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'[PIC] USART loading'
2005\12\01@161723 by Edward Cooper

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Hi guys,

I'm trying to use my PIC16F687's Tx port on the EUSART as a dual  
function pin, ie as a connect detect input and as a Tx channel.

So here's the question, what's the maximum safe loading on the Tx  
port when its doing RS232, I'm imagining its in the region of 2mA,  
but I can't find any details on the datasheet, and its seems unlikely  
it would match the 20mA of the GPIOs...

Thanks - you lot are the best....

Ed

Oh wont be running above 57.6k, incase that's at all relevant, and  
I'm at 3.3v

2005\12\01@164749 by David Van Horn

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> So here's the question, what's the maximum safe loading on the Tx
> port when its doing RS232, I'm imagining its in the region of 2mA,
> but I can't find any details on the datasheet, and its seems unlikely
> it would match the 20mA of the GPIOs...

No spec for minimum high/low output voltage into a given load?




2005\12\01@164913 by Harold Hallikainen

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> Hi guys,
>
> I'm trying to use my PIC16F687's Tx port on the EUSART as a dual
> function pin, ie as a connect detect input and as a Tx channel.
>
> So here's the question, what's the maximum safe loading on the Tx
> port when its doing RS232, I'm imagining its in the region of 2mA,
> but I can't find any details on the datasheet, and its seems unlikely
> it would match the 20mA of the GPIOs...
>
> Thanks - you lot are the best....
>
> Ed
>
> Oh wont be running above 57.6k, incase that's at all relevant, and
> I'm at 3.3v


I don't see why they'd use different pin drivers for different
peripherals, so I think it's the same for the USART as for general I/O. Of
course at 3.3V, you can't get the 25mA you can get at 5V, but that's not
the USART's problem.

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com

2005\12\01@165405 by Gerhard Fiedler

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Edward Cooper wrote:

> I'm trying to use my PIC16F687's Tx port on the EUSART as a dual  
> function pin, ie as a connect detect input and as a Tx channel.
>
> So here's the question, what's the maximum safe loading on the Tx  
> port when its doing RS232, I'm imagining its in the region of 2mA,  
> but I can't find any details on the datasheet, and its seems unlikely  
> it would match the 20mA of the GPIOs...

It /is/ a normal IO pin, even when being driven by the EUSART. The drivers
are the normal port drivers, just the logic is different.

I'm not so sure about the 20 mA though. I haven't worked a PIC in that
range yet. The data sheets mostly talk about 3 mA sourcing and 8.5 mA
sinking. You probably can go further, but that doesn't seem to be
guaranteed.

Gerhard

2005\12\01@170558 by Spehro Pefhany

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At 09:17 PM 12/1/2005 +0000, you wrote:
>Hi guys,
>
>I'm trying to use my PIC16F687's Tx port on the EUSART as a dual
>function pin, ie as a connect detect input and as a Tx channel.
>
>So here's the question, what's the maximum safe loading on the Tx
>port when its doing RS232, I'm imagining its in the region of 2mA,
>but I can't find any details on the datasheet, and its seems unlikely
>it would match the 20mA of the GPIOs...
>
>Thanks - you lot are the best....
>
>Ed

Ed:-

The same output buffer is used whether it's sending data or acting as a
general purpose I/O-- just the input to the buffer gets switched by a
2-input mux, as you can see in the block diagram for the relevant pin--
so the DC characteristics are the same.

Note that as someone else has hinted at, safe loading is not the same as
maximum loading to have the output valid 0/1, also note that the pin does not
really "do RS232" since it provides logic-level output (and inverted).

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\12\01@170916 by Edward Cooper

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On 1 Dec 2005, at 21:49, Harold Hallikainen wrote:

> Of
> course at 3.3V, you can't get the 25mA you can get at 5V, but  
> that's not
> the USART's problem.

Eh!?  Where does it say that on the datasheet!?  Why of course?

Cheers

Ed

2005\12\01@173742 by David Van Horn

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> > Of
> > course at 3.3V, you can't get the 25mA you can get at 5V, but
> > that's not
> > the USART's problem.
>
> Eh!?  Where does it say that on the datasheet!?  
> Why of course?

Because the output device is a FET, AKA a resistor.

Output current into any resistive load will always be lower, with lower
VCC.



2005\12\01@174044 by David Van Horn

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> Note that as someone else has hinted at, safe loading is not the same
as
> maximum loading to have the output valid 0/1, also note that the pin
does
> not really "do RS232" since it provides logic-level output (and
inverted).


That, and ESD protection is not what it should be, and 232 devices are
allowed to give you 24V on those pins, which could be interesting...

That's what they made 1489/1488 chips for :)



2005\12\01@174800 by Spehro Pefhany

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At 05:46 PM 12/1/2005 -0500, you wrote:
> > > Of
> > > course at 3.3V, you can't get the 25mA you can get at 5V, but
> > > that's not
> > > the USART's problem.
> >
> > Eh!?  Where does it say that on the datasheet!?
> > Why of course?
>
>Because the output device is a FET, AKA a resistor.
>
>Output current into any resistive load will always be lower, with lower
>VCC.

Draw enough current and it no longer acts much like a resistor and
becomes a (crummy) current source (once the output drop exceeds Vth,
approximately). This may be within abs. max. current specifications at
lower supply voltages.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\12\01@181450 by Harold Hallikainen

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>
> On 1 Dec 2005, at 21:49, Harold Hallikainen wrote:
>
>> Of
>> course at 3.3V, you can't get the 25mA you can get at 5V, but
>> that's not
>> the USART's problem.
>
> Eh!?  Where does it say that on the datasheet!?  Why of course?
>
> Cheers
>
> Ed


Well... I THOUGHT there were graphs in the datasheet showing source/sink
current versus supply voltage, but I sure can't find them. However, I've
had to add drive transistors to drive LEDs (digit select on 7 segment) on
3V systems while I did not on 5V systems. It also makes sense, since
there's not as much gate voltage on the FETs when the supply voltage is
not as high. But, I think you're right about it being on the datasheet (I
can't find it!).

Harold



--
FCC Rules Updated Daily at http://www.hallikainen.com

2005\12\01@195405 by Edward Cooper

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face

On 1 Dec 2005, at 23:14, Harold Hallikainen wrote:

{Quote hidden}

Exactly, I would of thought its on there because, if its not there,  
I'm not entirely sure how
I'm meant to know what current I can expect from each pin.  I'm  
currently setup to drive
some LEDs directly on a matrix, I may have to reconsider this has you  
had to.

Though I may be better off running at 5v and doing the level  
conversions on i2c and
serial (yes I know its not rs232 until it hits the maxim chip).

Anyway it appears I can do my light loading on the tx channel without  
any interference
in serial communications.

Cheers all.

Ed



>
>
>
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