Checking status of Output Pins

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Checking status of Output Pins

(7 posts) (3 voices)

Started 4 years ago by Silntknight
Latest reply from Silntknight

Silntknight
Moderator
Should this work:
```
#define randomPin 5
#define conditionalPin 6
void setup() {
 pinMode(randomPin, OUTPUT);
 pinMode(conditionalPin, OUTPUT);
}
void loop() {
 if(randomPin == HIGH) {
  digitalWrite(conditionalPin, HIGH);
 }
}
```
I am trying to set pins either HIGH or LOW depending upon the state of that pin or another pin. I don't think that a digitalRead() would work, so would it work the way I wrote the code?
Also, if you do a digitalRead() on an input pin, does HIGH count as true for a while loop?
Ex:
`while(digitalRead(randomPin)) {
//Stuff here
}

Thanks!
Posted 4 years ago #
snigelen
Member

Interesting question, I've never thought about that.

First

if(randomPin == HIGH)

will not work, it's about the same as "if (5 == 1)" which always will be false.

And we're talking about OUTPUT-pins here?

My short guess is it will probably work with something like

digitalWrite(conditionalPin, digitalRead(randomPin))

My longer spekulation/explanation is this.

digitalWrite will set a bit in the output data register (ODR) (by writing a bit to bit-reset or bit-set-reset register) and digitalRead will return HIGH if the bit in the input data register (IDR) is set, LOW if it's unset.

So you want to read the ODR-register, but that's no way to do that without going to lower levels.

But what happens with IDR for an output pin? The reference says (8.1.8 Output configuration)

● A read access to the Input Data Register gets the I/O state in open drain mode
● A read access to the Output Data register gets the last written value in Push-Pull mode

I'm not sure how to interpret that exactly.

I think it's like this

- If the pin is set to OUTPUT (then it will be set to push-pull mode) or OUTPUT_PUSH_PULL, the IDR and ODR will contain the same information. If that's the case you can use digitalRead() to get the value of the last digitalWrite().

- If the pin is configured as OUTPUT_OPEN_DRAIN, the relevant bit in the registers will be same if a pin is low. If you set the pin high, ODR will be high but IDR will be high if you have external pullup, low if you have external pulldown or have a random value if it's floating. But if you choose open-drain I guess you have some kind of external pullup. In that case ODR and IDR will have the same value.

And

while(digitalRead(randomPin))

will work for input pins and, if I'm correct above, for OUTPUT pins too.

Everything untested. Time to sleep.

Posted 4 years ago #
Silntknight
Moderator

Thanks, I definitely just realized that if(randomPin == HIGH) wouldn't work. I remembered that #define simply replaces "randomPin" with 5 at compile time. Anyway, 'digitalWrite(conditionalPin, digitalRead(randomPin));` probably won't accomplish what I am trying to do. I might be able to make a workaround, but it would be nice to see if it is possible to read the state of an output pin.

Posted 4 years ago #
gbulmer
Moderator
Silntknight - I believe snigelen's analysis is correct.

For your case, where the pins are:
```
pinMode(randomPin, OUTPUT);
pinMode(conditionalPin, OUTPUT);
```
The IDR == ODR, and digitalRead should 'Just Work' (TM)
Posted 4 years ago #
Silntknight
Moderator

Ok, I think I am understanding this more (after reading it 3-4 times). I meant that digitalWrite(conditionalPin, digitalRead(randomPin)); wouldn't work because I don't necessarily want them to have the same state. I see what you both are saying about the registers, so digitalRead() for an OUTPUT pin should give me a significant bit. (I like puns).

Posted 4 years ago #
gbulmer
Moderator
I think *I* may be getting confused.
What is it your trying to do?

Your original code
```
#define randomPin 5
#define conditionalPin 6
void setup() {
 pinMode(randomPin, OUTPUT);
 pinMode(conditionalPin, OUTPUT);
}
void loop() {
 if(randomPin == HIGH) {
  digitalWrite(conditionalPin, HIGH);
 }
}
```
could work, and set conditionalPin HIGH iff (if and only if) randomPin has been set HIGH (by code which is missing from this example) with this small change:
```
#define randomPin 5
#define conditionalPin 6
void setup() {
 pinMode(randomPin, OUTPUT);
 pinMode(conditionalPin, OUTPUT);
}
void loop() {
 if(digitalRead(randomPin) == HIGH) {
  digitalWrite(conditionalPin, HIGH);
 }
}
```
Is that what you want?

HIGH is always true, so
while (digitalRead(randomPin))
is the same as
while (digitalRead(randomPin) == HIGH)

(full disclosure: I am not a member of the LeafLabs staff)
Posted 4 years ago #
Silntknight
Moderator

Yes, I realized that my original code was incorrect, but it made sense to use if(digitalRead(randomPin)). You understood correctly.

Posted 4 years ago #

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