P82B96: No output on TXRX or TYRY

Ryan Goslin

Part Number: P82B96
Other Parts Discussed in Thread: ISO1640, ISO1640-Q1, P82B715, ISO6742, ISO7742

Hi, I'm using the P82B96 for a simple I2C extender, but I am not reading anything on the TXRX or TYRY. I've tried increasing the 3.3V that pull up on TXRX and TYRY to 5V and reducing the resistor values without any luck. Is there a reference design for a simple application design reference that I could look at for a similar implementation? Pullups too strong or weak?

Thanks!

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 244360 points

R2 are normal I²C pull-up resistors; R1 depends on the bus capacitance (see section 10).

What exactly do you mean with "am not reading anything"? Can you show an oscilloscope trace?

0 Ryan Goslin over 1 year ago

Prodigy 30 points

Here is the scope trace for the SCL(SX - Green) in and SCL (RX/TX - Yellow) out.

0 Ryan Goslin over 1 year ago in reply to Ryan Goslin

Prodigy 30 points

I think what might be causing this is the input logic low level is greater than the 650mV in the datasheet. This is due to the Isolator that precedes it. Would the ISO1640 be compatible? I don't think we can reduce the logic low voltage on it.

0 Clemens Ladisch over 1 year ago in reply to Ryan Goslin

Guru 244360 points

Yes, it looks as if the ISO1640-Q1's V_OL is above the P82B96's V_IH.

The P82B715 does not have switching thresholds. Alternatively, if your cable has a capacitance of less than 400 pF, it might be possible to use any simple I²C buffer.

0 Ryan Goslin over 1 year ago in reply to Clemens Ladisch

Prodigy 30 points

Just to clarify the waveforms use another IC for the isolation this has a OL of 700mV. The ISO1640 list's 450mV for its OL so I was suggesting it as an alternative. Does TI have any suggestions for an Isolation to I2C driver? Or are the ISO1640 to P82B96 compatible?

0 Clemens Ladisch over 1 year ago in reply to Ryan Goslin

Guru 244360 points

The ISO1640 has up to 710 mV on side 1, and 400 mV on side 2. So side 2 would be compatible with the P82B96.

Unidirectional signals are easier to handle. It might be possible to place the P82B96 on the other side of the isolator, and to use separate TX/RX signals with a plain digital 2+2 isolator like the ISO6742/ISO7742.

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P82B96: No output on TXRX or TYRY