Ethernet interface with dp83848c

Hello Nasser,

Thanks you for your help,

Jean-Marie

Thanks you Patrick for these explanations. I never had the good idea to use the bill of material you have on your article documentation page ! I will do now.

The microcoontrolleur I use is a PIC32, and mainly I will use Microchip gerber file and scheme I have to prepare my board. But after reading IBM example of interfacing DP83848C, I see some difference for example on line CRS/CRS_DV pin 40 and RX_ER pin 41 microchip did not put a serial resitor as IBM did.
An other difference is on the center tap of transformer : they are pulled to VDD, but microchip did not put a capacitor between ground as described in datasheet and IBM example.
So I will follow the datasheet preferably, but is this decision correct ?

Last question : microchip did not have a ground plane on its board. I would like to had one (second layer on four), excepted under the pulsejack connector as explained in your documentation. Could you confirm that correct ?

Best regards,
Jean-Marie

Jean-Marie,

The schematic looks very good.  I do not see any issues.

In the layout, it appears that power and ground are being routed to the RJ-45 connector pins (beneath the connector) as traces rather than being connected to the planes.  Is that correct?  Our recommendation is to void the planes beneath the magnetics, but it is OK to include power and ground planes beneath the remainder of the connector.  This could help simplify your connections to the connector.

Patrick

Hello Patrick,

I have received my board and I have on issu with it : it looks like my application send request to the DHCP server but the server did not answer.

With Wireshark I see that :

"4638","1787.335092","Microchi_75:e3:56","Broadcast","ARP","60","Yes","Who has 192.168.0.1? Tell 192.168.0.10

192.168.0.1 is the DHCP server and 192.168.0.10 is my application. 
During modification of the layout I try to have capacitance on the same layer than the DP, but by the way the 10 µ is not as close of the pin 23 (PFBOUT) as in the first layout. Can this small difference the issue ?


If yes, can I had a new 10 µ closer to the pin 23 by soldering the capacitance on the wire ?

Thanks you for your help,
Jean-Marie

Jean-Marie,

I would not expect a small change in the distance to the PFBOUT capacitor to prevent the DP83848C from operating. On how many boards have you seen this behavior?  

What measurement capabilities do you have?  Do you have an oscilloscope that could be used to probe clocks, supplies, and other signals on the board?  Do you have a multimeter that could be used to measure DC voltages and resistances?

Patrick

Patrick O'Farrell said:

Jean-Marie,I would not expect a small change in the distance to the PFBOUT capacitor to prevent the DP83848C from operating. On how many boards have you seen this behavior?  What measurement capabilities do you have?  Do you have an oscilloscope that could be used to probe clocks, supplies, and other signals on the board?  Do you have a multimeter that could be used to measure DC voltages and resistances?Patrick

Hello Patrick !

Thanks you for your answer. till now I had observed that on two boards (I have other, but I have to solder them).

I have a multimeter and an oscilloscope, so I can perform mesurement on the board but with this technologie I am not such what I have to test/check : operation timing are described in the datasheet.... but where did I have to start ? Only think I have controlled is oscillator frequency : it is as expected at 50 MHz.

Best regards

Jean-Marie

Jean-Marie,

I would suggest that you start by measuring the following DC resistances with the board powered down:

• RBIAS resistor - The resistor should be a 4.87 kOhm 1% resistor.
• Termination resistors - These should be 50 Ohm 1% resistors.  Typically 49.9 Ohm resistors are used.

I would then suggest that you measure the following DC voltages with the board powered up:

• VDD voltage - Preferably at each device pin to confirm the device is properly supplied.  Expected value is 3.3V.
• PFOUT voltage - At the PFOUT and both PFIN pins.  Expected value is 1.8V.
• RBIAS voltage - Across the 4.87 kOhm RBIAS resistor.  Expected value is 1.2V.

A significant variation from the expected value (more than 10% for the voltages or more than 1% for the resistors) could cause functional problems with the design. 

On a related note, do you have the capability to read out the registers from the DP83848C? 

Patrick

Thanks you Patrick for your help !

Patrick O'Farrell said:

Jean-Marie,

I would suggest that you start by measuring the following DC resistances with the board powered down:

• RBIAS resistor - The resistor should be a 4.87 kOhm 1% resistor.
• Termination resistors - These should be 50 Ohm 1% resistors.  Typically 49.9 Ohm resistors are used.

RBIAS is fine 4.88 k read, but it is not the case for the termination resistor... I have less than 1 ohm (about same value for the forth resitances) !!! I made the same verification on my second prototype : same observation.

As let me a little bit suspicious(I did not found the correct expression in English), I take resistors till on the band : 49.9 ohms as espected. I made a verification on an unpopulated pcb : resistance between the two resistances pad was not measurable. After soldering two resistors I found 49.9...

I would then suggest that you measure the following DC voltages with the board powered up:

• VDD voltage - Preferably at each device pin to confirm the device is properly supplied.  Expected value is 3.3V.
• PFOUT voltage - At the PFOUT and both PFIN pins.  Expected value is 1.8V.
• RBIAS voltage - Across the 4.87 kOhm RBIAS resistor.  Expected value is 1.2V.

A significant variation from the expected value (more than 10% for the voltages or more than 1% for the resistors) could cause functional problems with the design. 

On a related note, do you have the capability to read out the registers from the DP83848C?

Even if resistance value are not at all as expected, I measure the voltages as you recommande :

• VDD : 3.32 V
• PFOUT : 1.74 V
• RBIAS : 1.18 V

That fine. Now I have to found  what is wrong with the 49.9 ohm resistors : what I don't understand is how it is possible that I have the same trouble with 8 resistors...

I am not such that I can read the registers of DP83848C, I have to check that possibility.

Jean-Marie

Jean-Marie,

Now that you have corrected the problem with the 49.9 Ohm resistors, is the design operating correctly?  Without the 49.9 Ohm termination resistors, the DP83848C would not have been able to transmit or receive data.  Now that you have resolved the problem with the termination resistors, perhaps your overall problem has been resolved. 

Patrick

Patrick,

I don't have resolved that problem : I don't understand why I have these very low resistance value of 0.2 ohm instead of the 49.9. I only can tell that not coming from my pcb (there is not short) nor from an error in the resistance value : the value is as required (49.9). But till now I don't found explanation to explain these "strange values".

I am outside today, so I am not able to perform new trial, but I have a doubt about the way I proceed to solder component on the pcb :

• first I solder the lower layer which comprise the DP83848 with an oven : temperature profile go till 250 °C in 3 minutes and this 250 °C are maintained for 15 seconds.
• Then I solder the upper layer under the same condition, excepted I put a "dead pcb" under the pcb which is currently soldered. The dead pcb is spaced from the "working pcb" by spacer of 5 mm. I did that to protect the lover face during the second soldering operation.

I operate like this from a while, but perhaps this time the CI located on the lower layer (DP83848) did not appreciated this treatment and is partially dead. What did you think ?

Best regards

Jean-Marie

Hello again,

I said some error this afternoon : the value read with my ohmeter are normal the magnetic had a low resistivity !

That not my problem...

Hello,

I found the problem of my application : the MDIO was pulled up with a resistor of 30 ohms instead of 1.5 k . Modification solve the behaviour of my card : it is working properly now.

Jean-Marie

Hello,

 I have a little problem with DP83848. I have built a PCB using Atmel UC3A and 83848 PHY, with a PoE supplied board. The connector is a Tyco PoE conn with magnetics. The strange think is that the 75 ohm resistors on magnetics common wire is missing. (See drawings: http://www.tycoelectronics.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=6605834&DocType=Customer+Drawing&DocLang=English)

When I connect the PCB to a Notebook, the Auto-Negotiation finds the good configuration, but when connect to a router, switch or hub, the negotiation fails.

When I set up my Notebook's card to fixed 100BMit Full duplex, i can ping my panel property. 

I have tried the original Atmel firmware too, it works on original EVK1100, but my own panel with same firmware is wrong.

It is possible, the auto negotiation error because the connectors missing 75 resistors? I have found a documentation with DP83848, PoE and external Magnetics without 75 ohms. (http://www.national.com/assets/en/boards/dp83848_demo2_schematicB.pdf)

Thank you for your answers

David