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Topic: Can you tell me what this means and (Read 1 time) previous topic - next topic

raschemmel

#15
Nov 23, 2020, 04:57 pm Last Edit: Nov 23, 2020, 04:59 pm by raschemmel
The short answer is it means that pin #20 is a GPIO that doubles as the SDA I2C pin (Bi-Directional Serial Data signal) and pin#21 is a GPIO that doubles as the I2C SCL (Serial Clock). The typical pullup value used is 4.7k or 10k but as shown the value is not fixed because it is based on how many devices are on the bus. For a small number of devices 4.7k is fine. For larger numbers of devices you need to look up the value on the pullup reference page of the I2C standards doc.  That's pretty much all it means.

hextejas

#16
Nov 23, 2020, 06:15 pm Last Edit: Nov 23, 2020, 06:44 pm by hextejas
I have been reading more and more about this and came across this drawing.
Could you tell me if this is an accurate representation of what you mean by a pull-up resistor ?
I was planning on using this in my project where VDD is +5V.
What happens if the device already has a pull-up resistor ?


hextejas

There is no pulldown resistor on I2C, just a pullup.

The pullup is there because the open-collector can only pull down, so something else has to return the line high.

The only thing that indicates that a slave is present is the slave's response to the master.
How can that work ? Does the master issue a general purpose request to see if anyone is listening ? I was wondering how the master can respond to the slave if the master doesn't know the slaves address. Perhaps the slave announces itself when it attaches ? Very curious.

raschemmel

Quote
Could you tell me if this is an accurate representation of what you mean by a pull-up resistor ?
There is a clue: "RP"

hextejas

There is a clue: "RP

I guess this is not it.
Quote
RP
It's a
sexual roleplay game where you take on the role of a character and do whatever it takes to earn a sexual response. It is a sexual relationship designed to provide for a sexual relationship of similar character. It is not a family relationship.

raschemmel

#20
Nov 23, 2020, 06:28 pm Last Edit: Nov 27, 2020, 06:09 pm by raschemmel
Quote
Does the master issue a general purpose request to see if anyone is listening ? I was wondering how the master can respond to the slave if the master doesn't know the slaves address. Perhaps the slave announces itself when it attaches ? Very curious.
I don't think anyone said that it could. The master only talks to known slaves. Your code needs to have the I2C
address. This sometimes comes up as an issue for I2C LCD displays where the user doesn't know the address.
In that case they run the I2C scanner and it "goes out " and finds all the slaves and reports their address.
You could call it a "Slave Finder" but what with everything going on right now that might not be politically correct.
It's best just to call it the I2C SCANNER.

Quote
Rp
It's a
sexual roleplay game where you take on the role of a character and do whatever it takes to earn a sexual response. It is a sexual relationship designed to provide for a sexual relationship of similar character. It is not a family relationship.
I thought RP would obviously mean ResistorPULLUP, but that's just me.
And FYI, "RP" does NOT equal  "RP". (P is a SUBSCRIPT)

This reads verbally as "R sub P equals Resistor, pullup"

hextejas

There is a clue: "RP"

My snide response aside, actually it does not. RP could be either Resistor Pull-Up or Resistor Pull-Down, although member AWOL says that there is no such thing as a Pull Down.

TheMemberFormerlyKnownAsAWOL

No, I never said there is no such thing as a pull down, because that would be stupid.

I'm not stupid.
Please don't PM technical questions - post them on the forum, then everyone benefits/suffers equally

raschemmel

#23
Nov 23, 2020, 06:36 pm Last Edit: Nov 27, 2020, 05:44 pm by raschemmel
Quote
No, I never said there is no such thing as a pull down, because that would be stupid.
Quote
RP could be either Resistor Pull-Up or Resistor Pull-Down, although member AWOL says that there is no such thing as a Pull Down.
Actually it could NOT be a pulldown because it is connected to VDD , which by definition is "UP".
Just for the record, GND is "DOWN". So if the resistor labeled RP had the lower end connected to GND, then it
would indeed be a "PULLDOWN" , but the image you posted  shows it connected to Vcc ,(which is UP, just FYI)

hextejas

No, I never said there is no such thing as a pull down, because that would be stupid.

I'm not stupid.
I apologize for any offense AWOL, but that is how I read this.
Quote
There is no pulldown resistor on I2C, just a pullup.

raschemmel

#25
Nov 23, 2020, 06:46 pm Last Edit: Nov 23, 2020, 06:52 pm by raschemmel
Quote
The pullup is there because the open-collector can only pull down,
What AWOL is saying here is that "Open-Collector" is a reference to the COLLECTOR of a TRANSISTOR, WHICH
because it is the COLLECTOR, and the transistor is an NPN, it CANNOT be connected to GND (which is DOWN),
so , by definition, an NPN transistor collector can only be pulled UP (like to Vcc).
In order for this to make sense to you , you would first need to know what a transistor is and how it works.
And secondly, you would need to know what "open-collector" means and what it's application is. You would also
need to be aware that in electronics, the convention with respect to schematic illustration, is that the POSITIVE
supply (Vcc/Vdd) is at the TOP of the page and the GND or negative supply is at the bottom of the page. Hence
the origin of the "up" in "pullup" or the "down" in "pulldown".

Do you ?

hextejas

What AWOL is saying here is that "Open-Collector" is a reference to the COLLECTOR of a TRANSISTOR, WHICH
because it is the COLLECTOR, and the transistor is an NPN, it CANNOT be connected to GND (which is DOWN),
so , by definition, an NPN transistor collector can only be pulled UP (like to Vcc).
In order for this to make sense to you , you would first need to know what a transistor is and how it works.
And secondly, you would need to know what "open-collector" means and what it's application is.
Do you ?
No, I was hoping for clarity in the terms of the sketch, SDA, SCL, VCC, VDD, etc.
And thank you

raschemmel

#27
Nov 23, 2020, 07:03 pm Last Edit: Nov 23, 2020, 07:58 pm by raschemmel
Quote
No, I was hoping for clarity in the terms of the sketch, , SDA, SCL, VCC, VDD, etc.
If by "No" you mean you do not know what a transistor is or how it works or schematic convention or what open-
collector means and why it is used, then clarifying the below is not going to explain a whole lot because knowing
everything I just mentioned is almost a requirement for understanding SDA,SCL,Vcc, and Vdd, . (Vcc and Vdd are
functionally the same thing except Vdd is used for CMOS and MOS circuits that use mosfets , whereas Vcc is used
when the circuit uses transistors. (that's a simplistic definition)


Clarification
SDA: Signal name for the Serial Data I2C signal
SCL: Signal name for the I2C Serial Clock signal
Vcc (NOT shown in schematic posted)
Quote
VCC stands for "voltage at the common collector.". The letter "V" on a circuit stands for the supply voltage. The letters "CC" indicate that the supply voltage is positive or negative.
What does Vee stand for?
VDD : Drain Supply Voltage
etc:  et cetera et cetera ? (are there other terms you wanted clarified as well ?)

For single supply circuits Vee is replaced with GND. Vee was used with ECL that had a -5V supply and has not
been used since ECL was phased out. (it may still be in use somewhere but is largely obsolete.

Just for the record , I can barely remember the day I asked myself "What does Vcc mean ?" but unfortunately
there was no Google 40 years ago.

aarg

Vee is still in use for bipolar op amps negative supply.
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

raschemmel

I couldn't remember if op amps used -Vcc or -Vee.

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