a problem in my circuit

tl7705 circuit

Hi

I have a circuit that have some analog and digital parts + microcontroller and a dot matrix lcd.

When I strike with a metal rod (in my hands) to the ground trace of PCB of that circuit ,the system will be reset and LCD shows some strange data.

I have TL7705 as supply supervisor and reset generator.

Please say how Can I solve this problem.


Regarsd

Which processor are you using?

Can you post the circuit diagram (as a PDF)?

best regards

Check if any pin matter is floating.

hi

The micro is AT90S8515,I had similar problem with AT89C52 last year.

There is no float point.

If I strike with a metal rod(in my hands that have 50HZ noise) to GND this occurs ,(to other traces NO)


The circuit is simple you can imagine it,a micro with TL7705 and LCD .

I do not know how to solve it ! 8O

Hi

Surely the problem would be with your psu then?

Or perhaps your groundplane needs to be improved.

Good luck!

Did you put a bypass 0.1u capacitor accross REF and ground.
Have you connected GND (pin 4) to a low impedence ground track of your layout. TL7705 may be very sensitive, so usually it is located as closest as possible to psu. Try to cut pin 4 from ground track and replace with wire connected directly with a point closest to psu 5V ground.

Some useful tips here:
**broken link removed**

Obviously, your circuit is not well grounded.
Simply to as James told. Bypass caps will helps lot with your design.

Hi


If I bypass TL7705 with a simple reset circuit (resistor + cap),the problem exist but at lower rate,sometimes LCD lost the sequence and shows strange characters(if i strike).


I use 100n cap for all ICs and additional 10u for micro.
How can I solve or reduce this problem?


james pdf was good.

If you replaced TL7705 with simple res/cap reset and problem still happens (though its occurrence is statistically reduced) it is a further confirmation that noise you introducing will suffice to reduce signal integrity.
Again bad grounding could be the main reason. A missing pull-up may be the second one, or not connected pin, or again your psu is coming with low voltage or it's quite noised.

Which value of main clock are you using ? Are clock tracks short enough ?

From what I've understood your application is a single chip; no exram, so no ALE has been used. You said an LCD is connected to port. Which port ?

It's very difficult to help without see a full circuit and/or engineerization used. For sure if your LCD is printing funny characters when you touch ground it means int ram contents is destroied or who knows where the program counter is pointing to. Did you make use of watchdog ??

thanks james
your notes are correct and I checked them on the circuit ,I must redesign my pcb with good grounding and apply some rules to PCB design .

Regards