A valid 23-MHz to 26-MHz reference clock is required on the REFCLK pin 5 for precise frequency operation. The REFCLK frequency defines all internal clock timers, including the back channel rate, I2C timers, CSI-2 datarate, FrameSync signal parameters, and other timing critical internal circuitry. REFCLK input must be continuous. If the REFCLK input does not detect a transition more than 20 µs, this may cause a disruption in the CSI-2 output. REFCLK should be applied to the DS90UB662-Q1 only when the supply rails are above minimum levels (see Figure 9-1). At start-up, the DS90UB662-Q1 defaults to an internal oscillator to generate an backup internal reference clock at nominal frequency of 25 MHz ±10%.

As an option for mitigating EMI / EMC, the DS90UB662-Q1 is capable of tolerating a REFCLK with spread-spectrum clocking (SSC) profile with up to ±0.5% amplitude deviations (center spread) or up to 1% amplitude deviations (down spread) and up to 33-kHz frequency modulation from a clock source.

The REFCLK LVCMOS input oscillator specifications are listed in Table 7-2.