OIC, thanks Markus for the the explanation on the cap measurement method ! Its just I'm lazy to go through the source code !
As my question, so this thing can not be used on low voltage cap ?
You don't even have to decypher the source code. It's covered in the documentation that was posted on page 1 of this thread:
5.3.2 Measurement of big Capacitors
One side of the capacitor is connected to GND. The other side of the capacitor is connected with
the 680 resistor to VCC for a period of 10ms. Afterwards this probe pin is switched to Input (High
Impedance). After this 10 ms current pulse the voltage of the capacitor is measured without any
current. If the voltage has not reached a minimal value of 300mV, the load pulse is repeated up to
499 times. If after 127 pulses a minimum voltage of 75mV is not reached (about 2s), further load
is stopped, because never the 300mV can be reached with the remaining load pulses. Figure 5.21
shows the three phases of measuring the capacity value of a capacitor. The value of the capacity is
then computed with the count of load pulses and the reached load voltage from a table. The table
contains the factors to get the capacity in nF units from load time and the reached voltage with a
spacing of 25mV. Interim value of voltage will be interpolated.
Now I realize that VCC in this case is 5v, which is technically above the 1.8v you are concerned about. For all of 10ms. I don't really know much about such low voltage caps or if they are robust enough to take a brief pulse, but certainly the 6.3 volt caps are in no danger.
Personally, the highest voltage I saw on the scope during my own experimenting was about 3.5v, but I don't remember how big the cap was. It might well go higher with a low-value cap.