A time base circuit, for an oscillator and apparatus, defines a time interval in terms of a time taken for a capacitor to charge from a reference voltage level to a detection voltage level. The circuit operates by: supplying at the start of the interval a capacitor charging current, using a first semiconductor device supplied from a first power supply voltage, the device delivering the charging current according to a predetermined dependency on the first power supply voltage; and identifying the detection voltage level to signal the end of the time interval, using a second semiconductor device supplied from the first power supply voltage, the second semiconductor device identifying the detection voltage level value according to the same predetermined dependency on the first power supply voltage as for the first semiconductor device, the time interval being made substantially independent of variations of the first power supply voltage.