Electronic Design

Analog Switch Connects One I2C Bus Master To Multiple SFP Modules

Telecom and data-communications equipment commonly use small-formfactor pluggable (SFP) modules for the physical-layer interface. Also common in these systems is an I2C bus for the management data input/output (MDIO) interface. But when several SFP modules are used, they all have the same I2C address. As a result, they can't all connect to the same I2C bus. To avoid the need for multiple I2C bus masters in that situation, you can introduce an I2C-controlled analog switch to direct the data line to the appropriate SFP module.

In the figure, IC1 includes six SPDT switches and two SPST switches. It's configured as a 1-of-14 multiplexer with an I2C address of 68h. All of its COM lines connect to the bus master's SDA line. (SDA refers to the data input of a two-wire interface). Each NO (normally open) output can connect to an SFP module. (A 1-kΩ pull-up resistor at each SFP input ensures that the undriven SDA lines remain high.) The SFP modules retain their addresses, in the range A0h to A2h.

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