Patent ID: 12249989

DETAILED DESCRIPTION OF THIS DISCLOSURE

The present disclosure is directed to clock buffer. While the specification describes several example embodiments of the disclosure considered favorable modes of practicing the invention, it should be understood that the invention can be implemented in many ways and is not limited to the particular examples described below or to the particular manner in which any features of such examples are implemented. In other instances, well-known details are not shown or described to avoid obscuring aspects of the disclosure.

Persons of ordinary skill in the art understand terms and basic concepts related to microelectronics that are used in this disclosure, such as “voltage,” “signal,” “logical signal,” “clock,” “phase,” “duty cycle,” “period,” “common-source,” “power supply,” “ground,” “noise,” “CMOS (complementary metal oxide semiconductor),” “NMOST (n-channel metal-oxide semiconductor transistor),” “PMOST (p-channel metal-oxide semiconductor transistor),” and “cross couple.” Terms like these are used in a context of microelectronics, and the associated concepts are apparent to those of ordinary skills in the art and thus will not be explained in detail here.

Those of ordinary skill in the art can recognize a MOST (metal-oxide semiconductor transistor) symbol, and identify the “source,” the “gate,” and the “drain” terminals thereof, for both PMOST (p-channel metal-oxide semiconductor) and NMOST (n-channel metal-oxide semiconductor). Those of ordinary skills in the art can read schematics of a circuit comprising NMOST and PMOST, and do not need a verbose description about how one component connects with another in the schematics.

This present disclosure is disclosed from an engineering perspective. For instance, regarding two variables X and Y, when it is said that “X is equal to Y,” it means that “X is approximately equal to Y,” i.e. “a difference between X and Y is smaller than a specified engineering tolerance.” When it is said that “X is zero,” it means that “X is approximately zero,” i.e. “X is smaller than a specified engineering tolerance.” When it is said that “X is substantially smaller than Y,” it means that “X is negligible with respect to Y,” i.e. “a ratio between X and Y is smaller than an engineering tolerance and therefore X is negligible when compared to Y.”

A power supply node is a circuit node of a voltage that is approximately equal to a power supply voltage that is higher than zero but might have a small high-frequency fluctuation. A ground node is a circuit node of a voltage that is approximately zero but might have a small high-frequency fluctuation. Throughout this disclosure, “VDD” denotes a power supply node and “VSS” denotes a ground node.

A NMOST is configured in a “common-source” topology when an input is received from its gate and an output is output at its drain, while its source is connected to a ground node, e.g. “VSS.”

A PMOST is configured in a “common-source” topology when an input is received from its gate and an output is output at its drain, while its source is connected to a power supply node, e.g. “VDD.”

A schematic diagram a 4-phase clock buffer100in accordance with an embodiment of the present invention is shown inFIG.1. The 4-phase clock buffer100receives a 4-phase input clock comprising four phases: X1, X2, X3, and X4, and outputs a 4-phase output clock comprising four phases: Y1, Y2, Y3, and Y4. The 4-phase clock buffer100comprises: four PMOSTs PM1, PM2, PM3, and PM4configured in a common-source ring topology to dispatch Y1, Y2, Y3, and Y4, respectively; and four NMOSTs NM1, NM2, NM3, and NM4, configured in a common-source topology to control Y1, Y2, Y3, and Y4, in accordance with X1, X2, X3, and X4, respectively. PM1, PM2, PM3, PM4have a form of a 4-stage ring oscillator but cannot sustain an oscillation, because they can pull up but cannot pull down Y1, Y2, Y3, and Y4. Pull-downs of Y1, Y2, Y3, and Y4are carried out by NM1, NM2, NM3, and NM4in accordance with X1, X2, X3, and X4, respectively.

In an embodiment, a duty cycle of the 4-phase input clock is nominally 25%. An exemplary timing diagram of the 4-phase clock buffer100is shown inFIG.2. As shown, X1, X2, X3, and X4are four phases of the 4-phase input clock of a period T, and are evenly spread in time with T/4 spacing. Y1, Y2, Y3, and Y4are four phases of the 4-phase output clock of a period T and 75% duty cycle and are evenly spread in time with T/4 spacing. A low-to-high transition (L2H) E1of X1prompts NM1to pull down Y1, resulting in a high-to-low transition (H2L) E2of Y1. A L2H E3of X2follows the L2H E1of X1after a time of T/4 and prompts NM2to pull down Y2, resulting a H2L E4of Y2that consequently prompts PM1to pull up Y1, resulting in L2H E5. This way, Y1is pulled down by NM1and then pulled up by PM1with a time separation of approximately T/4, securing an approximately 75% duty cycle. The same analysis applies to Y2, Y3, and Y4. Since the H2L E2of Y1is driven by the L2H E1of X1and the ensuing L2H E5of Y1is driven by the L2H E3of X2, a time separation of the H2L E2the L2H E5of Y1and consequently a duty cycle of Y1is dictated by a time separation between the L2H E1of X1and the L2H E3of X2, which is T/4 due to the 4-phase nature of the input clock, and independent of the duty cycle of X1and X2, as long as a pull-up of PM1is stronger than a pull-down of NM1when X1is high and Y2is low. In other words, even if the duty-cycle of the 4-phase input clock (comprising X1, X2, X3, and X4) is not 25%, the duty cycle of the 4-phase output clock (comprising Y1, Y2, Y3, and Y4) is still approximately 75%, provided the pull-up of PM1, PM2, PM3, and PM4is stronger than the pull-down of NM1, NM2, NM3, and NM4. However, it is desirable that the duty cycle of the 4-phase input clock (comprising X1, X2, X3, and X4) is not greater than 25% to avoid contention between a pull-up of NM1, NM2, NM3, and NM4and a pull-down of PM1, PM2, PM3, and PM4, respectively.

In an optional embodiment, the 4-phase clock buffer100further comprises four additional PMOSTs PM5, PM6, PM7, and PM8, wherein PM5and PM7form a first cross-coupling pair (CCP) configured to cross couple Y1and Y3and PM6and PM8form a second CCP configured to cross couple Y2and Y4. Due to cross coupling, PM5and PM7ensure that Y1and Y3cannot be both low, while PM6and PM8ensure that Y2and Y4cannot be both low. This way, none of Y1, Y2, Y3, and Y4can stay in the low state for more than 50% of time, and the duty cycle of the 4-phase output clock is therefore ensured to be no greater than 50%, regardless of the duty cycle of the 4-phase input clock.

The 4-phase clock buffer100can be used to generate a 4-phase output clock of approximately 75% duty cycle in response to a 4-phase input clock that is approximately but not necessarily of 25% duty cycle, as explained earlier. If a 4-phase output clock of 25% duty cycle is needed, a 4-phase clock buffer300shown inFIG.3can be used. The 4-phase clock buffer300is adapted from the 4-phase clock buffer100by applying the following changes: (1) NMOST and PMOST are swapped, i.e., NM1, NM2, NM3, and NM4are replaced with PM1′, PM2′, PM3′, and PM4′, respectively, and PM1, PM2, PM3, PM4, PM5, PM6, PM7, and PM8are replaced with NM1′, NM2′, NM3′, NM4′, NM5′, NM6′ NM7′, and NM8′, respectively; and (2) the power supply node “VDD” and the ground node “VSS” are swapped. In addition, for distinction, X1, X2, X3, X4, Y1, Y2, Y3, and Y4are replaced by A1, A2, A3, A4, B1, B2, B3, and B4, respectively. Those or ordinary skill in the art can readily recognize that, the 4-phase clock buffer300is functionally the same as the 4-phase clock buffer100, but the high state and the low state are swapped (since the power supply node “VDD” and the ground node “VSS” are swapped). Therefore, the 4-phase output clock of the 4-phase clock buffer300, which is composed of B1, B2, B3, and B4, is of approximately 25% duty cycle, and the duty cycle of the 4-phase input clock, which is composed of A1, A2, A3, and A4, is nominally but not necessarily 75%.

By using the 4-phase clock buffer100followed by the 4-phase clock buffer300, one has a two-stage clock buffer that can output an approximately 25% duty cycle output clock in response to a 25% duty cycle input clock. By using the 4-phase clock buffer300followed by the 4-phase clock buffer100, one has a two-stage clock buffer that can output an approximately 75% duty cycle output clock in response to a 75% duty cycle input clock. In any case, a multi-stage clock buffer can be constructed from alternately instantiating the 4-phase clock buffer100and the 4-phase clock buffer300, such that one instance of the 4-phase clock buffer100is followed by one instance of the 4-phase clock buffer300, and/or one instance of the 4-phase clock buffer300is followed by one instance of the 4-phase clock buffer100.

In an embodiment, a two-stage 4-phase clock buffer comprises a cascade of a first stage and a second stage, wherein: the first stage is instantiated from the 4-phase clock buffer100and thus comprises four PMOSTs (such as PM1, PM2, PM3, and PM4) configured in a common-source ring topology to dispatch a first 4-phase clock (such as Y1, Y2, Y3, and Y4) and four NMOSTs transistors (such as NM1, NM2, NM3, and NM4) configured in a common-source topology to control the first 4-phase clock in response to a second 4-phase clock (such as X1, X2, X3, and X4); the second stage is instantiated from the 4-phase clock buffer300and comprises four NMOSTs (such as NM1′, NM2′, NM3′, and NM4′) configured in a common-source ring topology to dispatch a third 4-phase clock (such as B1, B2, B3, and B4) and four PMOSTs (such as PM1′, PM2′, PM3′, and PM4′) configured in a common-source topology to control the third 4-phase clock in response to the first 4-phase clock.

The 4-phase clock buffer100and the 4-phase clock buffer300are superior to inverter buffer in that, the duty cycle of the output clock is independent of (or at least highly insensitive to) the duty cycle of the input clock, therefore in a multi-stage clock buffer constructed from instantiating the 4-phase clock buffer100and the 4-phase clock buffer300, errors of duty cycle from constituent clock buffers will not be accumulated, and therefore the duty cycle of the output clock can be highly insensitive to process, supply voltage, and temperature variation.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the disclosure. Accordingly, the above disclosure should not be construed as limited only by the metes and bounds of the appended claims.