Lower voltage on SDA and SCL line(Bridge) in LPCXpresso board

[jayesh_joshi]

I am using the LPCXpresso54102 board in my system as a USB to UART bridge. I am running into an issue where Bridge is unable to communicate with the device. I have debugged the issue and found that voltage on I2C_SCL and I2C_SDA lines is lower than expected(2.5V). 

I have traced the issue to the LPCXpresso54102 board. I am attaching images of the board below.

I have taken the below steps to resolve the issue but I can not get 3.3V on I2C_SCL and I2C_SDA line 

1. Connected 2.2K pull-up on R19 and R20.

2. Connected JP7.2 and JP7.3 for 3.3V as suggested in SCH

Let me know if more Jumper needs to be connected for properly using LPCXpresso54102 board as USB to I2C bridge.

[xiangjun_rong · Accepted solution]

Hi,

Maybe I got the cause, pls refer to the following Fig, the LDO outputs 2.5V instead of 3.3V, but it is labeled as +3.3VLINK, it is wrong, that is why you get 2.5V logic voltage.

Pls remove the JS1 and JS2, populate the R19 and R20, close pin2&3 of JP7, I suppose that you will get 3.3V logic for the BRIDGE_SCL/SDA pins.

BR

XiangJun Rong

[jayesh_joshi]

@xiangjun_rong 

Thanks for pointing that out. With the above modification, I get 3.3V on BRIDGE_SCL and BRIDGE_SDA.

But with the above modification USB to the I2C bridge does not work and I think it is expected as we have removed JS1 and JS2. 

A possible solution I am thinking of is replacing TLV70225DBVR(2.5V regulator) with TLV70233DBVR(3.3V regulator). Can you please confirm if this will not any adverse effect on board? 

[xiangjun_rong]

Hi,

I think that it is okay to replace 3.3V LDO with the 2.5V LDO, no problem.

BR

XiangJun Rong

[xiangjun_rong]

Hi, Jayesh,

For the LPCXpresso54102 board, the BRIDGE_SCL and BRIDGE_SDA pins are connected to the LPC4322 also, I suspect that the LPC4322 take effect on the two pins, pls remove the JS1 and JS2 resistor, then have a check if the LPC54102 can drive the two pin with correct logic

Hope it can help you

BR

XiangJun Rong

[jayesh_joshi]

@xiangjun_rong 

Thank you for the reply.

I have one concern regarding removing JS1 and JS2. As I want to use the LPCXpresso54102 board as UART to I2C bridge, removing JS1 and JS2 disable that functionality. And we have to use the board as Bridge. 

Can you please share what could be the next step for achieving 3.3V on BRIDGE_SCL and BRIDGE_SCL bridge? 

[xiangjun_rong]

Hi,

Do you use oscilloscope to test the two signals: SCL and SDA? if it is, pls post the screenshot of the scope so that we can see the high logic voltage of the signal. You can not use the multimeter to test the voltage of the signals.

Hope it can help you

BR

XiangJun Rong

[jayesh_joshi]

Hi

Please find attached a screenshot of the SCL and SDA line.

Channel 1 is SDA and Channel 2 is SCL we are getting approx 2V. Currently, only LPCXpresso54102 is there we have not connected the I2C line to our unit.