Relationship betwen sink current and ability to pull the line low

AI Thread Summary
The discussion focuses on troubleshooting an I2C interface issue where the slave device is not pulling the data line to ground, violating I2C specifications. The problem is linked to the sink current capability of the slave, which affects its ability to pull the line low against the pullup resistor. It is explained that the maximum sink current determines the voltage drop across the pullup resistor, impacting the low output voltage. The slave's datasheet specifies a maximum sink current of 3 mA, which aligns with the observed issue. Adjusting the pullup resistor values may help achieve the desired low voltage level.
maverick202
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Hi,
I am trying to program an I2C interface and it's not working. The master is TI Cortex M3 processor and slave is ST Compass. I have captured the scope pictures and found that slave is not bringing the data line all the way to ground and this is violating I2C spec. I have sent the scope picture to TI. The reply I got was to verify the sink current on the slave as it may effect the slave's ability to bring the line low. Both ends have open-drain with 1.1K pullup.

Can someone please explain the relationship between sink current and the ability to pull the line low.

Thanks.
 
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maverick202 said:
Hi,
I am trying to program an I2C interface and it's not working. The master is TI Cortex M3 processor and slave is ST Compass. I have captured the scope pictures and found that slave is not bringing the data line all the way to ground and this is violating I2C spec. I have sent the scope picture to TI. The reply I got was to verify the sink current on the slave as it may effect the slave's ability to bring the line low. Both ends have open-drain with 1.1K pullup.

Can someone please explain the relationship between sink current and the ability to pull the line low.

Thanks.

The max sink current capability will determine how big the voltage drop can be across the pullup resistor. So if you can only sink 1mA and have a 1.1kOhm pullup, you will get a 1.1V drop across the pullup resistor (so the low output voltage value will only be Vcc - 1.1V).

What is the max sink current specified in the datasheet for your slave?
 
Thanks Berkeman that's a great explanation. According to slave spec, max sink current is 3 mA. Wit 1.1K it should be 3.3mA. This explains the reason for not reaching ground. I should play with pullup resistors.
 

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