Interfacing with microcontroller question

[hse]

Hi

I'm having a µC that is able to sink/source 3mA.
I have 2 inputs and 2 outputs that must be connected to some external devices through optocouplers that must be pretty fast.

First I thought of a 74OL6000 from fairchild, but they seems to be obsolete, or at least I can't find them anywhere. Then i looked through Fairchilds homepage for another one, that would be easier the get. The one i found was FOD0710 (12,5Mbit/s).

1.
Now I'm not sure if I need to connect any resistors on any side of optocouplers. I don't know, when controlling the output, and driving it low if the output should sink any current(for driving the optocoupler LED) or if the current runs from VCC to GND of the optocoupler, and no (mentionable) current runs through the output pin to GND.

2.
And for the inputs, do I need a current limiting resistor to protect the input, and if yes: in what range should it be? I thought maybe 47Ω?

3.
Am I correct when saying that µC inputs are high impedanced and almost no currents run into it, and therefore no resistor is needed?

4.
If in question 3 you state that no resistor is needed, what about the output current from the optocoupler that is stated at an average of 10mA??


best regards
HSE

 

[Sceadwian]

Pretty fast is not a value that you can design for, would you care to specify your design goals with real numbers so there's actually something to talk about?

What you connect to the optocoupler will depend on which device you end up using. Resistance may be required again you've posted no schematic of values to work from, but resistance will increase the RC time constant of the circuit, depending on how it's constructed it may limit your speed. With high speed micro controller inputs/outputs, the AC current (generally capacitive leakage to ground) can be quiet high.

The current limiting resistor you chose will depend on the specification of the input and output circuits, nothing may be required a gain no information to judge.

for 3, you would be incorrect, at high frequencies AC currents still flow, not always easy to calculate the effects of this on a circuit.

If you're using an optocoupler on a device that needs 10ma drive and your IC can provide 3ma obviously it's not going to work, you'll need an external driver.

 

[MikeMl]

My posts to this current thread in the other forum may be of some use to you.

 

[hse]

Hey Sceadwian, and thanks for tour reply

About the high speed i just need a short rise and fall time (<100ns) but there will be a very low no of changes in state. I guess the states will change a maximum of 2-3 times each seconds, and often they won't change for minutes (it depends on user inputs). All signals will be connected to a tristate TTL buffer 74HCTXXX on the other side of the optocouplers. .

regarding question 3, a leakage current at around +-3µA for input a logic 1, and -150µA for logic 0.

Regarding the 10mA that was what I thougt. Would a normal buffer do then?