Relationship betwen sink current and ability to pull the line low

In summary, when attempting to program an I2C interface, an issue was discovered where the slave device was not able to bring the data line all the way to ground, violating I2C spec. Upon further investigation, it was found that the max sink current on the slave device may be affecting its ability to pull the line low. The relationship between sink current and the ability to pull the line low is that the max sink current determines the voltage drop across the pullup resistor. In this case, with a max sink current of 3mA and a 1.1kOhm pullup, there is a 1.1V drop across the resistor, preventing the line from reaching ground. To solve this issue, the pull
  • #1
maverick202
2
0
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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  • #2
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?
 
  • #3
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.
 

1. What is sink current?

Sink current refers to the amount of current that can be drawn from a device or circuit when it is in a low state or logic 0. It is the opposite of source current, which refers to the amount of current that can be supplied when the device is in a high state or logic 1.

2. How is sink current related to pulling the line low?

In order to pull a line low, there needs to be enough sink current available to overcome any pull-up resistors or other sources of current on the line. The higher the sink current, the more easily the line can be pulled low.

3. What factors affect the sink current capability of a device?

The sink current capability of a device is determined by its internal circuitry and design. Factors such as the type and size of transistors used, the power supply voltage, and the load on the output pin can all affect the sink current capability.

4. Can a device with a low sink current capability still pull a line low?

Yes, a device with a lower sink current capability can still pull a line low, but it may require more time and energy to do so. This can result in slower response times and reduced efficiency in the circuit.

5. How can I determine the sink current capability of a device?

The sink current capability of a device is typically specified in the device's datasheet. It can also be measured by connecting the device to a load and gradually increasing the current until the output voltage drops below a specified level. The maximum current at which this occurs is the sink current capability.

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