Short circuit in a Picaxe chip?

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Amnesia67

New Member
Hello,

I'm French, so excuse my bad english:eek:.

I'm actually building my first electronic project, with two Picaxe 28X1. It's a sequential interrupter and source selector for a Hi-Fi installation.

It worked well for 3 or 4 weeks, but yesterday a Picaxe became warm and the programm run as I was permanently pushing a switch. The other picaxe was ice cold, but its programm reacted as an input was always high too.

After researching the failure, I found a short circuit between an input pin and the +Vcc pin of the warm picaxe.

I have to make clear that my circuit gets information from 4 switches and one infrared sensor, which are wired to the inputs of the defective picaxe. These same inputs are then directly connected to the inputs of the second picaxe, and pulled down via 10k resistors.

Is my circuit false, or happened something else to this poor chip?
I never heard about linking two µc together, so I count on your help.

Sorry for this long story :p. Thank you for reading ;)
 
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manuka

Senior Member
Bonjour- your Anglais is A1 ! It'd greatly help if you supply a circuit diagram, a layout picture, & your code. In ~7 years I have never lost a PICAXE, but most have been running on 3 x AA cells (4.5V). What supply are you using? Stan.
 
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eclectic

Moderator
Welcome to the Forum.

Please post
1. Your schematic / circuit diagrams.
2. A photograph of the circuit board.

e
 
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Amnesia67

New Member
Oh sorry, it's a 28X1 Picaxe. On the schematic, the damaged chip is the one on the left. The five input tracks (switches + IR) are the small one under the µc's.

The current is provided by a provisional 12VDC adaptor, then a 7805 regulator + capacitors

The photograph is the whole project board, the 19" casing isn't drawn yet.
I see we can't post more than 2 pictures, so if needed, I'll add other photographs on an other reply :)

Manuka: for "code", what do you mean, the program of both picaxes? How can I supply it? ( about 600 lines each)
 

Attachments

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westaust55

Moderator
A schematic (circuit) diagram would be easier to review than a PCB layout.

Do you have resistors (say 1 k Ohm) at the 28X1 inputs?
This will help prevent a short in the event that a port C pin is converted to an output and is shorted out - which will result in high current and pin logic failure.

Your BASIC program (code) can be posted by:

1. Copying the text into your post directly with [code] and [/code] markers

or

2. uploading the .bas file as an attachment. Click in the paper clip icon at the top of your post (first use the Go Advanced button if editing the post) then browse to the.bas program file and upload.
 
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Amnesia67

New Member
The hand wroten schematics are dispatched on many sheets lol, difficult to send something :D

But the principle is easy: I control the system with 4 switches or an infrared receiver (which will be enclosed in a separate casing & connected to the main board with sub D 15 plugs).

When I press a switch, it provides +5V to the corresponding input of the first picaxe. This same input is directly connected to an input of the second picaxe. The link is pulled down via a 10k resistor. Idem for the 3 other switches. Idem too for the infrared sensor, but without the pull down resistor.

But no, the inputs are not individually protected with these 1k resistors...

PS: I only use inputs that are definied by default as inputs. But on the picaxe which is still ok, I use two input pins as outputs (these pins are naturally not connected to the other chip)




As attached files: the picaxe #1 controls REED relays for audio sources selection (connects the audio in line level of the selected source to a unique output). It also controls the LED's corresponding to the selected switch/source.

The picaxe #2 controls 10 solid state relays of 8 & 16A, 230VAC for switching in a defined order the selected source, audio processor, filter, DIY volume controller and finally two amplifiers.

In both programms there are basic security to avoid inopportune source change while listening, and delays to synchronize the actions of both picaxe (so they are ready to receive external information always at the same time).

It's my first program, that's why it looks so easy :rolleyes::D
 
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boriz

Senior Member
“my first electronic project”

Wow. Ambitious. Fine job though.

Test the PCB with all ICs removed. Is the fault on the PCB?
 
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Amnesia67

New Member
No, the short circuit is between two pins of the chip (between an input and the +Vcc in, not on the PCB. It's an internal short circuit, so the program runs as the input was always high.

When I release the picaxe of its suport and plug an other one, the circuit works well. The circuit is OK...
 
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BeanieBots

Moderator
This is a classic symptom of an overloaded output.
When an output tries to source/sink current over it's specified maximum, the transistor(s) on the output stage melt and form a short either to Vcc or to 0v.

Double check that you are not exceeding it's maximum current (~25mA) and/or that there was no "event" which may have presented either a short or accidental connection to 0v and/or any supply rail.

Sorry, but I am not prepared to draw a diagram from your PCB layout to do the checks for you.
 
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westaust55

Moderator
In an earlier post, Amnesia67 stated:
After researching the failure, I found a short circuit between an input pin and the +Vcc pin of the warm picaxe.
Click to expand...
And has also subsequently stated it is a 28X1.

A 28X1 input can be changed to an output with a HIGH PortC pinX command, So yes, as BB indicates, the failure could be a result of an incorrect command on what is supposed to be an input making it an output and an overload situation eventuating.
 
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Amnesia67

New Member
If I understand, connecting the inputs of two picaxe directly together doesn't matter (like they say on a french forum: I MUST protect each input with a 10k resistor in my case, inputs of two chips should not be directly linked... are they right)?

The outputs are connected to darlington chips. ;)
 
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Andrew Cowan

Senior Member
The danger is if voltage is put into an output. If the pin is input only (ie pin3 on an 08M - it cannot be an output), then you are safe enough. The 10K stops any damage happening if the pin is accidentially made into an output.

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Amnesia67

New Member
I use only by default defined inputs as inputs. But I use 2 input pins as outputs (and use them only for outputs). Can the picaxe supply accidentally any voltage through an input pin (so the other picaxe which is directly connected will be damaged?)

On a french forum somebody told me that when I turn power on, the two picaxes don't start exactly at the same time, one can reset its pins before the other. Is this connection wrong? (from input to input, without resistor or diode?):(
 
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BeanieBots

Moderator
When the chip powers up, it will be INPUT so no damage.
It is only when IP is changed to OP (by code) AND connection to Vcc or 0v is made that damage will happen.
If an input has voltage applied when the chip is NOT powered then damage can happen.
Two inputs connected (that are always inputs) and both PICAXEs on the SAME power rail should not be a problem.
 
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Amnesia67

New Member
BeanieBots said:
When the chip powers up, it will be INPUT so no damage.

Two inputs connected (that are always inputs) and both PICAXEs on the SAME power rail should not be a problem.
Click to expand...
That's an answer like I like them!:)

On this french electronic forum (not specific for Picaxe), people shout like I've done the biggest error by connecting the inputs of two picaxe together!

"when they power up, they reset. Until the reset is done, pins can be either input or output. This way a short output voltage can damage the input diodes of the other chip... NEVER link inputs without resistors" :(
 
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BeanieBots

Moderator
I'd still recommend that you fit a resistor just in case your code has an accident.
If for example, you had a gosub with no return, the PICAXE stack would overflow sending it into a major 'brain haemorrhage'. This could result in anything happing which includes making an input into an output.
 
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Amnesia67

New Member
OK, let's suppose the program is tested, everything's OK and reacts correctly in each situation => nothing can happen? (the accident would come from an programmation error, not from the picaxe, am I right?)


In that case, in an electrical point of view, my actual circuit that links the inputs of two Picaxe withour any protection (the resistor you recommend, or diodes) isn't false and can work fine and reliable. And what somebody told me about potential risk at the reset of the chips (see previous reply) would be wrong (I hope..)

That's my main question...:confused:

Anyway, thanks for your great help! ;)
 
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BeanieBots

Moderator
Technically, you are correct and they are wrong wrt PICAXE.
However, if they are referring to a PIC, rather than a PICAXE it may be that they are just being cautious which is not a bad thing.

It does still leave the matter of what DID destroy one of your I/O pins!
 
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Amnesia67

New Member
Thanks for your help BeanieBots! I'll keep trying every possible situation with my circuit, then if everything works fine, I'll mount it in a 19" aluminium casing :)

Do people generally show the final project on the forum? ;)
 
hippy

hippy

Ex-Staff (retired)
The issue on connecting PICAXE inputs together is not always straight forward.

If all the inputs can only ever be inputs there is no need for resistors, and that's usually also true if only one of those inputs could ever be an output. There are however only a few pins which are genuinely input only on the PICAXE.

Most inputs can be outputs at the silicon level even if not allowed by PICAXE specification and operation. Under most circumstances those inputs will not become outputs so there will not be a problem without resistors, however there is the chance that they may be made output under program control, deliberately or by accident, and also a chance of that happening under adverse external influences. It is therefore recommended to fit protecting resistors.

It's like trying to answer the question, 'if two cars are driving alongside each other at the same speed, will they crash into each other, do we need a mattress between them to prevent damage' ? Normally no, they won't crash into each other. But if one driver chooses to change lane they will. If there's an unexpected gust of wind it might cause one to unwillingly change lane.

On showing final projects, not all people do but we do encourage that.
 
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