PCF8523 Disables I2C Interface When Dead Battery Used and Main Power loss occurs

[ermacv]

Hello!

We use PCF8523 in our custom board design and we got some issues related to I2C interface and SW reset/main Vdd power loss.

The first issue is that when a dead battery is used (voltage of the battery is about 0.5-0.7V) and you perform write to register CONTROL1 enabling switch over circuit (value is 0b000) and main power source (Vdd) is disabled then RTC enters the state when I2C interface is not accessible after main power source is back. To "revive" the interface you have to once again to disable and enable main power source and board starts to response to I2C commands.

The second issue is related to SW reset function and switch over circuit. If you enable switch over circuit (value is 0b000) and then you perform a SW reset in a loop then after some attempts (10-100) board stops responding to I2C commands. We used some delays before and after SW reset function but it doesn't work, board still enters this non-responding state after some SW reset cycles. And it makes SW reset useless.

[diazmarin09]

Hello ermacv,

I hope all is great with you. Thank you for using the NXP communities.

1. Please note that the device configuration, including the battery switch-over function, should be performed with a valid supply voltage. I mean, do not wait until the battery level is low.

Could you please share more information about your configuration?

2. The oscillator is also considered to be stopped during the time between power-on and stable crystal resonance. This time may be in a range of 200 ms to 2 s, depending on crystal type, temperature, and supply voltage.

Are you waiting such time after a power on event? I mean, in order to get the oscillator stabilized.

Regards,

David