DP83848J: DP83848JSQ/NOPB : Do I need external transformers ?

Jean Dupont

Part Number: DP83848J

Hi,

I need to communicate between a microcontroller that has an integrated MAC (datasheet : URL) and a microprocessor that has an integrated MAC and PHY (datasheet : URL). The pins of the MAC on the processor are already used by design, so I plan to use the DP838484JSQ/NOPB between the two. Here is the schematics :

The chips are on the same board. There will not be a RJ45 connector. The maximum distance between them will be some centimeters. I do not know yet if I will use 10 or 100 Mbps.

My questions are as follows:

- Do I need a transformer on the 10BASE-T/100BASE-TX lines? I think I do, based on the page 65/87 of the datasheet of the DP83848 : URL

- If so, would this one suits : URL ?

- Do I need a transformer on the RMII lines as well? BOM cost is a real concern for the project, so if it can work without a second transformer that would be great.

Thank you for your answer.

over 1 year ago

0 Gokul Koraganji over 1 year ago

TI__Mastermind 19180 points

Hi Victor,

Is this a new design? If so, can you use DP83826 instead?

For DP83848, magnetic is definitely needed where are for DP83826, you can use capacitive coupling which will reduce the BOM cost.

On RMII lines, you don't need any isolation or level shifters if the VDDIO level of the PHY and microcontroller are the same.

--
Regards,
Gokul.

0 Jean Dupont over 1 year ago in reply to Gokul Koraganji

Prodigy 30 points

Finally I will use the DP83848. Is it possible to ask for a quick schematics review? Here it is :

By chance do you see any mistakes ? I will use the chip in RMII mode.

Moreover I have a question : Are pull-up resistors also necessary on the left side of the transformer ?

Thank you.

0 Gokul Koraganji over 1 year ago in reply to Jean Dupont

TI__Mastermind 19180 points

Hello Victor,

Please give me until end of Wednesday to provide a detailed feedback.

--
Regards,
Gokul.

0 Jean Dupont over 1 year ago in reply to Gokul Koraganji

Prodigy 30 points

Hello Gokul. Were you able to take a look at the schematics ?

Thanks.

0 Gokul Koraganji over 1 year ago in reply to Jean Dupont

TI__Mastermind 19180 points

Hi Victor,

Sorry fot the delay in response. I am out sick and can give you the feedback in a day or two.

+1 Gokul Koraganji over 1 year ago in reply to Gokul Koraganji

TI__Mastermind 19180 points

Hi Victor,

Sorry again for the delay in response. Here is my feedback on the schematic.

Please place a 10uF cap on VCC_ETH
It is always recommended to isolate the connector ground (earth) and the PHY ground. These grounds can be shorted using 1Mohm and 4.7nF cap.
The center taps of magnetics on the cable side shouldn't ne connected to device supply rail. It can be left floating or be connected through a cap to the earth.
The connection of CRS_DV is correct in the schematic.

Please let me know if you need any other help.

--
Regards,
Gokul.

0 Jean Dupont over 1 year ago in reply to Gokul Koraganji

Prodigy 30 points

Thank you for your answer ! The communication is between two MCUs on the same board (there is no connector and cable). Do points 2. and 3. still apply in that case ?

0 Gokul Koraganji over 1 year ago in reply to Jean Dupont

TI__Mastermind 19180 points

Hi Victor,

You are correct. I missed that point. Since both the devices are on the same board, you can ignore points 2 and 3.

--
Regards,
Gokul.

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

Interface

Interface forum

DP83848J: DP83848JSQ/NOPB : Do I need external transformers ?