Dallas iButton Help

[suniljha]

Hi!
I have research and developed a security system based on Dallas ID only iButton DS1991. I have interfaced iButton with 1-wire software implementation on Motorola Microcontroller. The system reads 64-bit ROM ID from the iButton and validates the identity of person and activates the alarm system if wrong iButton is inserted or no ibutton is there while entering the secure place. The system is working fine but recently I discovered a big hardware bug with the system. Please help me in solving it so that I can go ahead with my plans. The problem is that iButton has physical two wire contact i.e. Ground (casing) and 1-Wire pulled up line and to bypass the security system, When any defaulter connected these two pins (I mean shorted) then system stops working as the pulled up line got ground connection (Vss got connected to vdd). In this way any physical interface to iButton when shorted together, hinders the microcontroller circuit functioning. Please help me in handling this bug by suggesting appropriate solution so that when interface pin shorted no problem will takes place with microcontroller circuit.<p>Waiting for your kind help and support...
regards,
Sunil Jha

[Edd]

Well its for sure that no one is going to do any bad data entry/hacking of the system in that latched down mode of your support U/P.
Now,considering that your I-button input line is using the common 5k pullup res you might look into an INITIAL sampling of that input line with voltage comparator circuitry to see if the nominal 5k ohm (pull up resistor)/500k ohm (I-button load) ratio is initially detected BEFORE a switching in and completing the input line circuitry to your U/P. <p>73's de Edd

[haklesup]

Use a different supply for the pullup/power to the iButton as you do for the rest of the circuits. You probably have a 5V regulator IC in there someplace. Put another one in right beside it and use that supply to power the interface (pullup resistor). Now if someone shorts the interface, only that supply collapses, the other isolated supply remains on. Use a common ground and it will seem as you have only one supply<p>I would also recommend additional input protection on the pins of the interface (pullup pin at least). Double diode protection is simple, effective and cheap. Reference the clamp diodes to the source and ground of the interface supply. This will prevent anyone from overstressing the inputs with an external source like a battery.<p>It is common on microcontrollers to have seperate power pins for the I/O pins and the core logic (same or different voltage). You may want to also segment those supplies for further isolation (if yours have those pins).<p>Without adding another supply, you may be able to increase the value of the pullup resistor such that when it is shorted to ground, the resulting current draw is not so high your supply shuts down (current limit or starving for power). For example, a 5K resistor with a 5V supply should only cause a 1mA short. In fact, I'm suprised this is not already the case since the iButton output would need to back drive this node to logic low and high.<p>Conversely, you could increase the current of the supply to keep up with the short but that might make a short circuit worse by making more heat in the conductors.

[Chris Smith]

Place a latch relay [SCR] in line so that when the short occurs, it locks out any attempts to debug, or over ride. Bias the latch to occur as a dead short, only. <p>Also place in a delay line to the set so that the latch can occur before the bug can act.

[bodgy]

if you can hunt someone down with a copy of February Elektor magazine (UK version) there was a security system using an i-button that solved this very problem.<p>Suggest you look in Tower Records or local library if you are near a University. I know Elektor is now hard to find in newsagents in the US.<p>Colin