To explore the possibility of soft intellectual property implementation, a fully digital smart temperature sensor without any full-custom device is proposed for painless VLSI or system-on-chip integrations. The signal is processed thoroughly in time domain instead of conventional voltage or current domain. A cyclic delay line is used to generate the pulse with a temperature-proportional width. The timing reference is just the input clock and a counter instead of voltage or current analog-to-digital converter is utilized for digital output coding. The circuit was implemented by field-programmable gate array chips for functionality verification and performance evaluation. Realized with as few as 140 logic elements, the proposed smart sensor was measured to have an error of -1.5 ~ 0.8 degC over the full commercial IC temperature operation range of 0 degC-75 degC for thermal self-sensing or monitoring. The effective resolution can be made better than 0.1 degC easily, and the power consumption is 8.42 muW at a sampling rate of 2 samples/s. The longest conversion time is around 260 s, and a conversion rate of 3 kHz at least is promised.