----- Original Message ----- From: "Peter Todd" To: "Microcontroller discussion list - Public." Sent: Friday, June 22, 2007 9:06 PM Subject: Re: [EE] High-current power/data one wire bus? > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On Thu, Jun 21, 2007 at 09:28:24AM +0100, Alan B. Pearce wrote: >> >Given that I need to have address discovery, then yes, 2 way >> >comms has to be possible. I might want to put sensors in the >> >network too later on. Being able to upgrade the firmware of >> >the devices via bootloader would be really cool too, except >> >that no 12F or 10F part supports it. >> >> Not sure if the small 16F parts support it. Would also give you enough >> pins >> to drive multiple colours of LEDs at a node. > > Yeah, they don't unfortunately. The cheapest part that supports > bootloaders is... > > Wait, now that's interesting. Using the paramatic search tool on > Microchip you would assume "Self Write" is the option to search for. But > the 16f818 chip doesn't show up, and I wrote a bootloader for it... > > I think the magic term is "Enhanced Flash" of which the 16f818 is the > cheapest part with it. > > Even then, a 6 pin 10f part is enough pins to drive a three color led, > 6 pins, 2 for power, one for communication and the other three for the > led rgb. > >> >My understanding is that I'd be doing voltage modulation >> >for both ways. The slaves would short Vcc/Data and GND to >> >indicate zero's. Given, say, a 5k pull up and a short through >> >a 200ohm resistor the slaves would be pulling the line >> >down to 0.2V >> >> OK, that is a similar system to what I am suggesting except you are >> clamping >> the voltage right down close to 0, which means you can use voltage >> sensing >> at the power supply end. However this will mean that you will need to >> have a >> power supply that has a sharp knee on its current limit, possibly even >> some >> form of foldback current limit, and the pass element has the possibility >> of >> getting very hot - remember this will have the whole power supply input >> voltage across it while the node is shorting to ground, ant will need to >> be >> current limited at a touch over 2A for 100 nodes at 20mA/node - >2A if >> you >> want expandability. Not only that but the transistor at the node will >> have >> to have this current capability as well. Note that unless you have some >> large capacitance at each node, isolated from the supply by a diode, you >> may >> see the transmission modulate the light level of any LEDs that are turned >> on >> due to the chopping of the supply voltage. > > Ahh ok. None of that will be needed, because when the master expects > communication from the slave, the master turns off the main, high > current source, and leaves the data line connected pulled up with > nothing more than a 4.7k resistor. A slave will easilly be able to short > that resistor. > > > That said, just realised that implies that all slaves must turn their > leds off to back communication to work... oops, as I said differently in > another post. > >> >I'm going to leave the checksum off as integrity isn't critical. >> >This is just a light show. :) >> >> But you will want a checksum if you start doing firmware downloads. >> Depending what you are using the LED display for, you may still want a >> checksum of some sort, there is nothing more frustrating than having such >> a >> system go AWOL in sequence, even if it is only a demonstration Christmas >> tree lighting display. > > I'm just assuming that given that updates are every second, any > incorrect color will only show up for a second anyway before it's > corrected. The occasional bit of weirdness will be interpreted as > character. :) > > Firmware downloads though, yes definetely. I'll likely even use a very > simple error correcting "two out of three" type system. With a checksum, > you can always just invoke the bootloader again, but a two out of > three system would even allow the bootloading to succeed in the > face of errors, as well as be very simple to program. > >> >Another problem with full cat 3 is visuals... it's big and ugly. >> >If anything I'll may very well have to find a source of jet black >> >twisted pair somewhere, skinny and unobtrusive will be important >> >during the day. >> >> I would see if I could find some dark green sheath CAT wire - like used >> on >> Christmas tree lights. As to the component cost, I would try hitting up >> some >> FAEs for chips - I am sure you could get a couple hundred CAN drivers & >> PIC >> chips for 'demonstration purposes' especially if you are prepared to >> write >> an app note of the experience (student task - part of the project!). You > > I'll definetely look into that, although the whole design can't > completely depend on such charity, as the idea is for other groups to be > able to reproduce our setup, even at normal market prices. It's why I > was quoting part prices from digikey for instance. > >> might even get the same consideration for a roll of CAT cable with a >> special >> coloured sheath - would the army use CAT5 with camouflage green sheath? >> With >> the right colour I think you would find that even CAT 5 would get >> reasonably >> invisible. > > Definetely. > > Having said that, I think I found my wire... Turns out digikey sells > solid two conductor twisted pair, with a pvc jacket, for $35/1000ft. > Basically telephone wire, which I'd totally forgotten about. P/N > C7023708-1000-ND. It's non-stock, but ships in a few weeks, and there's > gotta be more like it from other distributors, like local electrical > companies for telephone installation. > > That brings the per node wiring cost down to $1.75 and nobody has to > go around cutting up CAT3. > > - -- > http://petertodd.org > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.6 (GNU/Linux) > > iD8DBQFGfHH63bMhDbI9xWQRAt75AJwIKGOCpx4pV8G4V800XqFPLrd+dgCfXOLV > dWgD9+vOuHYQzj7TNKaUljo= > =TBX2 > -----END PGP SIGNATURE----- > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist