Power management & timing parameters of the ST25DV chip

[jokes]

 

 @JL. Lebon

 

 

Dear ST Engineer,

Thank you for your previous assistance. I have two further technical questions regarding the power management and timing parameters of the ST25DV chip:

1. Concerning the VCC Rise Rate Limitation The datasheet states in the Power Management section: "When the power supply is turned on, VCC rises from VSS to VCC. VCC must not rise faster than 1 V/μs."

In my ultra-low-power application, I connect the ST25DV VCC pin to a MCU GPIO pin, accompanied by a 10nF decoupling capacitor. During actual measurement, I observed that the initial rise rate of VCC is approximately 2 V/μs (specifically, the rise time from 0V to 1V is about 0.44 μs, as shown in the attached waveform).

 

 

 

 

Could you please clarify:

• What is the underlying physical reason or risk consideration for this 1 V/μs limit?
• Given my specific hardware configuration (GPIO-driven + 10nF cap), is the observed rise rate of ~2 V/μs acceptable for reliable operation, or does it pose a potential risk to the chip?

2. Concerning the tMINCD Parameter Regarding the parameter tMINCD mentioned in the datasheet:

• Data Transmission Window: The datasheet specifies tMINCD with a MAX value of 1 ms. Does this imply that no data transmission can be initiated within the first 1 ms after power-up?   
  • Since I am aiming to minimize the total communication time in my application, I would like to know if this 1 ms duration is a fixed worst-case scenario or if there is any possibility/method to shorten this mandatory waiting period.

 

 

Your insights on these points would be greatly appreciated.

Best regards,

 

[JL. Lebon]

Hello, 

The VCC rise time of 1V/us guarantees that the reset circuit inside the device will work correctly.
Above this value, the correct startup of the reset circuit can’t be guaranteed.
It may work, but you are out of the product specifications, and the device may not boot if the reset is not working.

Regarding the tMINCD: this value corresponds to the NFC forum and ISO15693 specifications. 
Those specifications require that the tag must be able to answer 1ms after the rise of the RF field. This tMINCD guarantees that the ST25DV will always be able to answer 1ms after the rise of the RF field.
Nevertheless, as you can see in the tbootRF value, the device is able to boot in less than 1ms (600us). So, it is indeed able to answer to an RF command when the boot is done, 600us after the Rf field rising.

Best regards.

[jokes]

Hello,Thank you for the clarification.

Regarding the VCC rise time, I plan to increase the decoupling capacitor from 10 nF to 100 nF, which is the value recommended in the datasheet. This should help ensure the rise time stays within the specified limit.

What are your thoughts on this approach?

 

Best regards.

[Andrew Neil]

I plan to increase the decoupling capacitor …  to ... the value recommended in the datasheet. 

What are your thoughts on this approach?

I think that following the datasheet recommendation is generally a good approach!

[JL. Lebon]

Yes, I think increasing to 100nF would be a good idea.

Best regards.