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# Interface ethernet voltage

#### FreshmanNewbie

##### Full Member level 6
I just want to know what are the voltage levels on the ethernet differential lines (both single ended voltage level wrto ground & different voltage between the pairs). I havent measured the signal and not able to get the right answer from Internet search

This is the PHY I am using

Hi,

Datasheet states differential output levels for
* 10BASE-TX and
* 100BASE-T
single endeld levels on these signal has no meaning.

Klaus

Hi,

Datasheet states differential output levels for
* 10BASE-TX and
* 100BASE-T
single endeld levels on these signal has no meaning.

Klaus
Just want to understand the voltage levels.

So, when measuring the voltage across the Tx- and Tx+ on the PHY pins, what would be the differential voltage? Also, any idea on would the voltage across Tx+ and ground? Because while measuring, the PHY is referenced to the board ground, right?

Also, curious to know why are there different output voltage levels for 10base and 100base? Are 100base and 1000base same differential voltage level?
--- Updated ---

Hi,

Datasheet states differential output levels for
* 10BASE-TX and
* 100BASE-T
single endeld levels on these signal has no meaning.

Klaus
1. Since the absolute maximum rating on the PHY pins are 4V, I am just trying to understand The peak voltage on Tx+ for 10BaseT. It would be 3.3 + (2.5V/2) = 3.3V + 1.25V = 4.25V, right? Which violates the stress levels of the pin right?

Last edited:

Hi,

Don´t get me wrong. I want to teach how to find the values on your own. This is not the fastet way for both of us. But it´s meant to help you for the future. It it helps you save time in the future.
So the question is: are you asking for help, or are you expecting others do the "reading job" for you? (meant as serious question, not offending)

So, when measuring the voltage across the Tx- and Tx+ on the PHY pins, what would be the differential voltage?
What does your datasheet tell? It´s quite clearly specified.
My help: The technical problem is, that you say "across the Tx pins". But the device is made to operate with transformers and therefore they specified the voltage at the output of the transformer (@ the ethernet connector). There are ethernet standards. And they don´t care about the IC pin voltages, they also refer to the (differential) voltage at the connector.

If you don´t use the recommended design - by omitting the transformer - then it´s your responsibility anyways.

It would be 3.3 + (2.5V/2) = 3.3V + 1.25V = 4.25V, right?
Where is this formula from?
I can´t find it in the datsheet.

Also, any idea on would the voltage across Tx+ and ground? Because while measuring, the PHY is referenced to the board ground, right?
Indeed some signal voltages are referenced to GND, others not. The Tx voltages are not referenced to GND.

An example:
Maybe you find a user here that measures the Tx+ voltage on his "KSZ..." device: 0.7V and 3.1V w.r.t. GND.
What do you gain from it? (especially for tranformer use.. the transformer "signal" usually is not referenced to GND at all)
Now maybe you measure at your device and it is 0.2V and 2.4V ... you can never say whether the one is wrong or not.
--> Important for ethernet communication simply is the difference. And as long as the difference voltage is within the specified range the device does it´s job well. Even if the voltage levels of Tx+ to Tx- differ a lot.

Klaus

Hi,

Don´t get me wrong. I want to teach how to find the values on your own. This is not the fastet way for both of us. But it´s meant to help you for the future. It it helps you save time in the future.
So the question is: are you asking for help, or are you expecting others do the "reading job" for you? (meant as serious question, not offending)

What does your datasheet tell? It´s quite clearly specified.
My help: The technical problem is, that you say "across the Tx pins". But the device is made to operate with transformers and therefore they specified the voltage at the output of the transformer (@ the ethernet connector). There are ethernet standards. And they don´t care about the IC pin voltages, they also refer to the (differential) voltage at the connector.

If you don´t use the recommended design - by omitting the transformer - then it´s your responsibility anyways.

Where is this formula from?
I can´t find it in the datsheet.

Indeed some signal voltages are referenced to GND, others not. The Tx voltages are not referenced to GND.

An example:
Maybe you find a user here that measures the Tx+ voltage on his "KSZ..." device: 0.7V and 3.1V w.r.t. GND.
What do you gain from it? (especially for tranformer use.. the transformer "signal" usually is not referenced to GND at all)
Now maybe you measure at your device and it is 0.2V and 2.4V ... you can never say whether the one is wrong or not.
--> Important for ethernet communication simply is the difference. And as long as the difference voltage is within the specified range the device does it´s job well. Even if the voltage levels of Tx+ to Tx- differ a lot.

Klaus
Thank you.

The thing is that, I just want help and want to learn. I don't want you to do the work.
But sometimes, in many of my questions, the answer seems to be either present in the datasheet which I didn't find because I didn't understand the terminologies properly or didn't get the things right. I always look for the datasheet points, do a google search and then come here.

1. So, you mean to say that the values given in the datasheet for "Differential output voltage" is applicable at the transformer pins, close to the connector side.
But my query is what would be the voltages on the Tx pins of the device? Because the PHY is connected to board ground, there's a possibility and a real value for a single ended voltage between Tx+ and Gnd and Tx- and Gnd, right? Can you tell me how to arrive at that answer? What would the single ended voltage between the pin and gnd and what would be the differential voltage across the pins before the transformer and after the transformer.

Dont mistake me. I don't want you to do the work. But sometimes, this type of answer is kind of simple from your experience that you actually hand out the answer to me.

2. 3.3V is the bias voltage on the centre tap of magnetics on the PHY side. So, when the data is transmitted between the PHY and the magnetics, the single ended voltage on Tx+ will be 3.3V + half the differential voltage, right? I did this for 10BaseT spec. Please tell me if its right approach to calculate the single ended voltage this way? If not, what's the right way?

Hi,

1. So, you mean to say that the values given in the datasheet for "Differential output voltage" is applicable at the transformer pins, close to the connector side.
Straight forward way to find the ouput voltage value in the datasheet:
* open the PDF
* go to chapter "Electrical specifications" (it´s only one page with voltage specifications)
* do a serach for "output" (since you want the information of output voltage)
find:
* "TTL Outputs" specifications --> isn´t what you are looking for. skip
* "LED Outputs" specifications --> isn´t what you are looking for. skip
--> * then "100BASE-Tx Transmit --> this is what you want .. or
--> * then "10BASE-T Transmit --> this is what you want
This way it takes just 5 seconds to find the desired values.
***

It´s not guess, not just an opinion:

It clearly says: "measured differentially after 1:1 transformer"
It also says about the 100 Ohms termination.
And it says, that it´s different for 100BASE-T vs 10BASE-T

Copied from the datasheet:

Single ended: Sadly it seems I can´t be clear enough. I just can repeat.
* The chip is not designed for single ended output at all
* the single ended voltage levels are not specified .. and thus not guaranteed at all
* measured single ended values are meaningless. they may vary with time with temperature with production batch, with chip version. Nothing you can rely on.

****

2)
Here you talk about the bias voltage on the center tap to be 3.3V.
I could not find this information in the datasheet.
Nor did I find any information where you did get this information from.

And here is the point: these missing informations take long time for us to find. It makes it hard to help.
Usually I don´t do this, but now I took the effort and did a seach through various manufacturer documents.

I was successful at he "HW design checklist".
(Why didn´t you tell? It´s so easy for you .. compared to the effort on our side)
Indeed there is a circuit with CT connected to 3V3. Now your math makes sense.
And your worries make perfectly sense. Now I can fully agree with you that the datasheet information is odd.
Trouble is: I can´t tell what information is correct.

The only way I see is to contact the manufacturer on this.

My guessing - nothing you can rely on:
* the circuit in the HW design checklist is correct (complies with "MICREL Evaluation board schematic" provided in the "Supporting Collateral" .ZIP file)
* the datasheet information "all outputs" should be "all outputs except Tx pins"

Klaus

Hi,

Straight forward way to find the ouput voltage value in the datasheet:
* open the PDF
* go to chapter "Electrical specifications" (it´s only one page with voltage specifications)
* do a serach for "output" (since you want the information of output voltage)
find:
* "TTL Outputs" specifications --> isn´t what you are looking for. skip
* "LED Outputs" specifications --> isn´t what you are looking for. skip
--> * then "100BASE-Tx Transmit --> this is what you want .. or
--> * then "10BASE-T Transmit --> this is what you want
This way it takes just 5 seconds to find the desired values.
***

It´s not guess, not just an opinion:

It clearly says: "measured differentially after 1:1 transformer"
It also says about the 100 Ohms termination.
And it says, that it´s different for 100BASE-T vs 10BASE-T

Copied from the datasheet:
View attachment 182948

Single ended: Sadly it seems I can´t be clear enough. I just can repeat.
* The chip is not designed for single ended output at all
* the single ended voltage levels are not specified .. and thus not guaranteed at all
* measured single ended values are meaningless. they may vary with time with temperature with production batch, with chip version. Nothing you can rely on.

****

2)
Here you talk about the bias voltage on the center tap to be 3.3V.
I could not find this information in the datasheet.
Nor did I find any information where you did get this information from.

And here is the point: these missing informations take long time for us to find. It makes it hard to help.
Usually I don´t do this, but now I took the effort and did a seach through various manufacturer documents.

I was successful at he "HW design checklist".
(Why didn´t you tell? It´s so easy for you .. compared to the effort on our side)
Indeed there is a circuit with CT connected to 3V3. Now your math makes sense.
And your worries make perfectly sense. Now I can fully agree with you that the datasheet information is odd.
Trouble is: I can´t tell what information is correct.

The only way I see is to contact the manufacturer on this.

My guessing - nothing you can rely on:
* the circuit in the HW design checklist is correct (complies with "MICREL Evaluation board schematic" provided in the "Supporting Collateral" .ZIP file)
* the datasheet information "all outputs" should be "all outputs except Tx pins"

Klaus
I truly appreciate you taking a lot of time for helping me out with this. Really appreciate it and thanks a lot!

I am sorry that I forgot to mention about the HW checklist document for the device.

1. Just to clarify, when they mentioned, "after" the 1:1 transformer, it means that the measurement is done close to the connector/cable side and not the PHY side, right?

2. In that case, the voltages on the transformer pins would be 1V typical for 100BASE, right? If the differential voltage is 1V on the pins, then Logic 1 is transmitted. If the differential voltage is 0V across Tx+ and Tx-, Logic 0 is transmitted. Am I correct?
Same logic applies for 10Base?

In that case, the voltages on the transformer pins would be 1V typical for 100BASE, right? If the differential voltage is 1V on the pins, then Logic 1 is transmitted. If the differential voltage is 0V across Tx+ and Tx-, Logic 0 is transmitted. Am I correct?
Same logic applies for 10Base?
Neither. 10BASE-T uses manchester encoding and a dedicated link integrity test pulse. 100BASE-TX (the most common 100 MBit variant over two twisted pairs) uses 4B/5B encoding and ternary signal level (MLT-3). https://en.wikipedia.org/wiki/MLT-3_encoding