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Discussion Groups | 68HC12 | current max on pin

Join our technical discussions about Freescale Microcontrollers: M68HC12. (Freescale Semiconductor is a Subsidiary of Motorola).

current max on pin - Pascal - Mar 25 12:23:00 2005

Hello, I want to know what is the current max that one pin of a port could deliver? I heard about 20mA but i find it important??!! Thank you very much Regards [Non-text portions of this message have been removed]

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Re: current max on pin - Nigel Johnson - Mar 25 14:53:00 2005

You need to consult the data sheet for the particular chip you are using. 20 mA is common, but there are caveats. For example, the absolute max rating on the data sheet is just that. For the 812 the absolute max is 25mA but that is the value beyond which the mfr does not guarantee that the chip will not be damaged. They also say they do not guarantee operation at this level. So the figure you should look for is the normal oparating current. In some chips, it depends on whether you are sourcing or sinking, in others, there are ports which are higher than others. In any case, you also need to consider the power dissitaion of the chip, looking at how many I/O pins are going to be able to be driven to that current and seeing if this will exceed the maximum power dissipation of the chip at the ambient temperature you are expecteing - in this case you must assume that the program will turn them all on at the same time, or you will end up with a software-destructible chip! Then, of course, at that kind of current, I have to wonder if you are trying to drive a small relay, in which case you need to take precautions against back emf. It's not an easy question! cheers, Nigel Pascal wrote: >Hello, > >I want to know what is the current max that one pin of a port could deliver? >I heard about 20mA but i find it important??!! > >Thank you very much > >Regards > >[Non-text portions of this message have been removed] > > >Yahoo! Groups Links -- Nigel Johnson MCSE, MIEEE VE3ID/G4AJQ http://nigel.homelinux.net If time travel ever will be possible, it already is. Ask me again yesterday [Non-text portions of this message have been removed]

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Re: current max on pin - Stephen Trier - Mar 25 15:09:00 2005

On Fri, Mar 25, 2005 at 05:23:35PM +0100, Pascal wrote: > I want to know what is the current max that one pin of a port could deliver? > I heard about 20mA but i find it important??!! I'm not sure which chip you're using, so I'll tell you how to find the answer for any chip. The number is typically in one of two places in the datasheet. The first place to look is under "Maximum Ratings", and the second is under "DC Electrical Characteristics". If there is a number under DC Electrical Characteristics, that is the number to use. This is the limit under which the chip will still operate correctly. The number under Maximum Ratings is the limit outside of which the chip will be permanently damaged. It may not operate correctly even if it is inside the Maximum Ratings limit, but at least it will still work correctly if you power it down and bring it back up, staying inside the limit. If the number isn't in one of those places, my favorite technique is to scan the units column of the whole electrical characteristics section, looking for anything with units of mA. It usually turns up there. On the 812A4, the maximum rating is +/- 25 mA per port pin. I don't see a specification under DC electrical characteristics, which is actually kind of surprising. Therefore, it would seem that your 20 mA is probably OK for the 812A4. Do note that the pins have a fairly high source impedance, so pulling a high current will pull the pin voltage down very low. It's not unusual to read back 0 from a port pin that is ostensibly set to 1, if the pin has to source lots of current. If in doubt, use an external MOSFET to handle high currents. Then the HC12's current source or sink burden will be nanoamps. Stephen -- Stephen Trier Technical Development Lab Cleveland FES Center

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RE: current max on pin - Allen, Nick - Mar 28 10:50:00 2005

It's always best to buffer the pins with an external driver. Protect the micro from unforeseen transients.... Nick > -----Original Message----- > From: Pascal [mailto:pascal.derex@pasc...] > Sent: Friday, March 25, 2005 11:24 AM > To: 68HC12@68HC... > Subject: [68HC12] current max on pin > > Hello, > > I want to know what is the current max that one pin of a port > could deliver? > I heard about 20mA but i find it important??!! > > Thank you very much > > Regards > > [Non-text portions of this message have been removed] > > > Yahoo! Groups Links

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Re: current max on pin - Pascal - Mar 29 3:26:00 2005

Thank you for help Regards ----- Original Message ----- From: Allen, Nick To: '68HC12@'68H...' Sent: Monday, March 28, 2005 4:50 PM Subject: RE: [68HC12] current max on pin It's always best to buffer the pins with an external driver. Protect the micro from unforeseen transients.... Nick > -----Original Message----- > From: Pascal [mailto:pascal.derex@pasc...] > Sent: Friday, March 25, 2005 11:24 AM > To: 68HC12@68HC... > Subject: [68HC12] current max on pin > > Hello, > > I want to know what is the current max that one pin of a port > could deliver? > I heard about 20mA but i find it important??!! > > Thank you very much > > Regards > > [Non-text portions of this message have been removed] > > > Yahoo! Groups Links Yahoo! Groups Sponsor ADVERTISEMENT ------------------------------------------------------------------------------ Yahoo! Groups Links a.. To [Non-text portions of this message have been removed]

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Re: current max on pin - pi eng - Mar 29 23:05:00 2005

Hi, Others have posted where to find the max current. Be aware that max current doesn't neccessarily mean max voltage. Ensure you have sufficient voltage to drive the required device at the the required current. Regards Eng Pi Pascal <pascal.derex@pasc...> wrote: Hello, I want to know what is the current max that one pin of a port could deliver? I heard about 20mA but i find it important??!! Thank you very much Regards [Non-text portions of this message have been removed] Yahoo! Groups SponsorADVERTISEMENT --------------------------------- Yahoo! Groups Links To ---------------------------------

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