Post by thread:Connecting iButton to PIC

Hi, Thanks for all the help on my previous RS485 question. I'll let the list know how things turn out. I have another little thing I would like to run by the list: I am using the Dallas iButton Serial Code buttons (DS1990A) to allow users to identify themselves. I have recepticals on my units and the users simply press their buttons on the receptical. Now, the obvious way of connecting the receptical to the PIC is to connect it directly with a 5K pull-up resistor, as is suggested by Dallas. But, is it a good idea to bring a pin of your microcontroller so directly out of the enclosure. What about static discharges onto the receptical, for instance? Would isolation by transistors by of any benefit? I was thinking of something like: +5V | R 5K R |--------+------------- Dallas Button PIC Tx |/c | -----------| | npn|\e | | | GND | R R 56K +5V | | | R | PIC Rx R 10K | -------------| | c\| | |------| e/| npn | GND This has the disadavantage of splitting up the transmit and receive lines, thus needing two instead of one pin (and it inverts the Ts and Rx signals, but that is not a problem), but at least the PIC pin is not directly brought out into the big bad world ;-) Comments? Niki

> I have another little thing I would like to run by the list: > I am using the Dallas iButton Serial Code buttons (DS1990A) to allow > users to identify themselves. I have recepticals on my units and the > users simply press their buttons on the receptical. > Now, the obvious way of connecting the receptical to the PIC is to > connect it directly with a 5K pull-up resistor, as is suggested by Dallas. > But, is it a good idea to bring a pin of your microcontroller so directly > out of the enclosure. What about static discharges onto the receptical, > for instance? Would isolation by transistors by of any benefit? I was > thinking of something like: How about trying this circuit instead: +Vcc +Vcc | | | R _ R 5K 1N914 A R | | PIC pin 100 ohm | | 100 ohm ------------RRR-----+---+---+------RRR------ Dallas Button | | _ A 1N914 | | GND This now only uses 1 PIC pin, and gives you protection for that pin. The 100 ohm resistors limit the current through the protection diodes and to the PIC's internal protection diodes. Also, the pair of diodes can be gotten in a single SOT23 surface mount device, like the BAV99 from Zetex and Vishay/Liteon (both in DigiKey cat.). Dave Reinagel Cisco Systems, Inc.

David Reinagel wrote: {Quote hidden}> > > How about trying this circuit instead: > > +Vcc +Vcc > | | > | R > _ R 5K > 1N914 A R > | | > PIC pin 100 ohm | | 100 ohm > ------------RRR-----+---+---+------RRR------ Dallas Button > | > | > _ > A 1N914 > | > | > GND > > This now only uses 1 PIC pin, and gives you protection for that > pin. The 100 ohm resistors limit the current through the > protection diodes and to the PIC's internal protection diodes. > Also, the pair of diodes can be gotten in a single SOT23 surface > mount device, like the BAV99 from Zetex and Vishay/Liteon (both > in DigiKey cat.). > We are using similar circuit: +Vcc | R R 5K R | PIC pin R1 | R2 --------RRR--+------------+--RRR--- Dallas Button | | _ A BZW06-5V8 | | GND R1 = R2 < 390 ohm (usualy 100 ohm). Works several years in hundreds units. Electrostatic discharge sometimes damages iButton, but PIC - never. Kestas Kaunas, Lithuania