Abstract:
A system and method for voltage sensing at active power gated cores of a multi core CPU wherein a Controlled Collapse Chip Carrier bump in a gating region for an associated core is isolatable from an ungated power region by a power gate to allow voltage sensing at a designated location with substantially no current passing there through.

Description:
FIELD 
       [0001]    The present disclosure is related to voltage sensing and regulation in power gated CPU&#39;s. More specifically, a system and method for voltage sensing at the active power gated CPU cores is disclosed. 
       BACKGROUND 
       [0002]    Voltage sensing and regulation become more complicated in a gated system. Power gating effectively closes off the leakage current path for CPU cores which are not active. In a power gated configuration, current travels into the CPU silicon at the power gate Controlled Collapse Chip Connection bumps (C4 bumps) on the ungated side, through power gating transistors, and into a gated power domain on the chip and package substrate. If the core&#39;s power gates are on, a conducting path exists between the ungated voltage domain and the gated domain. If the power gates are off, the gated CPU is isolated from the ungated supply. 
         [0003]    Multi core systems make use of power gating to reduce leakage power on inactive cores, and utilize that power for faster and more efficient operation of the active cores. Any core may go into an inactive state and be gated, in which its voltage supply is cut off by the power gates, and the on core voltage will decay to 0V. In that state, the remaining active cores must be supplied with an appropriate voltage level. 
         [0004]    Voltage sensing may be conducted through on die structures located in a variety of locations throughout the cores. An integrated on die regulator senses these locations and uses a digital algorithm to determine the correct regulation voltage. Power gating does not present a problem for this configuration, since the on die voltage regulator controller is able to logically determine the voltages to sense and those to ignore if power gating is enabled for some of the cores. 
         [0005]    However, in most instances external VR11 operation may be desired. In this case, a voltage sense line from the die is routed to the regulator to monitor the voltage and use it for regulatory feedback. Complications arise when the voltage at any gated core could fall to 0V, while other cores need a stable voltage supply. The voltage regulator needs to somehow ignore the voltage at cores with power gates turned off, while continuing to monitor and adjust to voltage changes at the active cores. 
         [0006]    Sensing the voltage upstream in the package ungated region is undesirable because the gated voltage is separated from this domain by package power routing, the power gates themselves, and any other parasitic impedance between the power gates and the core transistors. These series elements in the power path could account for up to 20-30 mV of voltage difference between the regulation point and the true core voltage. The loss in sensing accuracy of 2-4 MHz per mV is a problem which calls for an as yet unavailable means for voltage sensing at each core, which is able to comprehend the gating states to disable sensing at the gated cores. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic representation of the voltage domains on package and silicon for a typical power gated system according to the disclosure; 
           [0008]      FIG. 2  is a circuit diagram of a power supply pathway and sensing network for a power gated system according to the disclosure using an off die voltage regulator controller; 
           [0009]      FIG. 3  is a side view of a CPU mounting assembly according to the disclosure; 
           [0010]      FIG. 4  is a power flow block diagram for the CPU of  FIG. 3 ; and 
           [0011]      FIG. 5  is a diagram of the power gate transistors for the CPU of  FIG. 3 , having one pair of C4 bumps isolated for voltage sensing. 
       
    
    
       [0012]    Although the following Detailed Description will proceed with reference being made to these illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly. 
       DETAILED DESCRIPTION 
       [0013]    The disclosure of  FIGS. 1 through 5  provides a power gated system  100  and method, with only a package substrate change, to enable voltage sensing at a point  162  in the active CPU cores for providing correct voltage regulation. Referring to  FIG. 1 , Region A is the package substrate  102 . Region B is the Silicon Footprint  110  for a CPU having N cores, including Core- 1   106 A, Core- 2   106 B, Core- 3   106 C and so on, through Core-N  106 N. Region C is the ungated power region  108 , and Region D is the power gating region  104  for the CPU. In order to sense the voltages of the active cores of  106 A- 106 N and average them to provide an accurate regulation voltage, the power gates of Region D may be used along with a ‘wired OR’ in the package. 
         [0014]      FIG. 2  shows the schematic for the sensing circuit  130 , along with the standard core current pathway  134  through the power gates  124 A to  124 N.  FIG. 3  shows the mounting of CPU  152  of the disclosure to a typical CPU socket  154  of a typical motherboard  156 . The power gates  124 A- 124 N are shown as single devices for each gated power domain  110 , but are actually parallel sets of hundreds of devices to provide low resistance to current traveling through them. Current from the platform voltage regulator  150  enters the CPU silicon  152  through the ungated power supply section  108 . 
         [0015]    Depending on the activity state of the cores  106 A through  106 D in the CPU, all, some, or none of the power gating transistors  124 A to  124 D will be in an on state and conducting current. For those power gates  124 A to  124 D that are conducting, current will flow through the power gate transistors in region  104 , and into the gated regions  106 A to  106 D to power the transistors on the chip. If a power gate  124 A for a core is on, the gated power domain  106 A must be regulated to a specified voltage to ensure functionality of the core. If a power gate  124 N is off, its corresponding gated power domain will fall to zero volts. Power gates which are in the on state may observe and regulate the voltage of the cores which are active. If a power gate is off, its power domain may be ignored, since a zero voltage may corrupt the correct sensing voltage for regulation. 
         [0016]      FIG. 5  shows the isolation of one C4 bump pair ( 112 Ain/ 112 Aout) for voltage sensing, in the power gating of the disclosed CPU  152  of M bump pairs,  112 A through  112 M. Current enters the bump pairs  112  from ungated power region  108  region and exits though gated region  110 . To ensure that only the gated power domains in the on state are sensed, and to maintain design simplicity, one power gate  124 A per core may be allocated, or “sacrificed”, to a sensing pathway  160 . The power gates  124 A to  124 N are groupings of many parallel transistors that operate in unison. In the Bloomfield CPU design, there are approximately  150  individual pathways making up each power gate transistor represented in  124 A to  124 N. By isolating one power gating transistor  124 A from each group of  124 A to  124 N, and routing a sense line  160  through it, the problem of sensing only on state gated power domains is solved. If the power gate  124 A is turned off, the sensing voltage does not pass through the power gate. If the power gate  124 A is turned on, it passes the correct voltage from the gated power domain through the power gate. 
         [0017]    A single C4 bump  112 A in the ungated region  108  and a single C4 bump  112 A in the gated region  110  for each core are sacrificed for voltage sensing and are designated as the voltage sense bump pair  112 A. The designated sense bump pair is no longer available as a current providing pathway, and is isolated from the ungated power region  108  in the package  152  and on silicon power bussing. Substrate level isolation allows the bump voltage at pair  112 A to be observed with no significant current passing through it. With no substantial current passing through bump pair  112 A and its associated power gating transistor  124 A, the true core voltage is observable at the sensing point  162  of system  100 . 
         [0018]    Depending on design requirements, the voltage sensing point may be at C4 bump  112 A, or more commonly at an alternate location in the gated region  110  connected to C4 bump  112 A through a trace. Package traces  118  are routed from each of the designated sensing bumps  112 A for each core to a central point  162  in the package gating region  110 , and out of the package/socket through a merged trace  122 . The merging of the traces provides an adequate averaging function, so that the average voltage of active cores is observed. By averaging through merging, the regulation voltage will be appropriate for the loaded core and the unloaded core. The merged trace is routed to a high impedance amplifier input contained in voltage regulator  150 . The high impedance input implies that no significant current will flow through the sense line. 
         [0019]    When one or more of the power gates  124 A through  124 N turn off to isolate an inactive core, co-functioning sacrificial gates  132 A through  132 N simultaneously cut off the sense bump voltage connection into the inactive core. This provides that only the active cores are monitored for voltage regulation. Sacrificing a single pair of C4 power bumps and converting them to dedicated sense bumps eliminates the need for added die complexity to facilitate the voltage sensing on gated/ungated cores. 
         [0020]    Design simplicity is a key benefit of this invention. The necessary structures already exist on the power gated die to provide this capability. The package substrate is the simplest element to change in order to enable the off chip sensing capability. The sacrifice of the one C4 bump  112  for sensing in a 4-core package only reduces that path width by 0.6%, which is acceptable. 
         [0021]    Providing a unified sensing mechanism at the socket level allows a well established and trusted voltage regulation technologies, such as VR11 to support the CPU. Otherwise, a typical on die regulator controller may be used for validation. But, when such an on die controller is nonfunctional or less than fully functional, the validation of the entire CPU may be at risk. 
         [0022]    By implementing this simple package change, the system designer is able to regulate to the correct core voltage, and is freed from a requirement to determine the activity states of each core. 
         [0023]    It should be understood that the above disclosures are merely representative and that there are many possible embodiments for the present invention, and that the scope of the invention should only be limited according to the following claims made thereto.