MCF8316C-Q1: I2C No ACK

Hi Jean,

Schematic is fine, only question I had is about pull up on SCL, SDA which I dont see in the schematics, is this available on your host controller side?

I hope pull ups for all other pins are on your hist side which is not shown in this schematic.

Can you use EVM, blue wire to your hardware and test?

Thanks and

Hello, 

Yes SCL and SDA both have strong pull-up resistors of 1k, so the edges are sharp. nFault and FG outputs are pulled-up by the MCU inputs.

I made in-circuit measurements on the EVM board, and found some discrepancies with my circuit. I believe the buck regulator is malfunctioning, and that may cause the I2C issue. 

I realize that the voltage on FB_BK (pin 3) should be pretty clean, and averaging 3.3V, 4V, 5V or 5.7V. Instead, I measure 1.9V average with a 1kHz, 0.3V p-p ripple. The same ripple is visible on DVDD (pin 1), I should read a clean 1.5Vdc, but I read a 1.2V with 1kHz, 0.5V p-p ripple. However, it is not visible on AVDD (pin 27), where I have a clean 3.3V. 

I am now concerned with my layout, even though I did follow the guidelines and have experience with buck regulator designs. So here is a screenshot of L1 (mixed signals, ground and power) and L2 (power ground and signal ground). 

Do you think the layout could be the cause ? What would you suggest to try next ? Would replacing the resistor with an inductor make sense ? 

More importantly, would a buck regulator malfunction cause an issue with the I2C bus ?

Thanks !

Hi Jean,

Thanks for verifying the Buck stability issue. What is the component rating of Buck resistor used?

There is a configuration BUCK_CL value, required configuration is 150mA, default setting is 600mA.

Buck resistor rating should be > 0.5W.

Can you highlight the Net tie connecting power ground and AGND in your layout?

If you suspect, try blue wiring and verify again?

Thanks and Best Regards

Venkatadri S

R6 is a 2010, 0.75mW 22R. 

To read/write BUCK_CL, I need the I2C interface to work though. 

Here is the layout with the net tie highlighted. L2 is shown on top, it has an inner polygon for the power ground plane, and the rest is signal ground plane. They are both attached to the ground pin of the power supply connector J4. Therefore, I put the net tie close to that ground pin to split both planes as close as possible from the source. 

Would it help to replace R22 with an inductor ? 

What do you mean by blue wiring ? 

Thanks, 

Hi Jean,

Resistor rating is good. Blue wiring means try to wire ground path if we have to suspect that.

Can you also check if Speed pin is stable high? If device is sleep mode then device can drop to sleep state.

Let me recheck again schematics and layout.

Thanks and Best Regards

Venkatadri S