6
3 Hardware Configuration
3.1
Pull-up Resistors
I
2
C Bus systems are based on open-collector technology requiring pull-up devices
on each signal wire. These pull-up devices usually take the form of pull-up resistors
connected to bus power.
The I
2
C adapter includes a side-mounted slide switch used to enable or disable
internal 5V (or 3.3V) I
2
C Bus 1.8K ohm pull-up resistors attached to the SCL and
SDA lines. Every I
2
C Bus system must have at least one pull-up on the signal lines.
Use this switch to configure the pull-up resistors for your system.
3.2
Connecting a 5V i2cStick to a 3.3 Volt Target System
If you are connecting a 5 volt I
2
C adapter to a 3.3 volt target system, you should
follow these steps BEFORE applying power:
• Shut off the I
2
C adapter’s internal pull-ups (See Pull-up Resistor section). Use
external pull-ups to the target system’s 3.3V power. These pull-ups may already
be present in the target system.
• Disconnect the I
2
C con5V wire from the target system. The I
2
C adapter
will be powered from its USB interface, and the target system will be powered
by its own 3.3V power supply.
The I
2
C adapter is a 5-volt device. Any signal above 3.3V on the SCL or SDA lines
is high enough for the adapter to see a Logical 1.
3.3
Connecting to an SMBus Target System
If you are connecting the I
2
C adapter to a SMBus target system, you should follow
these steps BEFORE applying power:
• Shut off the I
2
C adapter’s internal pull-ups (See Pull-up Resistor section).
• Use external SMBus rated (appoximately15k ohms) pull-up resistors. These
pull-ups may already be present in the target system.
• Visit our I
2
C versus SMBus FAQ page (www.mcc-us.com/I2CSMBusFAQ.htm).
• See the SMBus Specification for additional details.
Special Note for SMBus Users
: MCC’s I
2
C adapters are designed to be I
2
C Bus
