mA. The CPC5902G/5903G drivers
at sideB are only rated for 3mA when used at VDDB
less than 4.5V. The sideB drivers are rated for 6mA
operation at up to 5.5V. At 5.5V, they will drive to
0.23*5.5=1.265V and will pull 4.4mA if the pull-up is
unchanged. At 5.5V, the pull-up resistor would need to
be larger than 705to limit IOL to less than 6mA.
