Double data rate dynamic logic

[0002] Embodiments of the present invention are directed to digital circuits, and more particularly, to dynamic logic gates.

[0003] Dynamic (or domino) logic gates are ubiquitous building blocks in many high performance digital circuits. In particular, for microprocessors, dynamic logic gates find their way into many functional units, such as multipliers and adders, among others. Dynamic logic gates allow for pipelining to increase throughput, and make use of nMOSFETs (Metal Oxide Semiconductor Field Effect Transistor) to speed logic evaluation.

[0004] An example of a four stage dynamic logic circuit (or domino circuit) is provided in FIG. 1. Each nMOSFET logic unit 101, 102, 103, and 104 denotes one or more nMOSFETs connected in various combinations of parallel and serial configurations so as to achieve the overall desired Boolean expression performed by the dynamic logic circuit. For simplicity, only one input port is shown for each nMOSFET logic unit, but there may in fact be several such input ports for each nMOSFET logic unit. Four clock signals, φ_i=1, 2, 3, 4, are provided in FIG. 1. These clock signals are staggered in phase by π/2, that is, for each i=1, 2, 3, φ_i+1lags φ_iby π/2 radians.

[0005] Considering stage 1 of the dynamic logic circuit of FIG. 1, the pre-charge phase begins on the falling edge of the clock signal φ₁, i.e., when φ₁transitions from HIGH to LOW, so that pMOSFET 106 turns ON and nMOSFET 108 turns OFF. With nMOSFET 108 OFF, nMOSFET logic unit 101 is isolated from ground (substrate), and with pMOSFET 106 ON, node 110 is pulled HIGH. Inverter 112 and pMOSFET 114 function as a keeper, so that node 110 is weakly held HIGH unless otherwise pulled LOW by nMOSFET logic unit 101 during the evaluation phase. Inverter 116 is a static inverter, so that the input to stage 2 of the dynamic logic circuit is LOW when stage 1 is in its pre-charge phase. The evaluation phase begins on the rising edge of the clock signal φ₁, i.e., when φ₁transitions from LOW to HIGH, so that nMOSFET logic unit 101 is now coupled to ground via nMOSFET 108. In the evaluation phase, nMOSFET logic unit 101 may pull node 110 LOW depending upon its input. If not, then keeper pMOSFET 114 keeps node 110 HIGH as mentioned earlier.

[0006] By staggering the phases of the clock signals, the various stages illustrated in FIG. 1 may be pipelined together to achieve a high throughput, so that input is provide, to the dynamic logic circuit at the clock rate.

[0007] FIG. 1 is a prior art dynamic logic gate pipeline.

[0008] FIG. 2 is a single-ended dynamic logic gate according to an embodiment of the present invention.

[0009] FIG. 3 is a dual-rail dynamic logic gate according to an embodiment of the present invention.

[0010] FIG. 4 is a single-ended dynamic logic according to an embodiment of the present invention.

[0011] FIG. 5 is a dual-rail dynamic logic gate according to an embodiment of the present invention.

[0012] Embodiments of the present invention provide for dynamic logic circuits with increased throughput over the dynamic logic circuit of FIG. 1 for the same clock frequency. An evaluation phase may be performed for each phase of a clock signal. As a result, embodiments of the present invention may realize twice the throughput as that of the dynamic logic circuit of FIG. 1 for the same clock frequency.

[0013] An embodiment is shown in FIG. 2, where nMOSFET logic unit 202 comprises a set of nMOSFETs configured in various parallel or serially connected combinations to achieve the overall desired Boolean function. As in FIG. 1, clock signal φ₁leads clock signal φ₄by 3π/2. A bar over a signal indicates its Boolean complement. For example, the clock signals φ_iand {overscore (φ)}_iare π radians out of phase with respect to each other. The circuit of FIG. 2 represents one stage of a dynamic logic circuit. For simplicity, only one input port to nMOSFET logic unit 202 is indicated, but in practice there usually is more than one input port.

[0014] The output signals are provided at output ports 204 and 206. An output signal is provided at output port 204 when clock signal φ₄is HIGH, and an output signal is provided at output port 206 when clock signal {overscore (φ)}₄is HIGH (φ₄is LOW). Transistors 236, 220, 216, and 238 may be considered forming a first dynamic latch, where the dynamic latch is switched (or clocked) by the clock signal φ₄. Similarly, transistors 240, 218, 222, and 242 may be considered forming a second dynamic latch, where the second dynamic latch is switched (or clocked) by the clock signal φ₄such that it is switched in complementary fashion with respect to the first dynamic latch.

[0015] Although the circuit of FIG. 2 comprises one stage, it is pedagogically convenient to decompose the single stage into sub-blocks or sub-stages, denoted as stage A and stage B in FIG. 2. Clock signal φ₁clocks (or switches) nMOSFET 208 and pMOSFET 210 in stage A, and its Boolean complement, {overscore (φ)}₁, clocks nMOSFET 212 and pMOSFET 214 in stage B. Clock signal φ₄clocks nMOSFET 216 in stage A and pMOSFET 218 in stage B, and its complement, {overscore (φ)}₄, clocks pMOSFET 220 in stage A and nMOSFET 222 in stage B. When the clock signal φ₁is HIGH, stage A is in its evaluation phase and stage B is in its pre-charge phase. When the clock signal φ₁is LOW, stage A is in its pre-charge phase and stage B is in its evaluation phase.

[0016] With stage A in its pre-charge phase, pMOSFET 210 pulls node 228 HIGH, and pMOSFET 224 and inverter 226 comprise a keeper so that node 228 is weakly held HIGH. With stage B in its pre-charge phase, pMOSFET 214 pulls node 234 HIGH, and pMOSFET 230 and inverter 232 comprise a keeper so that node 234 is weakly held HIGH. When stage A goes into its evaluation phase, nMOSFET logic unit 202 may pull node 228 LOW depending upon its input when the clock signal φ₁, is HIGH, and when stage B goes into its evaluation phase, nMOSFET logic unit 202 may pull node 234 LOW depending upon its input when the clock signal φ₁is LOW. The Boolean complement of the signal at node 228 is provided to output node 204 when the clock signal φ₄is HIGH, and the Boolean complement of the signal at node 234 is provided to output node 206 when the clock signal φ₄is LOW. In this way, outputs are provided at output ports 204 and 206 on alternate clock phases (π radians out-of-phase), so that the throughput of the dynamic stage in FIG. 2 is twice that of prior art dynamic logic.

[0017] The dynamic logic stage illustrated in FIG. 2 may be connected to other dynamic logic stages to form a pipeline. In one such embodiment, the output ports 204 and 206 are connected to each other, and to the input port of the next stage in the dynamic pipeline. If the stage in FIG. 2 is considered the zeroth stage in a dynamic pipeline, then the next stage, stage 1, in an embodiment dynamic pipeline is structurally equivalent to the zeroth stage, except that the clock signal φ₂replaces the clock signal A, and the clock signal φ₁replaces the clock signal φ₄. Stage 3 in the embodiment is structurally equivalent to stage 2, except that the clock signal φ₃replaces the clock signal φ₂, and the clock signal φ₂replaces the clock signal φ₁. This pattern repeats, where for stage i=0, 1, 2, 3, . . . , the clock signals clocking the nMOSFETs coupled to the nMOSFET logic unit in stage i are φ_kand φ_kwhere k=i mod(4)+1, and the clock signals clocking the dynamic latches in stage i are φ_mand φ_mwhere m=(i+3)mod(4)+1. In this way, the clock signals for the dynamic latches of stage i and the clock signals for the nMOSFET logic unit of stage i+1 differ in phase by ±π/2 so that race conditions are mitigated, and the pre-charge phase of one stage does not interfere with the evaluation phase of the next stage.

[0018] The circuit of FIG. 2 is a single-ended dynamic logic gate. An embodiment of a dual-rail dynamic logic gate is provided in FIG. 3. As illustrated in FIG. 3, the logic gate comprises a complementary logic path and a true logic path. The operation of each path in FIG. 3 is similar to that of the single-ended gate of FIG. 2, except that the keepers are realized by cross-coupling pMOSFETs. For example, pMOSFET 302 in the complementary logic path has its gate connected to node 304 in the true logic path, and the gate of pMOSFET 306 in the true logic path has its gate connected to node 308 in the complementary logic path. In this way, static inverters are not needed for the keepers, and there is no contention between the keepers and the nMOSFET logic units as in the single-ended gates. Consequently, the pMOSFET keepers in FIG. 3 may be sized larger than the pMOSFETs keepers in FIG. 2 so as to help increase noise robustness.

[0019] Another embodiment of a single-ended dynamic gate is provided in FIG. 4. The single stage shown in FIG. 4 may be decomposed into stage A and stage B, as was done for the embodiment of FIG. 2. However, clock signals φ₁and {overscore (φ)}₁are used to clock the various MOSFETs in FIG. 4. Stages A and B have their pre-charge and evaluation phases on alternate clock cycles, that is, the pre-charge and evaluation phases are π radians out-of-phase. When the clock signal φ₁goes HIGH, stage A enters its evaluation phase and stage B enters its pre-charge phase, and when φ₁goes LOW, stage A enters its pre-charge phase and stage B enters its evaluation phase.

[0020] With stage A in its pre-charge phase, pMOSFET 412 pulls output port 402 HIGH, and pMOSFET 406 and inverter 408 comprise a keeper so that output port 402 is weakly held HIGH. With stage B in its pre-charge phase, pMOSFET 410 pulls output port 404 HIGH, and pMOSFET 414 and inverter 416 comprise a keeper so that output port 404 is weakly held HIGH. When stage A goes into its evaluation phase, nMOSFET logic unit 418 may pull output port 402 LOW depending upon its input when the clock signal φ₁is HIGH, and when stage B goes into its evaluation phase, nMOSFET logic unit 420 may pull output port 404 LOW depending upon its input when the clock signal φ₁is LOW. In this way, outputs are provided at output ports 402 and 404 on alternate clock phases (π radians out-of-phase).

[0021] In the particular embodiment of FIG. 4, output ports 402 and 404 are connected to the input ports of static NAND gate 420 to provide an output signal at output port 422. Because output port 402 is HIGH when stage A is in its pre-charge phase, and output port 404 is HIGH when stage B is in its pre-charge phase, NAND gate 420 provides an output signal at output port 422 every alternate phase of the clock signal. In this way, the throughput for the dynamic logic gate of FIG. 4 may be twice that of the dynamic logic gates of FIG. 1 for the same clock frequency. In another embodiment, output ports 402 and 404 may be provided to other types of embedded static logic.

[0022] The dynamic logic gate of FIG. 4 may be pipelined with other dynamic logic gates having an equivalent structure. If the dynamic logic gate of FIG. 4 is considered the zeroth stage in a pipeline, then stage 1 in an embodiment is equivalent in structure except that the clock signal φ₁is replaced with the clock signal φ₂. Likewise, stage i is clocked by clock signal φ_kwhere k=1+imod(4).

[0023] Another embodiment of a dual-rail dynamic logic gate is shown in FIG. 5. As illustrated in FIG. 5, the logic gate comprises a complementary logic path and a true logic path. The operation of each path in FIG. 5 is similar to that of the single-ended gate of FIG. 4, except that the keepers are realized by cross-coupling pMOSFETs, as for the embodiment of FIG. 2.

[0024] Various modifications may be made to the disclosed embodiments without departing from the scope of the invention as claimed below. For example, the embodiment of FIG. 2 may be modified so that dynamic logic latches may be used in place of the dynamic latches in FIG. 2 to realize a Boolean function other than the inverse function, or the embodiment of FIG. 4 may be modified so that static logic elements may be used in place of gate 420 to realize a Boolean function other than the NAND function. Similar remarks apply to the other disclosed embodiments.