TM4C1294NCPDT: TM4C1294NCPDT Transformerless Communication Problem

Hello

Here has one of register function in the switch which is set current sinking mode class A or class B.
In general, switch is on class B, they are suggested to use class A for transformerless design.

Does you have the bit like as switch?

How can we change current sinking of MCU side?

May I know why the termination resistors pull up to 3.3V on datasheet shown, because almost PHY is supply by 1.8v.

Thank you
John

Hello all,

I have a workaround solution by adding a very low resistance into pull up of receiver side of Marvell switch about 6.8ohm.

The only one of key what i would like to know, how about internal current driving and internal termination resistors of TM4C1294

Thank you,

John

Hello John,

There is no specific termination control registers available in the TM4C129x integrated PHY. Can you please share your schematic and the change that you have made for me to understand what circuit components you have?

Hello Amit

Sure, i use message sent.

The circuit is very simply what is only adding pull up resistor (6.8ohm) for each single end and pull down (50ohm) for each single end the series one of capacitor (0.01uF) to ground. There are in the Marvell RX side.

MCU TX side is the same with TM4C Datasheet, there is only resistors (50ohm) pulled up to VCC.

I just would like to know the following questions:
1) is it current mode?
2) if the question is yes, how about its current be sinking?
3) has the termination be included into chipset?

Thank you,
John

Hello John,

Thanks for the schematic. Why is the pull up at 1.8V as the swing voltage is +1V/-1V. Can you please check the following application note?

www.ti.com/.../slla327.pdf

The termination is not there on the PHY, so all termination must be on the board.

Hello Amit,

The pull up at 1.8V is the suggestion from Marvell.

Regarding to the swing voltage is based on the current sinking multiplied by termination resistor at pulled up, so it will be 1V at differential pair.

As I know the termination on the pulled up is to create swing voltage and match impedance of signal end, so it is 50ohm.

But i'm not sure how many current be sinking from TM4C1294, Could you please help to confirm that?

Thank you,
John

Hello John,

Is it possible for you to capture the input and output swings on both TX and RX side?

Hello Amit,

Please refer to the file which included MCU side and swtich side.

Could you please help to confirm my question?

Thank you,

John

Waveforms.docx

Hello John,

Since the MCU TX side is driving a differential swing it should not be sinking current. And what is the waveform w/o the pull up on the Switch RX side.

Hello Amit,

You meant MCU isn't current mode, so it is voltage mode, right?

If it is voltage mode, why circuit recommendation has pull up resistor?

The second waveform is measured on switch RX side by pulled up resistor 50ohm what is based on Marvell's suggestion and application.

If you would like to see waveform without the pull up, i will measure it late due to the equipment is occupied.

Thank you,
John