FMS Forum

by **Merlin** » Wed Jun 26, 2002 11:01 pm

I have built the PIC interface according to the doc in the FMS install directory. It's not working properly though, and I have a question for PIC guru's: as far as I can remember, the Port A lines on a 16C/F84 don't have internal pull-ups (only Port B lines do, if configured properly). The original schematics doesn't contain an external pull-up resistor on RA4. When I look at RA4 with a scope (serial port connected, FMS started, the PIC is powered OK), I can't get any signal, that's why it's not functionning. I think it needs an external pull-up resistor.
What gives?

by **Isaac** » Fri Jun 28, 2002 12:57 am

You guessed it right, the pull-up resistor *is* necessary!
I noticed this a lot time ago and suggested the change to Vitaly, but he never updated the schematics.

In my opinion the best choice is the interface with PIC12C509A, because this PIC has only 8 pins, doesnÂ´t need a crystal and is cheaper. The fact it is not reprogramable isnÂ´t significant, since all interfaces I built never needed to be reprogramed.

In fact, I designed an interface using PIC12C509A (or PIC12C508A) with several improvements and that works with FMS *and* PreFlight. It autodetects the simulator and chooses the right protocol.

Happy flights!
Isaac

by **Merlin** » Fri Jun 28, 2002 1:23 am

Ok thanks, that's what I thought. Haven't had much time to work on it, but I'm gonna add the #### resistor and probably get started flying ;)
You're right, the 16C84 is definitely not a good choice for this interface, but I had a spare one lying around and wanted to build the interface, and possibly make changes to the firmware (so the flash reprogramming would be handy).
:)

by **Merlin** » Fri Jun 28, 2002 1:58 am

Well yes it does work perfectly now (I added a 4k7 pull-up resistor).

(running FMS 2.0 alpha 7.5.1 under Win2K)

by **gtrick90** » Mon Jul 08, 2002 9:38 am

I've built the circuit with the 16C84 but it doesn't work.
Can you tell me on which pin I must add the pull up resistor?
Thanks in advance

Luca

by **Mike Roberts** » Mon Jul 08, 2002 8:16 pm

Luca

The RA4 pin on the 18 pin PICs is an 'open collector' output. Hence it needs a pull up resistor. 10k should be fine.

Mike

by **gtrick90** » Thu Jul 11, 2002 7:15 am

Thaks a lot Mike.

by **odalaman** » Thu Jul 11, 2002 8:30 am

Hello Mike,

A question regarding the pull up resistor. I understand that one lead of the resistor shall be connected to RA4 pin of the PIC but I will appreciate if you could tell me where the other lead of the resistor should be connected to.

Thank you

Osman

by **Steve Lewin** » Thu Jul 11, 2002 9:57 am

"Pull up" means that it is connected to the +ve supply (Vcc).

The resistor pulls the pin up to Vcc, the PIC then switches it down to ground when it needs to.

Steve

by **odalaman** » Thu Jul 11, 2002 10:05 am

Steve, thanks a lot for the info.

Osman

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PIC 16C84 and RA4 pin issue

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