Battery operated device

[Agustina]

Hi everyone! I'm designing a battery operated circuit for automotive applications.

Everything is working fine! but... now I have a problem with some cars.

Sometimes, the LCD (displaytronic 16x2) hangs up!, the pic continues working ok, but the display dies!

I think the problem is the ignition noise... or something generating a very big spark. I don't understand why the pic doesn't hang up like the LCD, but its driving me crazy.

Some help please!!!

[PCM programmer]

See this thread:
http://www.ccsinfo.com/forum/viewtopic.php?t=32927

Also see this appnote:

Intel AP-125 -- Designing Microcontroller Systems for Electrically Noisy
Environments:
http://ecee.colorado.edu/~mcclurel/iap125.pdf

[Agustina]

I think i designed the pcb with a proper filtering...

the power supply:

9v bat
660uH (in series)
schottky diode (in series)
1000uF to ground
100nF to ground
5v regulator
100uF to ground
100nF to ground (close to pic and LCD)
PIC
LCD

MCLR circuit:

5v----10K----100nf (to ground) ---1k---- MCLR

IO ports:

Every input port have a 100nF cap close to the pin, then 10k pullup, then a 1K resistor (except LCD pins).

Thanks!

[FvM]

Do I understand right, that your circuit is powered by a 9V battery? Then it's very unlikely, that the problem has anything to do with the power supply.

You should rather think about the paths, that make the circuit susceptible to external interferences. You didn't tell about any external wiring, I guess the circuit has some.

Another interesting point is the wiring between PIC and display. What's the length? Does it have a good ground connection, at best multiple ground wires. A popular fault with alphanumeric LCD displays is to short the display frame to a metallic enclosure.

Even if the LCD display content or programming get's corrupted at times, it can be possibly refreshed by the controller. If you're operating the interface bidirectionally, you're most likely able to detect a display corruption by software and trigger a reinitialization. You have this as a last resort, if you can't overcome the interferences...

[Agustina]

Hi FvM! thanks for your answer.
answers below:

[treitmey]

Just for a test.
Tie all pins from the LCD also to a sp720 chip.
(including LCD chassis gnd and LCD gnd)

Then, as a separate test, tie 9V battery gnd to car gnd.

I bet the problem will go away.

RE: LCD refresh
Most programs have a main while(1) loop. It loops and loops real fast.
So replace all your update LCD messages with a update of two 16 char arrays. Call them int8 LCD1[16] and int8 LCD2[16].
In your main while(1) loop you then re-init the LCD and then lcd_putc the two arrays. That way each time though the loop the LCD is reset and updated with the current 2 lines.

[Agustina]

Something that I do often is to connect the chassis LCD to logic gnd, is this a bad thing?


I'll try the rest!

I will keep investigating the problem! if you have another suggestion please post!

Bye!

[Guest]

Hi

Maybe you can post the connection from PIC to LCD, or the better the complete schematic.

If not:

Is there unconnected pin on the LCD, what port on the PIC is it connected to. Because some pin is ST look PIC datascheet.

......

[Agustina]

hi! I don't have the schematics, sorry, I can try to draw it:

B0 -------- enable
B1 -------- rs
B2 -------- rw
B4 -------- D4
B5 -------- D5
B6 --vvv-- D6
B7 --vvv-- D7

B7 and B6 have a 4k7 resistor.

Vee pin is connected to gnd via a 1k resistor.
Vcc and gnd pins have a 100nf cap.

thanks!

[Guest]

Why the resistor 4k7 on B6/B7? Are you are using them for ISP. Try move the pin for LCD to other free PIC pin.

Have you checked the port B in the datasheet for ST logic level?

What pic are you using?

[Agustina]

I'm using a 18F252.

The resistor is for ICSP, but was used on the original design, now the icsp is replaced by a bootloader.

[mskala]

If the problem is related to ignition noise (high-frequency), then your cable
shield should be connected to ground at both ends.

[Agustina]

The thing is that the cable has a simple switch on the end. I can't connect it to ground.

Could some 5v varistors help? If I connect them on the input pin?

Thanks

[Agustina]

Anything? please I really need help, I have tried everything.

Thanks!

[PCM programmer]

If you want help, you will have to post links to a schematic and
a photograph of the device (showing the wiring and the construction).