Atually, the capacitor charges to about 12.7V because as it charges then the current from the output of the NOR gate drops to zero allowing its output voltage to reach Vcc which is +13V. The very low or no current in the diode allows its voltage drop to be only 0.3V so the capacitor charges to about +12.7V.
But the actual peak voltage of the capacitor does not matter because it is much higher than the threshold voltage of the opamp it feeds.
But the actual peak voltage of the capacitor does not matter because it is much higher than the threshold voltage of the opamp it feeds.