A temperature sensor circuit may include a ring oscillator being enabled according to an enable signal and outputting a square wave signal with a first frequency, a divider dividing the first frequency of the square wave signal from the ring oscillator to generate a pulse signal with a second frequency, a counter counting a time interval of the pulse signal outputted from the divider according to an external clock to generate a count signal, a latch temporarily storing a value of the counter signal according to the pulse signal and outputting a digital code, and a supply voltage monitor being enabled according to the pulse signal, comparing a reference voltage to one or more comparison voltages and generating a switching logic signal. The reference voltage is kept at a substantially constant level when a level of a supply voltage changes.