The problem of electromagnetic interference (EMI) has become increasingly important in the recent years. A high-speed data transmission usually causes severe EMI, and this influences the operation of neighboring circuits. Hence, to overcome these noises, spread spectrum clock generators (SSCGs) are proposed to reduce the EMI at a relatively low cost and a relatively simple design. There are several devices that have the defined specifications to restrict the EMI with a SSCG, such as Display Port , , which is a digital display interface and serial advanced technology attachment, which is a storage interface.
There are several modulation methods of SSCGs. Some SSCGs directly modulate the voltage controlled oscillator (VCO)  or the digitally controlled oscillator (DCO)  to achieve a spread spectrum. This modulation method can provide better EMI reduction performance, and it is a relatively simple modulation method to implement a SSCG. However, this approach usually requires a verylarge on-chip capacitor as a loop filter and occupies a relatively large chip area. Although an all-digital SSCG (ADSSCG) with DCO modulation and a frequency maintenance mechanism is proposed, in this approach causes large cycle-to-cycle jitters due to the rescheduling division triangular modulation. The other digital approach uses a digitally controlled delay-line (DCDL) cascaded with the all-digital phase-locked loop (ADPLL) to realize the ADSSCG. Nevertheless, the operation of DCDL is set at a high frequency, and thus, this architecture consumes more power.
The block diagram of the proposed ADSSCG is shown in Fig. 1. This ADSSCG is composed of a phase and frequency detector (PFD), a spread-spectrum clock (SSC) controller, a spreading ratio detector, a DLF, a DSM, a low-power monotonic DCO, and a frequency divider. The proposed ADSSCG uses the modulation method on DCO. Therefore, the SSC controller directly controls the DCO control code (control−code) to perform the center-spread triangular modulation on the output frequency. According to the SR (spread−range), which is calculated from spreading ratio detector, the SSC controller modulates the output frequency to achieve the corresponding spreading ratio. The proposed spreading ratio detector is for finding the SR with the proposed truly programmable spreading ratio decision method.