Connecting digital pin to digital pin.

[electricity86]

I'd like to connect digital output pin (3.3V) of CC2430 to digital input pin of CS5461 pin, which its VDD is also 3.3V (HIGH).
The mentioned pin in CS5463 is its \CS pin.
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So in order to know if i need to put a resistor on the line between the two pins, i need to know what is the input impedance of the CS5461 pin, right?

The problem is that i dont find any mention of this in the datasheet of the CS5461 datasheet.

What can i do about it?

Thank you.

 

[dknguyen]

WHy do you think you need anything between the pins? THey're both digital pins and they both are of the same voltage levels (though that does not always compatible logic threshold levels in which case you would need a bit extra).

Digital pins tend to have high input impedance already anyways. Input impedance is normally only an issue for analog systems that might have sampling capacitors or other things like that (and in those cases you'd never add a resistor since that's just making the problem worse unless you are trying to impedance match).

 

[electricity86]

Alirght, thanks.
How a newbie like me should know that input digital pins has already large input impednace if not from datasheet?

 

[dknguyen]

Alirght, thanks.
How a newbie like me should know that input digital pins has already large input impednace if not from datasheet?

Inputs tend to be the gate of a MOSFET, which is almost infinite impedance since that's the technology used for most digital circuits now. Even if it's BJT, tt's also just a matter of practicality. You have some ICs with hundreds of pins and it's a more than a bit inconvenient if the chip manufacturers required you to add something every time you connected one digital pin with another.

WIth digital pins, it's usually logic level thresholds that make the mismatch.

 

[electricity86]

Thanks i learnd alot now.
I noticed now that its written there input_current_max = +- 10mA (end of page 13 if its intersted you).
Does it mean that when Vdd is applied on an input pin, then current can reach 10mA?
Because my MCU pin can only push up to 4mA, so doesnt that mean that i'm in trouble if i dont add a resistor?

 

[dknguyen]

THe absolute maximum ratings of a part are not necessarily what it will naturally draw, but what it will tolerate.

THese are rail clamp diodes (ignore the one between Vdd and GND, that's not it). If the line voltage rises above Vdd+Vf, the diode becomes forward biased and turns on draining current in order to clamp the voltage. It is a similar thing for if the line voltage falls below Vss-Vf.
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THe specific type of diode is used is dependent on the type of overvoltage you want to protect against (ESD, inductive spike, bad voltage source, etc).

IN all acutuality, for these inputs it will probably take a voltage higher than the regular operating voltage to cause 10mA to flow. THere are probably rail clamp diodes with these pins that clamp the voltage to Vdd+Vf and Vss-Vf (where Vf is the forward voltage of the diode) during over/under voltage conditions. As the voltage gets higher, more current flows as the diodes work harder to clamp the voltage, and these diodes can only handle 10mA. There is also likely an internal resistance in series with the pins for the excess voltage to be dropped across (since diodes only have a constant voltage drop so in theory an infinite amount of current would have to flow through them to clamp a voltage). However, for practical reasons these resistances probably aren't outrageously large.

So if you were in a situtation where you knew these diodes existed (or in some cases added them yourself because you need them or needed them to handle higher power) you would add a resistor to limit the current (or further limit the current if an internal series resistance was already there) and give the diodes an additional resistor to drop the excess voltage across in order to allow the diodes to be able to clamp higher voltages.

But this probably isn't the case here since you are doing digital to digital. (BTW, this is how "5V-input tolerant" pins on 3.3V microcontrollers work.

 

[crutschow]

Alirght, thanks.
How a newbie like me should know that input digital pins has already large input impednace if not from datasheet?

You just have to know what to look for. The table on page 10 of the CS5463 data sheet lists the Input Leakage Current as 10µA maximum. That's the maximum current your source will have to provide to generate either a one or a zero. Obviously the CS5463 has a very high input impedance (330k ohm minimum).