A question about Standard mode and Fast mode CLK frequency

[cheseldai]

Hi NXP friends,

    I met a question, Standard mode CLK range is 0-100KHz, Fast Mode CLK range is 0-400KHz; I use the oscilloscope to measure the I2C BUS CLK is 97.6KHz, when I use the NXP MCU communicate with other I2C device; 

  Then I have measured the Thd(sta) is 3.6uS and Tsu(sta) is 4.6 uS；

On the Spec file from your Company, named <I2C-bus specification and user manual > , page 44/62 shows the spec is different between Standard mode and Fast mode, so how can I distinguish ?

If I choose the fast mode spec about thd(sta)0.6uS min,  tsu(sta) 0.6uS min, they are pass; Or I choose the standard spec(4.0uS，4.7uS) ,they are fail;

So whic is the correct value ?

Based on the CLK？ or based on the device its own ability?

Because the range is overlapping in 0-100KHz; They are not one is 0-100KHz, the other is 100-400KHz; It's a question to me;

Hope to recieve your reply;

[Pavel_Hernandez]

Hello, I believe have a little confusion, on the RM have a little commend about the spec, and the spec if this not the I2C.

System Management Bus

Check the Table 1: SMBus AC specifications.

Best regards,
Pavel

[cheseldai]

thanks for your reply,

https://www.nxp.com.cn/docs/en/user-guide/UM10204.pdf

In fact,I just have two question;

One is on what basis do we determine whether the device is operating in fast mode or standard mode?

Base the clk measured in real time or the device's datesheet capabilities statements?

The other is: if I measured the one master one slaver topology, the I2C bus clk is 55MHz real time，it belongs to stander mode or fast mode although the slaver support fastmode?

[Pavel_Hernandez]

Hello, my apologize even if the MCU could do more that the reference manual mentioned, need to stay under spec because in the other way the MCU could have a wrong behavior. 

The interface is designed to operate up to 100 kbit/s with maximum bus loading and
timing. The I2C device is capable of operating at higher baud rates, up to a maximum of
clock/20, with reduced bus loading.

Best regards,
Pavel

[cheseldai]

Thanks for your reply;

Maybe the question is no answer;

close this question

[Pavel_Hernandez]

Hello, my name is Pavel, and I will be supporting your case, could you let me know what is the target MCU?

Best regards,
Pavel

[cheseldai]

MKE04Z128VLH4R, I use this one;

The MCU itself is just an example, what I want to ask is more about IIC Santand mode and fast mode;

[cheseldai]

I just want to know the I2C device fast mode or standard mode decided by the BUS clk frequency measured by the oscilloscope, or decided by the devices its own ability(although it runs 100KHz, but it supports fast mode，should I still obey the fast mode spec?)

The spec said that I2C support backward compatibility, so although the device support fast mode but it runs 97KHz, At this moment, is it fast mode or standard mode? Will it be the same as the barrel effect? Depends on the slowest one on the bus?

Also the timing follow the slowest one's mode?

Thanks