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wildpossum

Junior Boarder 		Click here to see the profile of this user

2006/11/13 01:00

Sinking Current from a I/O Pin to drive a Tx.

Hi.

A general question reference the amount of current I can get out of one I/O pin on a LM3S101-1RN20 chip (as I am a little confused by the current documentation).

I have a application that need to turn off & on an external module. The external module draws a maximum (stable) of 13mA @ 3V3, with an inrush current of 17mA for ~1ms.

Can I safely use a I/O pin to sink (turn on ext module) 15mA?

Is it acceptable to parallel two (or more) I/O pins together (whilst ensuring switch bit(s) concurrently) to increase the total current sink, OR is this a big no no?

Any assistance appreciated. Thanks.

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LMI Eric

Moderator 		Click here to see the profile of this user

2006/11/13 13:38

Re:Sinking Current from a I/O Pin to drive a Tx.

The I/O pins aren't rated to sink that much current, so the answer is no. I would recommend using the I/O pin to drive a FET instead.

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wildpossum

Junior Boarder 		Click here to see the profile of this user

2006/11/13 18:06

Re:Sinking Current from a I/O Pin to drive a Tx.

Thanks Eric for your reply.

But you didn't answer the second part of my question. On CMOS based uP I have, at times in the past married two or three readily available outputs with a current limiting resistor in series and then combined the Output pins to derive the required current sink (but only in cases where I was dealing with a poorly spin artwork - for testing).

So, is this still a viable situation on LMI devices?

(Don't worry this is more a information request than a actual - I am doing to do so request)

Thanks.

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LMI Eric

Moderator 		Click here to see the profile of this user

2006/11/14 10:29

Re:Sinking Current from a I/O Pin to drive a Tx.

Sorry I missed the second part of your question.

In theory, it is possible to sink the amount of current you're describing by using multiple pins, but the thing to keep in mind here is that the pins are spec'd to source/sink 2/4/8mA at TTL levels (0.4 and 2.4V). With that said, by using the GPIO pins to sink current (or as the negative side of your supply for the external device), your "ground" may not necessarily be at 0V.

The best solution for what you're trying to do is using an external FET.

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