BACKGROUND 
     With the ever-increasing need for increased battery life for battery-powered devices, the need for low-power systems and SOCs (system on a chip) is also increasing. This gives rise to the need for power-managed designs with multiple power/voltage domains. In a design having a power domain, often there is a requirement to preserve state (of a flip-flop) even while the power domain is switched off. This state, often known as the standby state, helps reduce power-down and power-up time. To preserve this state, retention flops are widely used in almost all power-managed SOCs. 
     A typical retention flop includes a master latch and a slave latch, with the slave latch storing the state during power down. The slave latch of a retention flop must be powered by an always-on (AON) retention supply to store data when the associated device is turned off. The n-well and drain of the slave latch must to be connected to the always-on supply. To reduce leakage, the slave latch is often designed with high-voltage-threshold (HVT) transistors, while the master-latch is implemented using standard-voltage-threshold (SVT) transistors to target performance. Thus, such HVT transistors will at times be referred to herein as AON logic and such SVT transistors as switchable logic. 
     N-well leakage of SVT transistors is quite higher than that of HVT transistors. The n-well and drain of the storing latch of a retention flop must be connected to an always-on power supply, whereas the n-well and drain of the master latch of the retention flop should be connected to a switchable power supply in order to limit leakage current during retention/stand-by mode. Since the n-wells of the master latch and slave latch of the logic cell are connected to two different power supplies, such a logic cell must have two separate n-wells, which will also be referred to herein as a split n-well. 
       FIG. 1  is a schematic circuit diagram of an illustrative retention flop. Note that the retention flop implementation shown in  FIG. 1  is merely illustrative and that any number of different implementations are possible. The retention flop  100  of  FIG. 1  is a D flip-flop and includes a master latch  110  and a slave latch  120 . The illustrative master latch  110  of  FIG. 1  includes inverter  112 , inverter  114  and inverter  116 . Slave latch  120  includes inverter  122  and inverter  124 . The master latch  110  captures a new value D at the input to inverter  112 , while the slave latch  120  retains the value that was previously received by the master latch  110 . A pass gate  130  passes the value held in the master latch  110  to the slave latch  120  dependent upon a clock signal CK. Each of the inverters  112 ,  114 ,  116 ,  122 ,  124 , and  135  are illustratively composed of one or more transistors, such as metal oxide semiconductor (MOS) transistors. Together, the master latch  110 , slave latch  120 , pass gate  130 , and inverter  135  make up what is often referred to as a logic cell. In general, the term logic cell, as used herein, refers to a functional grouping of electronic elements such as transistors that form a logic element such as the D flip-flop of  FIG. 1 . 
     The master latch  110  is coupled to the always-on power supply V DDC  via a power switch  140 . The master latch  110  is illustratively disconnected from the power supply V DDC  by power switch  140  when the device is turned off or placed in a stand-by state in order to conserve battery power. In contrast, the slave latch  120  is connected directly to the always-on power supply V DDC  in order to maintain the data contents stored by the slave latch even when the device is turned off or placed in a standby state. Thus in device implementations wherein the master latch  110  and slave latch  120  are comprised of MOS transistors, the n-wells of the master latch transistors are coupled to the always-on power supply V DDC  via power switch  140 , while the n-wells of the slave latch transistors are connected directly to the always-on power supply V DDC . In alternative implementations, the master latch  110  is connected to a different, switchable, power supply, rather than connected to an always-on supply via a power switch. Since the n-wells of the master latch  110  and slave latch  120  of the logic cell  100  are connected to two different power supplies, such a logic cell must have two separate n-wells. 
       FIG. 2  is a schematic top view of an integrated circuit power domain.  FIG. 2  shows typical placement of standard logic cells in such a power domain. The power domain block  200  includes a p-well plane  220  comprised of positively doped (p+) semiconductor material. The power domain block  200  is conceptually divided into multiple rows, commonly referred to as cell rows  202 ,  204 ,  206 ,  208 ,  210 ,  212 . The power domain block  200  further includes a plurality of substantially parallel n-well drawings  230 ,  232 ,  234 , each comprising a layer of negatively doped (n+) semiconductor material deposited on top of the p-well-plane  220 . Such n-well drawings  230 ,  232 , and  234  will be alternatively referred to as n-well rows herein. Typically two adjacent cell rows share a common n-well drawing, with one cell row being flipped (sometimes referred to as a south row) and the other cell row not being flipped (sometimes referred to as a north row). For example, in  FIG. 2 , cell row  202  shares n-well row  230  with cell row  204 , cell row  206  shares n-well row  232  with cell row  208 , and cell row  210  shares n-well row  234  with cell row  212 . Thus standard logic cell  240  shares n-well  234  with standard logic cell  242 . Double-height standard logic cell  244  also makes use of n-well  234  as well as n-well  232 . Double-height power switch cell  250  utilizes only n-well  232 , which falls fully inside the footprint of the power switch cell  250 , as opposed to the power switch cell  250  sharing n-well  232  with an adjacent flipped cell row.  FIG. 2  also shows power tap cells  260 - 280  which are electrically coupled to the n-well rows  230 ,  232 ,  234  and also coupled to a power source to provide power to the n-wells  230 ,  232 ,  234 . For example, n-well row  230  is coupled to tap cells  260 ,  266 ,  270  and  276 ; n-well row  232  is coupled to tap cells  262 ,  268 ,  272  and  278 ; and n-well row  234  is coupled to tap cells  264 ,  270 , and  280 . Each n-well row  230 ,  232 ,  234  has tap cells placed at regular intervals along its length in order to minimize voltage drops resulting from resistive losses. 
     In the illustrative power domain block  200  of  FIG. 2 , all of the n-wells are connected to the same power supply. But in cases where a logic cell requires access to two different power supplies, such as in the example of  FIG. 1 , where the master latch  110  is powered by a switchable power supply and the slave latch  120  is powered by an always-on power supply, such a cell has to have two different n-wells that are separated from each other. Various solutions exist for implementing such n-well separation. One such existing solution is shown in  FIG. 3 .  FIG. 3  is a schematic diagram of a double-height logic cell  300  occupying two adjacent cell rows  310  and  312 . Logic cell  300  is commonly referred to as a “double-height” logic cell although the dimension occupied by the two logic cell rows  310  and  312  does not necessarily represent “height” in the traditional sense but may also represent other dimensions such as width or length. In the solution represented by  FIG. 3 , the standard-voltage-threshold (SVT) logic of the master latch of logic cell  300  is situated proximate, and utilizes, the n-well row  320 . N-well row  320  is coupled to a power switch  330  via one or more tap cells (not shown) that are placed at regular intervals along the length of n-well row  320  as shown in  FIG. 2 . The power switch  330  selectably couples the SVT n-well row  320  to an always-on power supply V DDC . Alternatively, the SVT n-well row  320  can be coupled to a switchable power supply that is wholly independent of the always-on power supply V DDC . A second n-well  340  is situated fully contained within, but isolated from, the n-well row  320 . The high-voltage-threshold (HVT) logic of the slave latch of logic cell  300  is situated proximate, and utilizes, this second n-well  340 . HVT n-well  340  is coupled to the always-on power supply V DDC . An AON tap cell  350  is placed inside each logic cell, illustratively situated within the footprint of the HVT n-well  340 , coupling the HVT n-well  340  to the always-on power supply V DDC . 
     The prior art solution represented by  FIG. 3  requires an area increase of the retention cell. The power domain of the n-well row  320  is switchable; thus to prevent a spacing requirement from an adjacent cell, the always-on n-well  340  is sandwiched between switchable n-wells. The AON tap connection requires extra area inside the cell  300  to accommodate the tap-cell  350  and maintain spacing between n-well  320  and n-well  340 . Such an implementation is also susceptible to latch-up issues due to the narrow n-well connection between the switchable n-well islands  360  and  370 . 
     Another prior art n-well separation solution is shown in  FIG. 4 .  FIG. 4  is a schematic diagram of a double-height logic cell  400  occupying two adjacent cell rows  410  and  412 . In the solution represented by  FIG. 4 , one side of logic cell  400  has a switchable n-well  420  while another side of the cell  400  has an always-on n-well  440 . The standard-voltage-threshold logic of the master latch of logic cell  400  is situated proximate, and utilizes, the switchable n-well  420 , which is coupled to a switchable power supply  430  via a tap cell (not shown) that, in some implementations, is coupled to the n-well  420  outside of the footprint of the logic cell  400 . The high-voltage-threshold logic of the slave latch of logic cell  400  is situated proximate, and utilizes, the always-on n-well  440 , which is coupled to an always-on power supply V DDC  via a tap cell (not shown) that, in some implementations, is coupled to the n-well  440  outside of the footprint of the logic cell  400 . An advantage of the n-well separation solution of  FIG. 4  is that adjacent cells can be arranged “flipped” relative to each other. For example, a cell to the right of logic cell  400  can be arranged such that it shares the always-on n-well  440  with logic cell  400 , and a cell to the left of logic cell  400  can be arranged such that it shares the switchable n-well  420  with logic cell  400 . But since the n-wells  420  and  440  are not continued, the solution of  FIG. 4  requires tap cells between standard logic cells, which increases the area required by this solution. Additionally, the n-well spacing between the two n-wells  420  and  440  needs to be maintained within the cell, further increasing the area requirement. 
     SUMMARY 
     An illustrative aspect of this disclosure is directed to an integrated circuit device having a p-well plane, a plurality of substantially parallel n-well rows, and a logic cell. The p-well plane is comprised of p-type semiconductor material. Each n-well row comprises an n-type layer disposed on the surface of the p-well plane. The plurality of n-well rows includes a first n-well row and a second n-well row. The logic cell is arranged on the p-well plane and the footprint of the logic cell encompasses both the first and second n-well rows. 
     Another illustrative aspect of this disclosure is directed to an integrated circuit logic cell that includes a p-well plane, a first n-well row, a second n-well row, and first, second, third and fourth parallel and contiguous cell rows. The p-well plane is comprised of p-type semiconductor material. The first n-well row is comprised of an n-type layer disposed on the surface of the p-well plane. The second n-well row is substantially parallel to the first n-well row and is comprised of an n-type layer disposed on the surface of the p-well plane. 
     The first and second logic cell rows are parallel to and share the first n-well row, and the third and fourth logic cell rows are parallel to and share the second n-well row. 
     Another illustrative aspect of this disclosure is directed to an integrated circuit device having a p-well plane, a plurality of substantially parallel n-well rows, and a logic cell. The p-well plane is comprised of p-type semiconductor material. Each of the plurality of n-well rows is comprised of an n-type layer disposed on the surface of the p-well plane. The plurality of n-well rows includes a first n-well row coupled to a switchable power supply and a second n-well row coupled to an always-on power supply. The logic cell is arranged on the p-well plane, with the footprint of the logic cell encompassing both the first and second n-well rows. The logic cell includes at least one standard-voltage-threshold (SVT) transistor and at least one high-voltage-threshold (HVT) transistor. The at least one SVT transistor utilizes the first n-well row and the at least one HVT transistor utilizes the second n-well row. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of an illustrative retention flop. 
         FIG. 2  is a schematic top view of an integrated circuit power domain. 
         FIG. 3  is a schematic diagram of a double-height logic cell occupying two adjacent cell rows. 
         FIG. 4  is a schematic diagram of a double-height logic cell occupying two adjacent cell rows. 
         FIG. 5  is a schematic diagram of a quad-height logic cell occupying four adjacent cell rows in accordance with illustrative aspects of the present disclosure. 
         FIG. 6  is a schematic diagram of three adjacent quad-height logic cells occupying four adjacent cell rows in accordance with illustrative aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments are described with reference to the drawings, wherein like reference numerals are used to designate similar or equivalent elements. Illustrated ordering of acts or events should not be considered as limiting, as some acts or events may occur in different order and/or concurrently with other acts or events. Furthermore, some illustrated acts or events may not be required to implement a methodology in accordance with this disclosure. 
       FIG. 5  is a schematic diagram of a quad-height logic cell  500  occupying four adjacent cell rows  510 ,  512 ,  514 ,  516  in accordance with illustrative aspects of the present disclosure.  FIG. 5  shows two substantially parallel n-well rows  520  and  540  disposed on a p-well plane  505 . P-well plane  505  is comprised of positively doped (p+) semiconductor material. The n-well rows, or n-well drawings,  520  and  540 , each comprise a layer of negatively doped (n+) semiconductor material deposited on top of the p-well plane  505 . In the n-well separation scheme of  FIG. 5 , the n-wells  520  and  540  are spaced apart “vertically,” as opposed to “horizontally” in the prior art. Those of skill in the art will recognize that the terms “vertically” and “horizontally” are used here to describe their spatial relationship as represented in figures such as  FIG. 5  for purposes of explanation, and may not necessarily describe the actual physical spatial relationships in a given integrated circuit embodying the described aspects of the present disclosure. Thus, logic cell  500  has a footprint that encompasses four contiguous cell rows  510 ,  512 ,  514  and  516 , as well as two adjacent and parallel n-well rows  520  and  540 . In the illustrative embodiment of  FIG. 5 , n-well row  520  is coupled to a switchable power supply  530  and n-well row  540  is coupled to an always-on power supply V DDC . Logic circuitry that can be turned off when the device is turned off or placed in standby mode is placed in cell rows  510  and  512  and uses switchable n-well  520 . Logic circuitry that needs to remain powered up at all times is placed in cell rows  514  and  516  and uses always-on n-well  540 . For example, in an illustrative embodiment, the logic cell  500  is a retention flop that includes a master latch and a slave latch. In such an embodiment, the standard-voltage-threshold (SVT) logic of the master latch of logic cell  500  is arranged proximate, and utilizes, n-well row  520 . The high-voltage-threshold (HVT) logic of the slave latch of logic cell  500  is arranged proximate, and utilizes, n-well row  540 . Because each n-well  520  and  540  stretches all the way across the logic cell  500 , there is no horizontal n-well spacing requirement within the cell as there is with the prior art solutions of  FIGS. 3 and 4 . 
       FIG. 6  is a schematic diagram of three adjacent quad-height logic cells  600 ,  605 ,  610  occupying four adjacent cell rows  620 ,  622 ,  624 ,  626  in accordance with illustrative aspects of the present disclosure.  FIG. 6  demonstrates other aspects of the n-well separation scheme shown in  FIG. 5 .  FIG. 6  shows two substantially parallel n-well rows  630  and  640  disposed on a p-well plane  602 . Adjacent logic cells  600 ,  605  and  610  each have a footprint that encompasses cell rows  620 ,  622 ,  624  and  626 , as well as n-well rows  630  and  640 . In an illustrative embodiment, n-well row  630  is coupled to a switchable power supply and n-well row  640  is coupled to an always-on power supply. Logic circuitry that can be turned off when the device is turned off or placed in standby mode is placed in cell rows  620  and  622  and uses switchable n-well  630 . Logic circuitry that needs to remain powered up at all times is placed in cell rows  624  and  626  and uses always-on n-well  640 . In an illustrative embodiment, the logic cells  600 ,  605 ,  610  are retention flops that each include a master latch and a slave latch. In such an embodiment, the standard-voltage-threshold (SVT) logic of the master latch of each logic cell  600 ,  605 ,  610  is arranged proximate, and utilizes, n-well row  630 . The high-voltage-threshold (HVT) logic of the slave latch of each logic cell  600 ,  605 ,  610  is arranged proximate, and utilizes, n-well row  640 . Tap cells, such as tap cell  650  in cell row  622 , are coupled to n-well row  630  at regular intervals to provide power to the n-well  630  and the drains of its connected transistors. The tap cells, such as tap cell  650 , that are coupled to n-well row  630  are coupled to a switchable power supply (not shown). Tap cells, such as tap cell  660  in cell row  626 , are coupled to n-well row  640  at regular intervals to provide power to the n-well  640  and the drains of its connected transistors. The tap cells, such as tap cell  660 , that are coupled to n-well row  640  are coupled to an always-on power supply. 
     With the n-well separation scheme of  FIG. 6 , since n-welt row  630  is continued across cell rows  620  and  622 , and n-well row  640  is continued across cell rows  624  and  626 , there is no need to put extra tap cells between logic cells  600 ,  605  and  610 . Placement of tap cells such as tap cells  650  and  660  at regular intervals is sufficient. Thus, with this approach there is no area wastage inside the logic cells and no cell placement overhead resulting from a need for extra tap cells. 
     While the logic cells described with respect to  FIGS. 5 and 6  include two n-well rows and four cell rows, the present disclosure is not limited to these embodiments. The present disclosure contemplates logic cells encompassing any plural number, i.e., greater than or equal to 2, n-well rows, and a commensurate number of cell rows. 
     Commercially available place-and-route (PNR) tools support placement of multiple-height cells and such placement does not result in any overhead in logic cell placement. Such PNR tools can be employed to create a placement site encompassing four cell rows for logic cells such as those described in  FIGS. 5 and 6 . 
     With integrated circuit applications demanding increasingly low-power designs, the use of multi-voltage-threshold, split n-well designs is likely to become ubiquitous. The aspects of the present disclosure differentiate over existing solutions in that there is no overhead attendant to implementing the multi-voltage-threshold, split n-well designs described with respect to  FIGS. 5 and 6 . Thus the logic cells such as those described with respect to  FIGS. 5 and 6  can serve as a fundamental building block for any such low-power integrated circuit applications. 
     Existing integrated circuit designs are generally very frugal in their use of retention flops due to the power and area overheads associated with them. But with high voltage-threshold transistors reducing leakage current by amounts on the order of 100× compared to standard voltage-threshold transistors and the proposed solution eliminating the area overheads, designs implementing 100% retention flops are feasible. This makes ultra-fast power-down and power-up times possible, with ultra-low leakage currents during power-down. This, in turn, makes the sleep and power-down states more lucrative than ever before and extends battery life. 
     It is noted that the embodiments disclosed herein are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure. Furthermore, in some instances, some features may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the broad inventive concepts disclosed herein.
An integrated circuit device having a p-well plane, a plurality of substantially parallel n-well rows, and a logic cell. The p-well plane is comprised of p-type semiconductor material. Each n-well row comprises an n-type layer disposed on the surface of the p-well plane. The plurality of n-well rows includes a first n-well row and a second n-well row. The logic cell is arranged on the p-well plane and the footprint of the logic cell encompasses both the first and second n-well rows.