Source & Sink Capability of GPIO Pins

What do you mean "general specification"? The spec is going to depend on the chip maker, the product line, the model chip and even the specific pin. The only thing remotely like a spec were the various TTL variations from which a de facto standard emerged for 5 volt systems which can also be used for 3.3 volt logic. Below that each voltage has come up with its own levels which are not really standard at all.

Usually the standards only care about voltages anyway, so the current drive of outputs vary a great deal.

What I need to know, if the user chooses to apply a signal to my chip from a GPIO, will my chip having an internal 100uA pull-up affect the accuracy of the applied voltage? ...Jim Thompson

Den fredag den 6. juni 2014 00.21.06 UTC+2 skrev Jim Thompson:

same as any logic IC I'd say,

V_OH, V_OL at a few currents from tens of uA to several mA

-Lasse

Unless you can say *which* GPIO the question is like, "how long is a piece of string?" I suggest that you spec your input and let the user drive it with a suitable output. I assume you can figure out how to write the I/O spec? That's not intended to be offensive, I'm seriously asking since I think it is at the level of the question you are asking.

I have requested the client clarify. I'm guessing that GPIO pins, when outputting analog, don't have buffers... just raw DAC outs. ...Jim Thompson

My needs are analog... no 1's and 0's allowed ;-) ...Jim Thompson

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Lots of FPGAs have weak pullups, in that sort of ballpark, on their i/o pins, to keep them from floating and hogging a lot of shoot-through current.

But "accuracy"? Just tell them the min/max logic levels and note the weak pullup. Most logic chips that might drive your part can easily source/sink milliamps.

GPIOs on most chips are complementary CMOS, HC-logic sorts of stuff. Where does a DAC come in?

I was told they (the customer) would provide a "DAC output via a GPIO pin". (Keep in mind the customer representative is a sales type who fathoms himself an engineer ;-)

I suspect that what I'm to be handed is actually PWM (analog ;-) ...Jim Thompson

Den fredag den 6. juni 2014 01.20.47 UTC+2 skrev Jim Thompson:

if it is from an MCU it might be a real DAC, it just on a pin that could also be a gpio

The DACs I've seem are rather wimpy unless you turn on a buffer but then the output range is reduced

-Lasse

It could be a real DAC, overloaded onto a GPIO pin. There's no standard for DAC outputs; might be buffered, might not. Might be a string DAC with impedance varying with code. Might be a current.

Better try to talk to a real engineer.

Den fredag den 6. juni 2014 01.20.47 UTC+2 skrev Jim Thompson:

if from an MCU it might be a real DAC it is just multiplexed on a pin that can also be a digital IO

some of them are rather wimpy, often you can enable a buffer but that reduces the output range

as an example stm32f407, the DAC has 15K output impedance, 5K with buffer and the buffer eats 0.2V at each end of the range

-Lasse

There's usually a source current rating and a sink current rating. In the analog world I guess it's just considered load current for bipolar stuff.

Cheers

No.

Your 100uA is far below any thing to worry about.

After reading more of this, the IO in question is coming from a MPU ? if so, normally up to 5mA is a safe bet.

Maybe you should get the IO spec's from the client or at least request the chip package they are supplying the IO from.

Jamie

There are such specs on every processor and FPGA I've seen.

No, GPIOs on many uCs are multi--function (*really* general purpose). They mix analog and digital functions onto these pins.

I'm not sure if I understand better or if this is just leading me further down the rabbit hole. lol

Ok, so your client is sending you an analog level from an MCU. Some MCUs dedicate analog I/O pins, others share them with the digital I/O. I doubt the digital circuitry gets in the way at all, so you are just looking at an analog output. I would highly recommend that you get the datasheet for the part involved if you are presenting anything other than a high impedance input.

When you say a 100 uA pullup, is this *really* a current source or a FET wired as a resistor or really a resistor? When does it draw/source the

This is an already existing chip... designed (by me) for another purpose... natch; but now the customer wants to use it for another purpose in spite of it is not really a match :-( ...Jim Thompson