TCAL9539-Q1: TCAL9539RTWRQ1 Holding SDA Line Low. Measuring 120Ohm to Ground

Hi Kevin,

Is the measurement of 120 ohms to GND while the device is unpowered? This would indicate some internal damaged to the SDA pull-down driver. 

If the TCAL9539-Q1 is actively in a stuck low condition, and then the resistance is measured, it makes sense that the output impedance is measuring 120 ohms to GND as the device is actively pulling low. 

Does replacing the device fix the issue? I.e. replace the bad device with a new TCAL9539-Q1 to see if the issue follows the IC. 

Kevin Richardson said:

• 8 Debug Blue LEDs via 1kOhm resistors
• 4 3.3V feedback signals (toggle high or low)
• 2 analog load switches. The expander controls the EN pin to turn the switch on/off

I initially don't see a problem with this implementation. A schematic would be helpful for the full picture. 

Do you have an oscilloscope capture of SDA and SCL to the TCAL9539-Q1? I am wondering if there is a double clock edge that is causing TCAL9539 to be stuck low. 

I want to understand if this stuck low condition is due to a double clock edge from the controller, or if SDA is stuck low due to internal damage of the device. 

Regards,

Tyler

Hey Tyler, 

Yes, the 120Ohm to GND measurement on SDA is while the device is unpowered. That would explain why the SDA line on an oscilloscope is just held low the whole time. After severing the device from our MCU, the I2C from the MCU is working fine again. The SDA line is still measuring 120Ohm to GND on the severed trace connected to the SDA pin of the TCAL component. Below is the I2C expander schematic. The Door/Latch input are from the circuit on the right in a source-follower configuration. This circuit measures ~3.0V when the input to the gate is driven to 5V. The other inputs to the IO Expander are connected to optocouplers that drive a 3.3V to the input dependent on other status inputs. 

I am suspect of the circuit on the right however as it is a low-impedance input without any series resistance to the TCAL IO expander input.

Hi Kevin,

So it seems there is clear damage to the SDA pin. I assume that if you replace the TCAL9539-Q1 with another device, it would work properly i.e., the issue would follow the problematic IC. 

The schematic looks fine. 

Any ESD risk to the SDA pin? 

SDA is open-drain, it has the ability to sink up to 20 mA of current through its open-drain driver. Is there a chance that the pull-up resistor on SDA is sized incorrectly as to draw too much current through SDA? 

Regards,

Tyler

Hey Tyler,

Yeah, it is pretty clear that the part is damaged.

The board is nonconformally coated with test points/vias exposed so it is a possibility that an ESD event could have damaged the trace.

We pull 10k Pullups to the SDA/SCL lines, so at the most .33mA.

Does the source-follower circuit above look like it could cause this sort of damage through the TCAL input pin?

Hi Kevin,

We are still looking into this and will provide a response soon.

Regards,

Jack

Hi Kevin,

Usually an ESD event on the IO side would damage the IO either its ESD cell, or part of the output driver (the pFET or nFET). 

If the damage is on SDA, I am not sure how likely an ESD event on the IO side would couple through the chip to damage the SDA pin specifically without damaging any other part of the chip. 

Door status is on P16. This pin is quite far from SDA.

It is more likely that some solder bridge existed between SDA and the thermal pad causing a short. 

Regards,

Tyler

That could be possible. It would be a pretty uncommon situation for this to occur, but this does make sense. If the SDA was shorted to the thermal pad, would that have caused this issue?