1. Field of the Invention
The present invention relates in general to the field of integrated circuits. More specifically, the present invention relates to improvements in the testing of frequency divider circuits implemented in various embodiments of integrated circuits.
2. Description of the Related Art
With semiconductor process technology scaling down to 45 nm or beyond, the maximum frequency (fmax) achievable for an on-chip phase-locked loop (PLL) is more than 30 Ghz using CMOS process. The fmax limitation is mainly due to the fmax of the voltage-controlled oscillator (VCO) and frequency divider circuitry used in the PLL. In prior testing techniques for testing the fmax of a divider, a high-frequency on-chip signal source such as a VCO is used since an external high-speed clock generator is not available. As a result, the fmax obtained from such testing is a combination of the operating characteristics of the VCO and the divider. To increase the fmax of modern PLL, it is necessary to is necessary to obtain more accurate measurements of the fmax of the divider to determine whether the VCO or the divider is the bottleneck. An improved technique for measuring the fmax of a divider is provided by embodiments of the present invention, as described in greater detail hereinbelow.
Embodiments of the invention provide a system and method for measuring the fmax of very high speed frequency divider.
Embodiments of the invention use a PLL and a high frequency generator outside the loop to obtain the fmax of the divider. The divider in the PLL loop is fed by a VCO and its operation range is characterized by measuring the PLL lock range. An identical copy of the same divider is used outside the PLL loop and it is fed by a higher frequency clock. The high frequency clock is generated by the multiple phase of the VCO. By characterizing the outputs from both dividers, the fmax of the divider is obtained.
The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element.
Various illustrative embodiments of the present invention will now be described in detail with reference to the accompanying figures. It will be understood that the flowchart illustrations and/or block diagrams described herein can be implemented in whole or in part by dedicated hardware circuits, firmware and/or computer program instructions which are provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions (which execute via the processor of the computer or other programmable data processing apparatus) implement the functions/acts specified in the flowchart and/or block diagram block or blocks. In addition, while various details are set forth in the following description, it will be appreciated that the present invention may be practiced without these specific details, and that numerous implementation-specific decisions may be made to the invention described herein to achieve the device designer's specific goals, such as compliance with technology or design-related constraints, which will vary from one implementation to another. While such a development effort might be complex and time-consuming, it would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. For example, selected aspects are shown in block diagram form, rather than in detail, in order to avoid limiting or obscuring the present invention. In addition, some portions of the detailed descriptions provided herein are presented in terms of algorithms or operations on data within a computer memory. Such descriptions and representations are used by those skilled in the art to describe and convey the substance of their work to others skilled in the art. Various illustrative embodiments of the present invention will now be described in detail below with reference to the figures.
In some embodiments of the invention, the frequency multiplier 114 generates a clock signal, 2f_clk, that is twice the frequency of the clock signal that is provided as an input to the frequency divider 110. In principle, the multiplication factor used by the frequency multiplier 114 can be an quantity that generates a clock signal that is larger than the frequency of the output signal generated by the VCO 108. For example, it can 1.5 times or 2 times of the VCO frequency. For simplicity, the discussion herein will illustrate an example embodiment of the invention where the signal, “2f_clock,” generated by the frequency multiplier 114 is twice the frequency of the output signal generated by the VCO 108.
In one embodiment of the invention, the VCO 108 is a five-phase VCO with the five available phases shown in
The circuitry for measuring the maximum operating frequency of a frequency divider described herein is embedded in a plurality of data processing circuits in integrated circuits that are used in information handling systems and in a wide range of other applications. Those of skill in the art will understand that the embodiments described herein will result in improved performance and an increased effective lifetime for such products. Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.