Hi! I've been experimenting for a while to get good results with a DS2406 configured for input, utilizing the edge detector latch feature (this would probably apply to DS2408 which also has this feature). Since this haven't been as simple as "connect the switch between GND and PIO", I'd thought I ask around to see what your successful solutions look like?
Also, I thought I'd share my findings and schematics, and ask for any advice or input on it. I'm no EE, just a hobbyist, so my know-how is somewhat limited.. First some background (TLDR below): I have a 1-Wire network with 26 temperature sensors (DS18B20, DS18S20), and 4 inputs via DS2406. The master is a LinkUSB dongle, and 5V VCC is provided by an external PSU. The bus is ~80 meters (estimated), in a almost-perfect bus shape, i.e. no stars and almost no stubs longer than 1 meter (I think there are 2 or three exceptions). The devices are distributed along the bus at uneven places. All temperature sensors are powered, and temperature conversions are executed using simultaneous convert every minute. Two of the DS2406's have been on the net for a while, both configured with a pull-up resistor, and a push-button switch which shorts to ground. I have a script which scans the alarm directory a few times per second, and if anything is found, reads the status of the latches. Most of the time this gives acceptable delay. When executing temperature conversions, there naturally is a bit of delay, but usually not more than a second or so. Mostly this has been working good, with reliable triggering of the latch. However, one problem with pull-up based circuit is that I get erroneous triggers if VCC disappears, for example due to a bus disconnect, either unintentionally or intentionally (me adding/removing devices or rerouting, or something). My first thought was that this should be easily avoided by using pull-down resistors instead, and bringing the input high by closing the input to VCC. I do have a circuit which seems to be working fine now, but let me guide you through my experimental steps/thoughts: During this experimentation I added another two DS2406, one powered and one unpowered. The rest of the ones in the net are unpowered. At first I just tried with a simple resistor pull-down (PIO<->GND), quite high though, 60k.. In one location, with short leads between PIO and switch, this was quite successful, but some random triggers too. In another setup, with a ~50cm cable between the PIO and the switch, the latch triggered all the time. Tried to lower the pull-down resistor value, and at 12k it still fired of all the time. Removing the cable from the PIO immediately removed the invalid triggers, at least the constant ones.. Every now and then it decided to trigger anyway.. Why the cable? Mainly for serviceability.. Also, tried a shielded cable and connecting shield to GND it; no effect at all. To avoid this, I tried to connect a 100nF ceramic capacitor (C1) between GND and PIO, that is parallel with the pull-down resistor. Immediately this resolved the above problems, even with the cable connected. However, this introduced another problem.. On the powered DS2406 (lets call it IC2) which was a few meters of cable away, I now started to get random latch triggers, when I pressed the button on IC1. My guess is that when C1 is charged, this gives a small bump in the 5V VCC power line, enough to fool the powered DS2406.. I have not scoped this on this exact circuit, but in a separate lab setup (same circuit but other PSU/no long power cable), this was easily seen. Simple mitigation was to add a 10uF electrolyte cap between 5V and GND. With this in place, none of the sensors have triggered without me pressing a button, and none have missed any real inputs. I do have some further experimentation to do, mainly trying to use a larger resistor to lower the amount of power used when button is pressed.. This is not so important for a push-button, but for switches which are NC it might be more important. >From the data-sheet, which I'm not really sure I'm reading correct, I read that logic 0 is max 0.3V (0.4 on PIOB), and with a input leakage current of 1.9uA, that would mean ~150k would be OK (0.3v / 1.9uA = 150k)... Totally wrong or correct? TLDR: The current version of the circuit, which seems to work, is now a unpowered DS2406, with a 12k resistor and a 100nF cap between PIO and GND, and then a piece of cable to the switch connecting to VCC. Schematic: http://www2.stromnet.se/~johan/schematics/1wire/ds2406-input-v3.png This has worked perfectly for 2 days or so now (the previous ones triggered wrongly of a few times per day). So.. my questions would be: a) what does your DS2406/08-setup look like, and have you had any similar problems? b) any ideas for improvements on the schematic described above? Any flaws or common EE practice which I've missed? c) Any comments on data-sheet based guesses? Thank you! Johan ------------------------------------------------------------------------------ "Accelerate Dev Cycles with Automated Cross-Browser Testing - For FREE Instantly run your Selenium tests across 300+ browser/OS combos. Get unparalleled scalability from the best Selenium testing platform available. Simple to use. 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