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PICList Thread
'Simple PIC to PIC comms'
1999\04\08@145754 by Stuart Meier

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I have a project with a PIC testing sensors, and displaying results remotely
(2 metres away), on 6 leds. The environment is avionic, with little room for
multiway ribbon cable.

I plan to use a 12c508 to read/analyse sensors, and transmit results (an 8
bit word) to another 18pin PIC on the display board, talking serially (TX
only) the PICs linked by 3 ways (+5,0, data) RS232-like.

Firstly, I would welcome advice on synchronising the PICS. My current
thought, given there is no need for results to update more than say four
times per second, is to hold data line low until

1.5 units start bit (high)
8x1 unit data bits
16 units low (synchronising again)

and repeat three times to validate.

Secondly, how much trouble do you think I might have with EMC. Do I need to
load the data line capacitatively to slow rise rate to reduce emc?

Many thanks for any advice.

Stuart Meier

1999\04\08@152459 by Wagner Lipnharski

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Is this "second" PIC at the panel (with the 6 leds) just converting the
8 bits received from the first PIC and driving LEDS?  Is this just a
level indication? I mean, each LED correspond to a level step like a
voltage? or inclinometer or pressure? If yes, why don't convert to
analog right at the first PIC and using the 3 wire just to feed a LED
driving chip like the LM3914? it works directly with 12V as VCC (goes up
to 25V), and an adjustable reference, so pretty nice for battery
operation, probably can save a wire between the 3915 and the PIC "dac".
The LM3915 can drive LCDs or even vacuum fluorescent displays. The DAC
at the PIC could be done easily with a R-2R configuration.
Wagner

Stuart Meier wrote:
{Quote hidden}

1999\04\08@160706 by MEDICINTEKNIK KB

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The EMC problem (YOU SAID AVIONIC - OH DEAR..):

1: Slow down. use least possible data rate and definitely slowest clock possible on the PIC (not it's nax of 4MHz ), and "can" the PIC well.

2: Get a dedicated expensive cable, or, use a ribbon cable, and make sure that you ground every other lead. The trick to do so, has of course a serious loading effect (capacitance) but the price has a very good reward: Dramatically reduced EMC. Better than you would guess.

3: Put 1 (or even 10) kOhms in series to reduce drive current. Now you are slowed down. But radiate only little. I don't think you need extra capacitance now, but reducing the current is healthy.


Sven
-----Ursprungligt meddelande-----
FrŒn: Stuart Meier <spam_OUTsmeierTakeThisOuTspamPAVILION.CO.UK>
Till: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU <PICLISTspamKILLspamMITVMA.MIT.EDU>
Datum: den 8 april 1999 20:58
€mne: Simple PIC to PIC comms


{Quote hidden}

1999\04\08@163016 by Dave VanHorn

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>1: Slow down. use least possible data rate and definitely slowest
clock possible on the PIC (not it's nax of 4MHz ), and "can" the PIC
well.

>3: Put 1 (or even 10) kOhms in series to reduce drive current. Now
you are slowed down. But radiate only little. I don't think you need
extra capacitance now, but reducing the current is healthy.



These two are my favorites. DONT add capacitance, you're only
increasing the high speed spikes on transitions. :-P


>>Secondly, how much trouble do you think I might have with EMC. Do I
need to
>>load the data line capacitatively to slow rise rate to reduce emc?


Keep the uP xtal lines short, run the xtal caps directly back to the
pic ground pin.
Murata makes a nice EMI filter that will stomp it flat on the VCC
line, for most projects I use the one with the highest C. Select from
the catalog at http://www.digikey.com

1999\04\08@172957 by Stuart Meier

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Sven said :

1: Slow down. use least possible data rate and definitely slowest clock
possible on the PIC (not it's nax of 4MHz ), and "can" the PIC well.
?Plan was to use internalRC osc on the 12C508 - ie no external crystal/caps
etc. BUT can't then slow it down...

3: Put 1 (or even 10) kOhms in series to reduce drive current. Now you are
slowed down. But radiate only little. I don't think you need extra
capacitance now, but reducing the current is healthy.
?to reduce PIC drive current - ie resistor in series with PIC Vdd?

Many thanks for your help

Stuart

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