16bit I/O expander and mosfet driver behaviour

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fuzzzy

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HI

I'm intended to use an I2C I/O expander to drive some N-Mosfet by an High side mosfet driver.
I already used PCA9555 to drive some relays by ULN2003. But when I power up my siystem, PCA9555( at startup is set as Input) puts at ON-State, for a moment, all relays.Untill MCU sets PCA9555 pins as Output.

Do you think Mosfets will be at ON-state for a moment during power-up? If it happens how can I prevent this problem?

Thanks a lot for helping me
 

Pins configured as inputs have weak internal pullups, so on power-on you have to pull pins down for a period of time required to configure them as outputs ..

This can be done by connecting (1N4148) diodes to each I/O pin (A) and (K) to a OC/OD transistor driven by Power-on-Reset circuit ..
If you don't have one you can use DS1232 and connect all diodes (Ks) to the DS's /RST pin .. see picture below ..
**broken link removed**

Regards,
IanP
 

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