PCA9518: communication error with 1:8 configuration

Taketo Sato

Part Number: PCA9518

Hello team

I have two PCA9518s to configure 1:8 I2C expand like datasheet figure3. However I see comunication error with the configuration.

Looking at SCL1 of the device1, I found clock waveform has glitch like depression during high state.

When I don't connect each EXPSCLy, EXPSDAy and operate as 1:4, then the issue resolve.

Could you please suggest how to improve the behavior when operating 1:8 with using two devices?

Pullup resistor value what I use now is as below.

SDA0, SCL0: 10k ohm to 3.3V

SDA1-4, SCL1-4: 2.7k ohm to 5V

EXPSDA1-2, EXPSCL1-2: 6.8k ohm to 3.3V

Best regards,

over 7 years ago

0 Clemens Ladisch over 7 years ago

Guru 320590 points

Can you show the waveforms of the master, EXPSCL, and SCL1?

0 BOBBY over 7 years ago in reply to Clemens Ladisch

TI__Guru* 78241 points

Hello Taketo,

In addition to Clemen's response, are you seeing any signal integrity issues/NACKs when you have both devices connected or are you only seeing a small glitch on the EXPSCLx lines?

"When I don't connect each EXPSCLy, EXPSDAy and operate as 1:4, then the issue resolve."
^I'd like to know if the SCLx lines on the connected hub (device 2 in figure 3 as you suggest your schematic is like) are fine or if they also see a glitch/integrity issue.

Also, are you expect any of your slaves to perform clock stretching?

Thanks,
-Bobby

0 Taketo Sato over 7 years ago in reply to BOBBY

TI__Genius 10425 points

Hello Clemens-san, Bobby-san,

additional info.
We see glitch on device 1 side in figure 3 and also EXPSCL line.
Howver there is no glitch on device 2 side bus in figure 3.

best regards,

0 Miguel Robertson over 7 years ago in reply to Taketo Sato

TI__Guru 62920 points

Sato-san,

Would it be possible to capture this glitch on an oscilloscope so that we can review the waveform?

Regards,
Max Robertson

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PCA9518: communication error with 1:8 configuration