PCM1681: min tr and tf in Standard-mode

Youhei Miyaoka

Intellectual 2365 points

Part Number: PCM1681

Hi experts,

My customer have a question about following requirements t(SCL-R) to t(SDA-F).

They want to know why these min and Max values are.

Because I2C parameter doesn't show minimum value about tr and tf in standard-mode. Is there no limitation?

Regards,

Youhei MIYAOKA

over 1 year ago

0 Arash Loloee over 1 year ago

TI__Mastermind 23920 points

Hello Youhei,

It is true that usually we spec the max tr/tf , as if it tr is too slow it may never reach the needed threshold for detection. So in this case I think this is the minimum (fastest slope) that the device can handle. probably it working as an edge detection rather than level detection.

Regards,

Arash

0 Youhei Miyaoka over 1 year ago in reply to Arash Loloee

TI__Intellectual 2365 points

Hi Arash-san

Thanks for your reply.

OK, I understood max tr and tf. And how about t(SCL-R) to t(SDA-F) min value?

Could you kindly tell me why min value is 20 + 0.1CB ?

Regards,

Youhei MIYAOKA

0 Arash Loloee over 1 year ago in reply to Youhei Miyaoka

TI__Mastermind 23920 points

Hello,

Without looking at the internal design I can not tell for sure; but since it is related to a specific relationship (20+0.1Cb , the Capacitive load for SDA and SCL lines , i guess it is the inherent minimum limit of the internal design.

Regards,

Arash

0 Youhei Miyaoka over 1 year ago in reply to Arash Loloee

TI__Intellectual 2365 points

Hi Arash-san

Thank you for your early reply. Customers understood of your answers.

And they are considering to use FAST MODE Plus as I2C host, so it is hard to fulfill min 20 + 0.1CB.

So, could you kindly tell me what happens if a signal less than 20ns is input internally to the PCM1681.

Customers said they will be OK if it just increases the current.

Regards,

Youhei MIYAOKA

0 Jeff McPherson over 1 year ago in reply to Youhei Miyaoka

TI__Mastermind 32955 points

Hi Miyaoka-san,

To add on to Arash's point, the I2C FAST MODE definition lists 20ns as the minimum. The additional 0.1Cb term is from TI to account for bus capacitance. Referring to NXP's app note on the I2C timing https://www.nxp.com/docs/en/user-guide/UM10204.pdf it looks like the minimum was established to improve EMI performance. Going below the minimum rise time may end up repressing the signal and causing errors.

Theoretically there's no issue going below the spec from a protocol standpoint ( this is why standard mode has no minimum), but the customer will have to accept the risk that they are out of I2C spec.

Thank you,
Jeff McPherson

0 Youhei Miyaoka over 1 year ago in reply to Jeff McPherson

TI__Intellectual 2365 points

Hi Jeff-san (Arash-san)

Customer is thinking "FAST MODE Plus" as I2C host, so I understand tr(min) is no minimum and tf(min) is 20×(VDD/5.5V).

And they concerned about 20 + 0.1CB requirements because signal fall earlier than 20ns.

Is this signal fall affecting the IC ?

Regards,

Youhei MIYAOKA

0 Arash Loloee over 1 year ago in reply to Youhei Miyaoka

TI__Mastermind 23920 points

Hi Miyaoka-san, I too don't think it will affect the operation of the IC , but whenever the customer goes outside datasheet specs of any IC, they should be willing to accept the risk.

Regards,

Arash

0 Youhei Miyaoka over 1 year ago in reply to Arash Loloee

TI__Intellectual 2365 points

Thank you Arash-san. I just replied for customer you said. But, let me confirm to make sure.

In "FAST MODE Plus", tf(min) is 20×(VDD/5.5V) and VDD is recommended between 3V and 3.6V in PCM1681 datasheet.

So, using for calculation VDD=3.3V, tf(min) is 13.2[ns]+0.1CB.

Is my interpretation correct ?

Regards,

Youhei MIYAOKA

0 Arash Loloee over 1 year ago in reply to Youhei Miyaoka

TI__Mastermind 23920 points

Hi Miyaoka-san,

Although I follow your reasoning in calculating the 13.2ns,the data sheet states the min is 20+0.1CB and that is what needs to be used.

Regards,

Arash

0 Youhei Miyaoka over 1 year ago in reply to Arash Loloee

TI__Intellectual 2365 points

Arash-san

Yes, of course I know. It was just confirmation. Thanks for your reply.

0 Arash Loloee over 1 year ago in reply to Youhei Miyaoka

TI__Mastermind 23920 points

Hi Miyaoka-san,

Although I follow your reasoning in calculating the 13.2ns,the data sheet states the min is 20+0.1CB and that is what needs to be used.

Regards,

Arash

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PCM1681: min tr and tf in Standard-mode