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Hi Group,

The schematic below (in ASCII art, please view with Fixed font) shows how my friend connects a switch and an LED to one port, so he can make use the port to read a switch in input mode and then changes the port direction to light up the LED.

My questions are:

  1. Would this design cause the port to overload and eventually get destroyed if the switch is pressed too long?

  1. Isn't it advisible to have a diode or resistor added between the switch and the port to prevent the above from happening?

  2. If 26 LEDs each consuming 15mA, were connected to the PIC, and all were switched ON during startup test for a few seconds. Would this endanger the PIC (say 16F877 as an example)?

  1. I have seen in schematic examples that 10 LEDs were connected to a

4017 without any current limiting resistor. Does the CMOS output has some special properties to limit its output current?

  1. The PIC datasheet says that each port can source or sink 25mA. Is this the Max current or working current?

Thank you for reading. Any feedback is welcomed.

Merry Christmas and regards.

Allen Bong

VCC | |---o---| .-----------------------. | | | | | \\ o\\ o\\ o | PIC | \\ \\ \\ | | \\. \\. \\. '-----|------|-----|----' sw1o o o | | | |sw2|sw3| o------|-----|---------o | | | o-----|---------|---o | | | o---------|---|---o | | | | | | V -> V -> V -> | | | -LED1 -LED2 -LED3 | | | | | | | | | | | | | | | .-. .-. .-. .-. .-. .-. | | | | | | | | | | | | 220| | 220| |220| | | | | | | | '-' '-' '-' '-' '-' '-' | | | 10k|10k|10k| | | | | | | o------o-----o---------o---o---| | | GND (created by AACircuit v1.28.6 beta 04/19/05

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Reply to
Allen Bong
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If the output is low and you hit the switch then you could damage the output. You'd force the LED on too when a switch is pressed, I assume that is acceptable? Although it is possible to have the software ensure that the output is never activated *and* low at the same time.

Only if the softare does not do as described above..

Could very well do. The PIC will have a maximum rating for its power pin. 200mA for the

16F88 for example. Why do you need 15mA for each LED anyway, that is a lot for most purposes. 5mA is usually enough, some LEDs even work fine on 1mA or less. Each "port" on the PIC also has it's own maximum rating.

Yes, it has a higher output resistance. Probably not the best practice though!

That is the absolute maximum figure before damage could occur.

Dave :)

Reply to
David L. Jones

If the output is low and you hit the switch then you could damage the output. You'd force the LED on too when a switch is pressed, I assume that is acceptable?

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Yes, it's supposed to light the LED when the switch is hit. Then the PIC would take over using its internal latch. The LED would flash at 1Hz and become permanently on once the PIC receives acknowledgement from another PIC on the Remote end.

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Although it is possible to have the software ensure that the output is never activated *and* low at the same time.

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How would you do it if it were your project, Please ? OR some pointers would be appreciated! I am using PIC Basic Pro.

Allen Bong

Reply to
Allen Bong

If the pin is set to be an output pin and it is set to low (0), then yes it could damage the port pin. This is not a very good design.

IMHO, yes. Resistors are cheap.

Yes. It would greatly exceed the maximum capacity of each port and of the entire device.

Just some resistance. Depending upon the design, this resistance might be very low. This would be yet another example of how not to do something.

I think you mean that each individual pin of a port can source/sink that much current. AIR that device has a limit of 100mA per port and a total device limit of 200mA. I didn't check the data sheet, but you should. ;-)

Reply to
Anthony Fremont

Ok, sounds good then.

This is a common technique and works as follows: Simply write a "HIGH" to all the output pins and leave it like that. Then you acually toggle the output state by setting the mode enable line for that pin. So the output pin actually toggles between "Output-HIGH" and "Input", and never has a chance to go "Output-LOW" i.e. you never actually write a LOW to the output pin.

Hope that makes sense?

Dave :)

Reply to
David L. Jones

This is a common technique and works as follows: Simply write a "HIGH" to all the output pins and leave it like that. Then you acually toggle the output state by setting the mode enable line for that pin. So the output pin actually toggles between "Output-HIGH" and "Input", and never has a chance to go "Output-LOW" i.e. you never actually write a LOW to the output pin.

Hope that makes sense?

Dave :)

got it. Thanks very very much, Dave!

Allen

Reply to
Allen Bong

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