Tiny Analog Switch Alleviates I2C Address Conflicts
Abstract: A small SPDT switch can be used to alleviate 2-wire addressing conflicts. This simple circuit depicts how a simple switch can provide proper address to two components with identical addresses.
To avoid address conflicts, every peripheral on an I2C™ bus must have a unique address. Sometimes, however, peripherals may be assigned the same address. The circuit of Figure 1 resolves address conflicts by enabling the I2C bus to select between two peripherals with the same address.
Figure 1. A dual analog switch (IC1) and a single controller line (SELECT) enable this I2C bus to select between two peripherals with identical addresses.
The popular I2C bus is an open-collector, 2-wire interface that includes a clock line and a bidirectional data line. It allows a controller (the master) to select a particular device (the slave) by first issuing a serial address on the data line, then issuing appropriate commands or data. Master and slave can send data in both directions by pulling the data line low, and slaves can generate wait states by pulling the clock line low. Bus switching, however, is complicated by the open-collector architecture—it cannot be accomplished with the CMOS outputs of AND gates or 74HC157 data selectors.
The peripherals shown in Figure 1 are a Philips I2C real-time clock (PCF-8583) and a large I2C EEPROM (Microchip M-24LC16). Both have an internal, hexadecimal slave address of A0. (The EEPROM takes up the entire address range, making it impossible to avoid.) The analog switch connects either one device or the other. Selection involves the data line (SDA) only, because an I2C start condition requires that the SDA signal goes low before the clock goes low. To select between the devices, the master device sets a port pin to control the state of the dual SPST analog switch.
IC1 is a CMOS chip well suited to this function. Its normally open switch and normally closed switch perform the 2:1 selector operation with no additional inverters or port lines. It features low on-resistance (33Ω) and low supply current (1µA), and is specified for operation below 3V. Also, its tiny 8-pin SOT package (µMAX) is only one-half the size of an SO-8 package.
A similar idea appeared in the 6/23/97 issue of Electronic Design.
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