Connecting open drain output to another output of PIC

Hasan

Hi,
Here is a question to all of you PIC experts from a half engineer.

I have a project where I have a bunch of slave controllers/sensors based on PICs connected to a central controller which is based on Javelin Stamp. One of these slave controllers is a ultrasonic ranger used to sense fluid level. This ranger is designed around pic16f628. Skipping the irrelevant design details here, this PIC has a couple of comparators which I use at the last stage of the ultrasonic reflected pulse detector. Compartor's output is digital and is open drain. Here is the quesiton.

I want to connect this open drain output to another port pin which is always output in my case (CCP1/RB3 pin). Will this be OK? i.e., connecting an open drain output to another regular, non-open drain output? I know two or more open drains can be connected together thru a resistor to Vcc but not sure about mixing the two types of outputs.

Also, the data sheet says that "If the RB3/CCP1 is configured as an output, a write to the port can cause a capture condition." This is good news for me since I don't have to change status of ccp1 pin. Does any one have an idea how this writing to output, works?

LoopyByteloose

The Art of Electronics explains how open-drains are properly interfaced.
You really need to work through all the appropriate cases they present in order to avoid a bad result.

I would have to go back to the book and guess what you want to do. Try to locate a copy at your library and I am sure it will clear up your questions.

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G. Herzog in Taiwan

Beau Schwabe

If you need the driven output for other parts of your circuit and can't change it in software, a diode connected off the pin will "approximate" the functionality
of an open collector for the parts of your circuit that are already open collector.

                     10K
       +5V >---------/\/\----o
                             |
                             |
Output Pin >-----o------|<---o------> Open Collector Output with pullup
                 |
                 o------------------> Driven Output

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
Latest Company News
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

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Hasan

Hi,
Thanks for the replys guys.

I do have the Art of Electronics Book. I couldn't find the solution to my problem in it though.

ICeMAN's simple solution will actually work for me. I should have thought of that too...

How do you draw these ascii based circuit diagrams?

-Hasan

Beau Schwabe

Hasan said...

How do you draw these ascii based circuit diagrams

I usually draw them in Windows Notepad (non-proportional font)

Then cut and past them into a reply as if they were CODE.

{code}
ASCII Stuff
{/code}


Substitute { and } with [noparse][[/noparse] and ]

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
Latest Company News
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

---

LoopyByteloose

I believe it is in The Art of Electronics under 'open collector' not 'open drain' [noparse][[/noparse]I maybe wrong].
I knew that possibly both diodes and pull up resistors were required, but unsure of the logic.

Thanks for the tip on ASCII schematics. Having seen a lot of those, I really appeciate knowing how easy they are to create.

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G. Herzog in Taiwan