555 timer causing PIC to reset - Electronic Circuits Projects Diagrams Free

gabeNC · 26th May 2009, 01:25 PM

Howdy,

I have a 555 timer wired as a astable, flashing a LED every second or so. I also have 16f876a wired on the same breadboard and working through a seven segment tutorial. Why both? I dunno... cause it's cool to have a bunch of flashing lights. Makes my kids think I know what i'm doing.

The PIC is programmed to display 0 through 9 on the seven segment and then loops back to 0 with a delay of a second or so. The display randomly jumps back to 0 part way through the sequence, to me indicating a reset.

If I disconnect the power to the 555 circuit, the PIC is fine. I have a 4.7K resistor tied to MCLR and I disconnect the pickit2 before flipping the power supply switch.

Power is supplied from a 9V wall wart into a 7805. I just noticed I do not have any power decouping capacitors. Is that the idea behind it? For lack of a better term, "insulating" one circuit from another? Preventing noise or interference?

thanks.

ericgibbs · 26th May 2009, 01:40 PM

hi gabe,

Caps on a 7805 are essential for PIC work.
A 100nF and 470uF on the input side of the 7805 and a 100nF and say 100uF to 220uF on the output side.

Sceadwian · 26th May 2009, 01:43 PM

The 7805 needs capacitors to be stable. A random 7805 PDF I pulled up suggested .33µ for input and .1µ for output. Decoupling caps directly on the 555 or micro controller circuit may be required as well. Make sure you read the recommended circuits in the PDF's for your chips as it's covered in most PDF's.

gabeNC · 26th May 2009, 02:13 PM

Thanks gents for the replies. I believe you but i'm curious, is there a way to measure what is happening to the pic/power supply without an oscope? Is it a transient "spike" when the 555 discharges?

I'm going to add those caps but I really wanted to understand what is actually happening.

ericgibbs · 26th May 2009, 02:19 PM

Quote:

Originally Posted by gabeNC

Thanks gents for the replies. I believe you but i'm curious, is there a way to measure what is happening to the pic/power supply without an oscope? Is it a transient "spike" when the 555 discharges?

I'm going to add those caps but I really wanted to understand what is actually happening.

hi gabe,
All the active devices on your pcb when switching tend to pull the supply rail down a little, as you say its a transient switching spike.

If a number of devices switch at the same instant they will be the rail down even further.

This transient pulldown can cause others devices to mistrigger and the system could latch up.

The caps provide the little extra current when a device switches on and so tends to keep the supply rail voltage steady.

The 100nF cap can also help to 'absorb' transients on the rail by providing a low impedance path to 0V..

Does this help.?

gabeNC · 26th May 2009, 02:25 PM

Thanks Eric. The term "absorb" makes more sense to me. So if you continue to have issues do you just keep increasing the size of the cap?

**Nigel Goodwin** · 26th May 2009, 02:25 PM

You shouldn't EVER use a 7805 without the required capacitors, they aren't optional, they are an essential part of the circuit.

But 555 timers cause large spikes on the power rail, it's a VERY well documented problem with them, and is something you should take account of in any design.

You should also have decoupling capacitors close to the PIC, and the 555 - it's standard practice.

Sceadwian · 26th May 2009, 02:34 PM

The output cap is required for stable voltage regulation of the 7805 as well. There's really no point in trying to study the circuit to understand what's going on, you're basically trying to understand why the car isn't driving so well without any tires on it. Put the tires on and see what happens next =)

ericgibbs · 26th May 2009, 02:37 PM

Quote:

Originally Posted by gabeNC

Thanks Eric. The term "absorb" makes more sense to me. So if you continue to have issues do you just keep increasing the size of the cap?

hi,
As Nigel points out the early TTL version of the 555 does cause a transient on the supply rail when switching, the later CMOS 555 is not a problem.

Rather just added more caps, you should as explained decouple as close as possible to the PIC's power pins.

The 7seg led you are using could draw about 7 * 0.01 = 70mA when switching all the segments. I would always have a 470uF across the +5V, others dont agree..

.. placed near the high current device, the LED in your case.

The 7805 isnt blazing fast in responding to current surges and also the inductance/resistance due to the supply rail wiring can cause the voltage to drop. The 470uF will source that extra current for the device.

gabeNC · 26th May 2009, 03:44 PM

Quote:

The 7seg led you are using could draw about 7 * 0.01 = 70mA when switching all the segments. I would always have a 470uF across the +5V, others dont agree..

.. placed near the high current device, the LED in your case.

So are you saying put a 470uf cap next to the 7 segment? I'm using a common anode and so each cathode segment is tied directly to PORTC (which dims the display without a transistor). That's actually next on my list of things to do, wire it up like Nigel's tutorials, but i'm just experimenting and screwing around at this point.

Thanks for the admonishment gents, i'm sure there will be plenty more stupid questions to follow.

ericgibbs · 26th May 2009, 04:07 PM

Quote:

Originally Posted by gabeNC

So are you saying put a 470uf cap next to the 7 segment? I'm using a common anode and so each cathode segment is tied directly to PORTC (which dims the display without a transistor). That's actually next on my list of things to do, wire it up like Nigel's tutorials, but i'm just experimenting and screwing around at this point.

Thanks for the admonishment gents, i'm sure there will be plenty more stupid questions to follow.

hi,
I havnt seen your layout, so all I am saying place the 470uF cap near the LED rail rather than directly on the 7805 output.

Sceadwian · 26th May 2009, 04:20 PM

Gabe are you using input/output caps on the 7805 yes or no? You said you weren't using any bypass caps but I'm not clear on weather you consider those bypass caps or not.

gabeNC · 26th May 2009, 04:53 PM

Quote:

Originally Posted by Sceadwian

Gabe are you using input/output caps on the 7805 yes or no? You said you weren't using any bypass caps but I'm not clear on weather you consider those bypass caps or not.

I'm not using any bypass caps. I will now.

I was under the impressions that the cap Eric was talking about was in addition and directly for the 7-segment?

Sceadwian · 26th May 2009, 05:37 PM

Yes, that's what eric was talking about. I just wanted to make sure you're going to add the 7805 bypass caps as well, as Nigel said, they're not really a suggestion they're required for proper predictable operation. Good luck =)

ericgibbs · 26th May 2009, 06:06 PM

Quote:

Originally Posted by Sceadwian

Yes, that's what eric was talking about. I just wanted to make sure you're going to add the 7805 bypass caps as well, as Nigel said, they're not really a suggestion they're required for proper predictable operation. Good luck =)

Quote:

hi gabe,

Caps on a 7805 are essential for PIC work.
A 100nF and 470uF on the input side of the 7805 and a 100nF and say 100uF to 220uF on the output side.

I had already advised gabe about these capacitors.

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26th May 2009, 01:25 PM	#1
gabeNC	555 timer causing PIC to reset Howdy, I have a 555 timer wired as a astable, flashing a LED every second or so. I also have 16f876a wired on the same breadboard and working through a seven segment tutorial. Why both? I dunno... cause it's cool to have a bunch of flashing lights. Makes my kids think I know what i'm doing. The PIC is programmed to display 0 through 9 on the seven segment and then loops back to 0 with a delay of a second or so. The display randomly jumps back to 0 part way through the sequence, to me indicating a reset. If I disconnect the power to the 555 circuit, the PIC is fine. I have a 4.7K resistor tied to MCLR and I disconnect the pickit2 before flipping the power supply switch. Power is supplied from a 9V wall wart into a 7805. I just noticed I do not have any power decouping capacitors. Is that the idea behind it? For lack of a better term, "insulating" one circuit from another? Preventing noise or interference? thanks.

26th May 2009, 01:40 PM	#2
ericgibbs	hi gabe, Caps on a 7805 are essential for PIC work. A 100nF and 470uF on the input side of the 7805 and a 100nF and say 100uF to 220uF on the output side. __________________ Eric " Good enough is Perfect " I will NOT answer PM's requesting technical help, please use the Forum PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

26th May 2009, 01:43 PM	#3
Sceadwian	The 7805 needs capacitors to be stable. A random 7805 PDF I pulled up suggested .33µ for input and .1µ for output. Decoupling caps directly on the 555 or micro controller circuit may be required as well. Make sure you read the recommended circuits in the PDF's for your chips as it's covered in most PDF's. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har." Last edited by Sceadwian; 26th May 2009 at 01:44 PM.

26th May 2009, 02:13 PM	#4
gabeNC	Thanks gents for the replies. I believe you but i'm curious, is there a way to measure what is happening to the pic/power supply without an oscope? Is it a transient "spike" when the 555 discharges? I'm going to add those caps but I really wanted to understand what is actually happening.

26th May 2009, 02:25 PM	#6
gabeNC	Thanks Eric. The term "absorb" makes more sense to me. So if you continue to have issues do you just keep increasing the size of the cap?

26th May 2009, 02:25 PM	#7
Nigel Goodwin	You shouldn't EVER use a 7805 without the required capacitors, they aren't optional, they are an essential part of the circuit. But 555 timers cause large spikes on the power rail, it's a VERY well documented problem with them, and is something you should take account of in any design. You should also have decoupling capacitors close to the PIC, and the 555 - it's standard practice. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

26th May 2009, 02:34 PM	#8
Sceadwian	The output cap is required for stable voltage regulation of the 7805 as well. There's really no point in trying to study the circuit to understand what's going on, you're basically trying to understand why the car isn't driving so well without any tires on it. Put the tires on and see what happens next =) __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har." Last edited by Sceadwian; 26th May 2009 at 02:35 PM.

26th May 2009, 04:20 PM	#12
Sceadwian	Gabe are you using input/output caps on the 7805 yes or no? You said you weren't using any bypass caps but I'm not clear on weather you consider those bypass caps or not. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

26th May 2009, 05:37 PM	#14
Sceadwian	Yes, that's what eric was talking about. I just wanted to make sure you're going to add the 7805 bypass caps as well, as Nigel said, they're not really a suggestion they're required for proper predictable operation. Good luck =) __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."