Post by thread:Open drain

At 04:07 PM 4/16/97 PDT, you wrote: >Hi, > If RA4 is open drain, can I put a higher voltage onto it to pull it up. > Eg. can I use a 12V relay between +12V and RA4? >Thanks for any help in advance, Brian, I think you would be asking too much current of the PIC device. You should use a transistor to drive the relay, with a field collapse diode as well. Take a look at our tutorial, 'Device Control via the PC', for details on how to wire this up correctly. You can download a copy at our web site at http://www.netins.net/showcase/elab in the 'Intergated Circuits' section under 'EDE300'. If your were merely using the relay as an example and just wondering if you could use +12V to drive a lower current load, take a look at the 'max' specs for the chip - they specify .6V above Vdd. I suspect this is because of the clamp diodes that are there for protection. So if you can drop enough of the voltage across your load to make sure the PIC stays within this range (and most of it should be dropped across most any load), and stay within the current specs for the device, I think you should be O.K., as the PIC would never know it were connected to the +12V, assuming it was dropped before it got to the PIC. I am, however, curious to hear other's opinion on this, as I have never tried it. -Todd Peterson E-Lab Digital Engineering, Inc.

Yep, that's why its open drain. Must limit to +14V and if driving a relay, watch out for back EMF. Rgds, Brian. ______________________________ Reply Separator _________________________________ Subject: Open drain Author: "Clewer;Brian" <spam_OUTbrian.clewerTakeThisOuTTELEMATICS.COM> at Internet_Exchange Date: 4/16/97 4:07 PM Hi, If RA4 is open drain, can I put a higher voltage onto it to pull it up. Eg. can I use a 12V relay between +12V and RA4? Thanks for any help in advance, Brian.

At 10:13 AM 4/16/97 -0700, Brian Boles wrote: > Yep, that's why its open drain. Must limit to +14V and if driving a > relay, watch out for back EMF. > > Rgds, Brian. > > Brian: On RA4 on a 16C62x part, does 'open drain' still apply if the part is powered down? Or will the part load RA4 down (try to draw power from) if a voltage appears on RA4 and NOT on the VCC pin? I can eliminate a part in one of my designs if it truly is a open drain. It was my impression that the diodes to VCC and GND still existed since RA4 could also be used as a input. Hence, the part would only be "open drain" between GND and VCC (+-0.6v). Regards, ------------------------------------ | Jeff King Aero Data Systems | | .....jeffKILLspam@spam@mich.com P.O. Box 510895 | | (810)471-1787 Livonia, MI 48151 | |F(810)471-0279 United States | ------------------------------------

Todd Peterson wrote: {Quote hidden}> > At 04:07 PM 4/16/97 PDT, you wrote: > >Hi, > > If RA4 is open drain, can I put a higher voltage onto it to pull it up. > > Eg. can I use a 12V relay between +12V and RA4? > >Thanks for any help in advance, > > Brian, > > I think you would be asking too much current of the PIC device. You should > use a transistor to drive the relay, with a field collapse diode as well. > Take a look at our tutorial, 'Device Control via the PC', for details on how > to wire this up correctly. > > You can download a copy at our web site at > http://www.netins.net/showcase/elab in the 'Intergated Circuits' section > under 'EDE300'. > > If your were merely using the relay as an example and just wondering if you > could use +12V to drive a lower current load, take a look at the 'max' specs > for the chip - they specify .6V above Vdd. I suspect this is because of the > clamp diodes that are there for protection. So if you can drop enough of > the voltage across your load to make sure the PIC stays within this range > (and most of it should be dropped across most any load), and stay within the > current specs for the device, I think you should be O.K., as the PIC would > never know it were connected to the +12V, assuming it was dropped before it > got to the PIC. > > I am, however, curious to hear other's opinion on this, as I have never > tried it. > > -Todd Peterson > E-Lab Digital Engineering, Inc. If the current is not flowing through the port pin there is no drop so the potential at the port pin is still 12v.

re: Max voltage on any pin. John, I just went back through my data books and only MCLR is specified to handle +14V. The other pins are 7.5V. When I saw that reply it set off an `alarm' so I had to check it out. Use a transistor or MOSFET buffer for a higher voltage load. - Tom At 08:54 PM 4/16/97 -0500, you wrote: >[re port pin RA4] >| Yep, that's why its open drain. Must limit to +14V and if driving a >| relay, watch out for back EMF. > >Is the +14 limit documented anyplace? All of the PIC books I've seen >document the max on "any pin other than /Mclr" as Vdd. Does Microchip >"officially" allow RA4 higher? > >

Dear PIC.ers, Open drain on RA4 of the 16C84 / F84 has generated a lot of debate. .. FWIW I've been using RA4 at +12v, dropped through a 10k resistor to the +12v line. This node drives a 555 configured as a schmitt trigger buffer to the gate of a mosfet, for PWM drive on a dc motor armature. .. The voltage on RA4 pin does reach +12v when the bit is turned off (output high). PIC doesn't complain tho', and I have a few of these in service. My verdict is it's OK to impress a voltage somewhat > Vdd on the pin. email from John Sanderson / JS Controls, Boksburg, RSA Manufacturer & purveyor of laboratory force testing apparatus and related products and services. Tel/fax: Johannesburg 893 4154 Cellphone 082 453 4815