TCAL9539-Q1: Questions about tRT, tFT, tTRR_GND and tTRR_POR50.

All four parameters are limits of the device. When you have a supply voltage dip that goes beyond these limit, a reset is not guaranteed.

Hi Leo,

Leo Ma said:

How to understand the tRT, tFT, tTRR_GND and tTRR_POR50?

tRT = rise rate, the power supply ramp rate from GND to VCC

tFT = fall rate, the power supply falling rate from VCC to GND. 

tTRR_GND = the time to hold at GND or 0.2V before rising back to a VCC again. 

tTRR_POR50 = The time to hold the VCC at a voltage below the power-on reset level before ramping back up the supply voltage. 

Leo Ma said:

tRT, tFT (in red rectangular) are the features of TCAL9539-Q1 itself, that can’t be controlled by user.

Sometimes it can be controlled by the user depending on how they are supplying the VCC to the I/O expander. External capacitance on the power supply lines also affects this ramp rate. 

Leo Ma said:

tTRR_GND and tTRR_POR50 (in green rectangular) aren’t the features of TCAL9539-Q1, they need to be controlled by MCU when in using.

I would tend to agree here. The amount of time that the MCU waits to enable the power supply, LDO, etc. to the I/O expander is usually determined by the controller. 

regards,

Tyler