topic Re: For PCF8523, how to calculate backup time when supercap is 0.22F? in Other NXP Products

For PCF8523, how to calculate backup time when supercap is 0.22F?

maxxcheng — Fri, 17 Mar 2017 07:24:48 GMT

For PCF8523, how to calculate backup time when supercap is 0.22F?
I've checked UM10301.pfd, there is a formula to calcuate it, but I don't know

what Vcbackupend is?

Please help to point it out? 1.8v or 1v....

Using C·V = I·t: C⋅(V ) − V Cbackupstart Cbackupend = t backup I RTC

Regards,

Maxx

TomasVaverka — Fri, 17 Mar 2017 09:56:26 GMT

Hi Maxx,

For PCF8523, V_Cbackupend= 1.0 V.

Then considering V_Cbackupstart = 3.3 V, I_RTC = 150 nA and C = 0.22 F you will get:

t_backup = 0.22 F . (3.3 V - 1.0 V) / 150 nA = 3373333 s

As one day contains 86400 seconds, this corresponds to 39 days.

I hope it helps.

Best regards,

Tomas

maxxcheng — Mon, 20 Mar 2017 01:49:40 GMT

Hello Tomas,

Thanks, one more question for I_RTC = 150 nA, it is RTC working current when PM[2:0] = 111, VDD active.

But in a actual case, it should be VDD inactive, VBAT active, PM[2:0] = 000, isn't it?

Regards,

Maxx

maxxcheng — Thu, 23 Mar 2017 07:07:57 GMT

Hello Tomas,

Sorry to bother you, we met a abnormal case in RTC application.

The reading of control_1 register is 0xFF, but we did not set this register,

do you have a idea, what event will cause the reading of control_1 become 0xFF?

Regards,

Maxx