RE: [PIC] Battery backup by Chen Xiao Fan .

I think the circuit will be easier to understand if the names of the pins and the internal structure of the PIC port pins are in the picture. I am not so sure if I understand the circuit correctly but I will give it a try. The one connected to the decompling capacitor (pin14) is the Vdd pin. Normally it is connected to the supply, but not in this case. Port pins have diode protection to Vdd and Vss. Therefore the external power (5.5V and limit at 50mA) will charge the Vdd decoupling capacitor through the port protection diode to about 5.5V-0.6V=4.9V. This will make the PIC happy and running. Pin 8 then will be configured as an output pin and output high (to turn the high sider FET on). Now we can pulse-charge the two backup batteries or supercaps by configuring pin6/pin7 as output pins and outputing high to charge and configuring as input to stop charging. To detect DC voltage on Pin 8, we need to configure it as an input pin from time to time (thus turn the high side FET off). Once it reads low, we know that the DC voltage drops quite a lot and we'd better turn off the high side FET totally. In this case, the backup batterie(s)/supercap will take over. Since pin 8 should not sink more than 25mA, the total current consumption should be less than this. The voltage should not be higher than maximum allowed input voltage (Vdd+0.3V). That is why the external DC voltage to pin 8 is typically 5.5V and can drive 25mA at most. I think this is quite non-standard. ;-) Regards, Xiaofan {Original Message removed}