5653 - iCE40 Ultra/UltraLite: What is the accuracy of the 48MHz internal oscillator (FCLKHF) at room temperature 20 deg C and 25 deg C as well as 50 deg C? In the datasheet, I only found +- 10% but I need more information that is specific.
If you refer to FPGA-DS-02008 otherwise known as iCE40 UltaPlus Family Data Sheet, Section 4.15. Internal Oscillators, it is described in Table 4.11 in the parameter description of the said temperature ranges such that for Commercial Temp, the +/- 10% tolerance is applicable to Tj ranging from 0degC to 85deg C.
In conjunction with that, kindly refer to FPGA-TN-02008, iCE40 Oscillator Usage Guide, section 4, it is noted that oscillators cannot provide accurate frequency. For applications that require more accuracy, it is recommended to use a calibration circuit to support the oscillator used as a clock source. In summary, frequency value increases with two factors: Vcc and temperature. It starts to change trend at hot temp with Vcc > 1.2V.