GPIOs

Thread Starter

chandu13

Joined Oct 15, 2008
22
Hay
I am using the LPC2114 micro controller, on RESET all GPIO pins of the controller will be INPUTS & the logic on the pins is 0 (low state).
If the micro controller burns or fail or die then what will be the status of the GPIOs

Regards
chandu
 

scubasteve_911

Joined Dec 27, 2007
1,203
The GPIO for any modern microcontroller is a complex structure. When something burns, you never really know which part will fail.

From practical experience, I have blown either the upper or lower transistor when the device was configured as an output. This is because I was either trying to drive a short high or pull a low-impedance high line low.

You have less to worry about when it is an input. If the input is high impedance, these can tolerate a lot of overvoltage because the input diodes clamp the signal.

Steve
 

John Luciani

Joined Apr 3, 2007
475
You have less to worry about when it is an input. If the input is high impedance, these can tolerate a lot of overvoltage because the input diodes clamp the signal.
For the protection diodes to work for large overvoltage conditions you need to limit the current with a series resistance. There is usually an absolute maximum input current specification in the datasheet.

(* jcl *)
 

Thread Starter

chandu13

Joined Oct 15, 2008
22
Hay thanks for the reply

The micro controller burns or fails or dies because
1. The GND pin & Vcc pin short
2. Applying high voltage to the Vcc
3. Applying high voltage to the GPIOs (INPUT mode)

I am controlling C2H2&N20 solenoids using controller, when controller pin high (1) solenoid will ON if controller pin low (0) solenoid will OFF
When the controller is RESET both solenoids will be OFF.
If micro controller burns or fails or dies then what will be the status off the solenoids
Regards
chandu
 

scubasteve_911

Joined Dec 27, 2007
1,203
John,

I was ambiguous in my statement, I meant if the source impedance is high, then the clamp diodes should handle higher voltage. You're right though, in regards to the current limitation.

We told you. The transistor that is responsible for sourcing or sinking will no longer work. Sometimes both of them get destroyed. Depending on the failure mode, the transistor could be stuck open or shut. Which results in a steady low or high signal.

Steve
 

eblc1388

Joined Nov 28, 2008
1,542
These diodes are never supposed to pass any current. In fact I have seen Microchip's document mentioned that current through these protection diodes are out of spec. of the PIC. They don't even put a max. current value for them in the datasheet.

At least this is my understanding. Higher input voltages can be dealt with using clamp diodes and current limiting resistors.
 

scubasteve_911

Joined Dec 27, 2007
1,203
These diodes are never supposed to pass any current. In fact I have seen Microchip's document mentioned that current through these protection diodes are out of spec. of the PIC. They don't even put a max. current value for them in the datasheet.

At least this is my understanding. Higher input voltages can be dealt with using clamp diodes and current limiting resistors.
Many microcontrollers allow it, so it probably varies by manufacturer. I have even seen zero-crossing detectors straight from 120V into the analog inputs through a high impedance.

Steve
 

eblc1388

Joined Nov 28, 2008
1,542
I have located the document that warns against doing so for PICs by Microchip. Funny enough some of their application notes are doing just that.

But the document is quite recent so maybe it doesn't apply retrospectively.

Edited: A definitive answer from Microchip regarding this issue is presented here in this thread: http://forum.allaboutcircuits.com/showthread.php?p=107510#post107510
 

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John Luciani

Joined Apr 3, 2007
475
I have located the document that warns against doing so for PICs by Microchip. Funny enough some of their application notes are doing just that.

But the document is quite recent so maybe it doesn't apply retrospectively.
That note warns against passing *sustained* currents through the diodes. ESD
is transient. Usage of the diodes will vary by application.

Unless a manufacturer specifies the ESD performance of the protection diodes
I would use an external TVS (e.g. Littelfuse SP72x).

(* jcl *)
 
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