TCA9548A: TCA9548A：Regarding the I2C low level

user4658448

Part Number: TCA9548A
Other Parts Discussed in Thread: TCA9803

Tool/software:

Hi Sir,

In the configuration below, why does the L-level voltage of CLK and SDA between TCA9548A and Slave drop from approximately 300mV to approximately 0mV for a certain period of time?

over 1 year ago

+1 Clemens Ladisch over 1 year ago

Guru 323430 points

Such a step happens when there are multiple I²C devices with different drive strengths, or when multiple devices pull the line low at the same time.

Please note that the B side of the TCA980x must not be connected to pull-ups, and that the TCA9548A does create a direct connection between the two sides. You must exchange the A and B sides of the TCA9803.

0 user4658448 over 1 year ago in reply to Clemens Ladisch

Prodigy 50 points

Hi Clemens Ladisch,

Isn't the Master outputting CLK?
Should the Slave also be set to Low?

0 Clemens Ladisch over 1 year ago in reply to user4658448

Guru 323430 points

This might be clock stretching by the slave.

0 user4658448 over 1 year ago in reply to Clemens Ladisch

Prodigy 50 points

Hi Clemens Ladisch,

The same phenomenon is occurring with the data, so I don't think it's clock stretching.

Is there any reason for these steps to occur in the internal circuitry of the TCA9548A?

0 Clemens Ladisch over 1 year ago in reply to user4658448

Guru 323430 points

The TCA9548A does not really have much of an internal circuitry; the switches are simple N-channel MOSFETs with the gate connected to V_CC. The TCA9548A never actively pulls any I²C line low, expect when you address it directly to configure it; the line must be pulled low by the TCA9803 or the slave.

0 user4658448 over 1 year ago in reply to Clemens Ladisch

Prodigy 50 points

Hi Clemens Ladisch,
In the above configuration, is it possible to remove the pull-up between TCA9548ASlave rather than swapping the A and B sides of TCA9548A?

+1 Clemens Ladisch over 1 year ago in reply to user4658448

Guru 323430 points

The TCA9548A must have some kind of pull-up on both sides. The TCA9803's B-side current source works as a pull-up, but the TCA9803 does not allow any other pull-up connected to its B side. So you must not connect the TCA9548A to the TCA9803's B side.

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TCA9548A: TCA9548A：Regarding the I2C low level