HOW TO MULTIPLEX A 1-WIRE MASTER INTO NUMEROUS CHANNELS

Optionally, the mux (U₂) has external, post-mux, pullup resistors (R_P4 and R_P5) to provide power for idle 1-Wire slaves when the switches are open. If this is not done, each time a channel switch is connected, the µC must wait the maximum wake-up time of the connected slaves on that channel (usually 2ms) before beginning communication.

However, it is important to consider the effects of the mux’s R_ON parameter during a pulldown event by the 1-Wire master when using an external pullup resistor on each channel. Any effects can be considered negligible by selecting a small R_ON to avoid violating the highest 1-Wire input low (V_IL) parameter of the 1-Wire slaves. So, for a given post-mux pullup resistor of R_P and a given mux resistor of R_ON, the post-mux output low voltage is expressed as follows:

V_{OL_MUX} = V_OL+ (V_CC – V_OL) × ^R_ON/(_RON+RP)

Additionally, it is important to consider the flexibility of the 1-Wire master used. Maxim recommends the DS2477 1-Wire master for any 3.3V system because the DS2477 timing, input triggering levels, and internal pullup resistors are very adjustable. The DS2477 can also be set to a high impedance mode, which can be helpful when using the external resistor option. However, if a system needs 5V then the next best option is to use the DS2484.

Lastly, during this examination, some systems require a mix of overdrive only and standard/overdrive 1-Wire slave devices. If the overdrive only and the standard/overdrive devices reside on the same 1-Wire bus, communication faults occur. One simple solution is to use a mux that places overdrive only devices on different channels than the overdrive/standard devices. The DS2477 can then simple switch to overdrive mode or standard mode between the selection of channels for proper communication.

Analog Mux Selection
There are many requirements a designer considers when selecting the analog mux. These requirements can be the number of channels, interface type, cost, package type, and performance. Table 1 lists the Maxim recommended analog muxes for 1-Wire applications. All of the recommended analog muxes handle rail-to-rail analog signals, have a small R_ON, and come in various package types. The µC that controls the selected channel must have spare GPIO pins. If the µC does not have any spare GPIO pins, it is possible to use the MAX14661 or a similar device that can be tied to the same I²C bus used by the DS2477.

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