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1-Wire networks with many 1-Wire nodes can require devoted 1-Wire channels. This text discusses a technique for utilizing just one 1-Wire host in a community whereas having quite a few 1-Wire channels.
1-Wire networks are initially designed for communication with a single 1-Wire host and quite a few 1-Wire nodes on a single 1-Wire bus. Ideally, a linear topology, which incorporates insignificant stubs, is greatest for a 1-Wire community. Nevertheless, a star topology, which incorporates lengthy stubs, is usually unavoidable and makes it tougher to find out the efficient limitations. A way to get rid of these difficulties is to interrupt up a star topology into quite a few channels through the use of an analog multiplexer (mux). Benefits of utilizing quite a few channels embrace accelerating particular person 1-Wire node entry time, bettering community robustness, and mixing overdrive-only nodes with commonplace/overdrive nodes on totally different channels. These benefits will be gained whereas nonetheless having a single 1-Wire host.
Association
When configuring a 1-Wire community to have many channels, the overall strategy taken makes use of an Analog Gadgets’ 1-Wire host linked to the frequent sign of an analog mux. The mux has digital channel choose alerts to attach the 1-Wire frequent sign to the specified I/O that incorporates a channel of 1-Wire node units. With this association, many extra 1-Wire nodes will be networked over the restrictions of a single 1-Wire bus. That is as a result of elimination of stubs and a lower within the variety of 1-Wire nodes per channel pushed by the 1-Wire host.
Examination
Determine 1 reveals a 3.3 V system when utilizing the DS2485 1-Wire host. The microcontroller controls each the DS2485 and the mux channel to be chosen. In a 1-Wire community, it’s important that the mux used can deal with rail-to-rail analog alerts. In any other case, sign distortion can happen and the VPUP parameter requirement of the 1-Wire nodes will be violated. The mux RON parameter should even be as small as attainable to keep away from altering the DS2485 energetic pull-up impedance (RAPU). If this isn’t taken under consideration, the 1-Wire nodes won’t obtain the required present to function throughout a robust pull-up occasion.
Determine 1. Typical utility circuit. (Supply: Analog Gadgets)
Optionally, the mux (U2) has exterior, post-mux, pull-up resistors (RP4 and RP5) to supply energy for idle 1-Wire nodes when the switches are open. If this isn’t executed, every time a channel change is linked, the microcontroller should wait the utmost wake-up time of the linked nodes on that channel (often 2 ms) earlier than starting communication. Nevertheless, it is very important take into account the results of the mux’s RON parameter throughout a pulldown occasion by the 1-Wire host when utilizing an exterior pull-up resistor on every channel. Any results will be thought of negligible by deciding on a small RON to keep away from violating the very best 1-Wire enter low (VIL) parameter of the 1-Wire nodes. So, for a given post-mux pull-up resistor of RP and a given mux resistor of RON, the post-mux output low voltage is expressed as follows:
2404_adi01-eq-01.jpg
Moreover, it is very important take into account the pliability of the 1-Wire host used. Analog Gadgets recommends the DS2485 1-Wire host for any 3.3 V system as a result of the DS2485 timing, enter triggering ranges, and inside pull-up resistors are very adjustable. The DS2485 will also be set to a excessive impedance mode, which will be useful when utilizing the exterior resistor possibility. Nevertheless, if a system wants 5 V then the subsequent most suitable choice is to make use of the DS2484.
Lastly, throughout this examination, some methods require a mixture of overdrive-only and commonplace/overdrive 1-Wire node units. If the overdrive-only and the usual/overdrive units reside on the identical 1-Wire bus, communication faults happen. One easy resolution is to make use of a mux that locations overdrive-only units on totally different channels than the overdrive/commonplace units. The DS2485 can then merely change to overdrive mode or commonplace mode between the number of channels for correct communication.
Analog Mux Choice
There are numerous necessities a designer considers when deciding on the analog mux. These necessities will be the variety of channels, interface kind, value, bundle kind, and efficiency. Desk 1 lists the really helpful analog muxes for 1-Wire purposes.
Desk 1. Analog Muxes for 1-Wire Gadgets Selector Information
Half Quantity
Channels
Provide Voltage (Min) (V)
Provide Voltage (Max) (V)
RON (Max) (Ω)
Microcontroller—
Channel Choose
Packages
4-to-1
1.8
5.5
4
2 GPIO
10-lead µMAX, 10-lead TDFN-EP
(3 mm × 3 mm)
4-to-1
1.6
3.6
0.8
2 GPIO
10-lead µMAX, 12-lead TQFN
(3 mm × 3 mm)
8-to-1
2
5.5
10
3 GPIO
16-lead PDIP, 16-lead SOIC,
16-lead TSSOP
8-to-1
1.8
5.5
3.5
3 GPIO
16-lead TQFN (3 mm × 3 mm),
16-lead SOIC, 16-lead TSSOP
8-to-1
1.8
3.6
1
3 GPIO
16-lead TQFN (3 mm × 3 mm),
16-lead TSSOP
16-to-1
1.8
3.6
8
I2C/SPI
28-lead TQFN (3 mm × 3 mm)
*The AB_ and COM_ pins present +10 kV ESD safety (HBM). Any 16 AB_ pins will be linked to both COM_ pins.
The entire really helpful analog muxes deal with rail-to-rail analog alerts, have a small RON, and are available varied bundle varieties. The microcontroller that controls the chosen channel will need to have spare GPIO pins. If the microcontroller doesn’t have any spare GPIO pins, it’s attainable to make use of the MAX14661 or the same gadget that may be tied to the identical I2C bus utilized by the DS2485.
Conclusion
This text gives a technique to interrupt up star topography 1-Wire networks through the use of an analog mux from the really helpful record. As with the number of any digital part, the supporting system ought to fastidiously look at all gadget specs below all use circumstances to make sure dependable operation.
Stewart Merkel has been an purposes engineer for over 10 years. Previous to becoming a member of Analog Gadgets, Stewart labored at Maxim Built-in, concentrating on embedded safety, 1-Wire® units, and safe/nonsecure telecommunications tools. He holds a B.S. diploma in electrical engineering from Binghamton College in New York.
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