Posted on March 05, 2013 at 16:17If you connected a supecap diectly to Vbat you are going to have some problems. Until the cap is charged Vbat is pulled low, so you can't use anything in the backup power domain, like the backup registers or backup SRAM (F2 and later). I doubt if the LSE oscillator will start since it's powered from backup also.
I use a Nesscap 10F supercap with an 'F205. I charge the cap from the 3.3V supply with a blocking diode (1N4448) and current limiting resistor (100 ohm 1/4 watt, watch out for how high the charge current is through the resistor) to the cap. Most supercaps have a voltage rating around 2.5V so you need to drop the 3.3 supply. The diode and resistor will drop the charge voltage to a safe level. The cap will charge slowly (over several minutes) but it doesn't stress the 3.3V supply that way.
The diode is necessary to isolate the cap from the rest of the board. You need to minimize power from the cap leaking into the board circuitry.
The real problem is not dragging down Vbat while the cap is discharged. I use a pair of diodes (BAT54A) to wire-OR the supercap output and the 3.3V supply to Vbat. While the ssytem is powered Vbat draws from the 3.3V supply, but switches to the cap when the 3.3V supply shuts down.
On an 'F205 with the LSE running and backup SRAM enabled the cap will keep the backup power domain running for at least 3 weeks (longest I've tried so far).
Jack Peacock
