Abstract:
In general, in one aspect, the disclosure describes a localized IC thermal sensor. The thermal sensor includes an array of analog thermal sensors distributed across a circuit die to provide localized thermal measurements across the circuit die. The analog thermal sensors generate a frequency which is functionally dependent on temperature.

Description:
BACKGROUND 
     Integrated circuits (ICs), such as microprocessors, continue to increase circuit densities to support higher levels of performance and functionality. The growth in transistor density has resulted in transistors having shorter gate lengths while power supply voltages have not been scaled proportionally. The increase in performance may include an increase in clock speeds and accordingly an increase in the power required to operate the ICs. The increase in power leads to increased temperature of the ICs. The temperature of the ICs needs to be maintained within certain parameters to ensure the IC does not malfunction or burn-out due to overheating. The temperature may be maintained by utilizing cooling systems (e.g., fans). The temperature of the IC should be monitored and the cooling system adjusted based on the temperature of the IC (e.g., air flow increases as the temperature increases). In addition, if the temperature gets to high the IC may be throttled down or off to bring the temperature down. 
     On chip temperature sensors may be used to monitor the temperature of the IC. Present thermal sensors include diode elements and inverter based oscillators. Diode thermal sensors require large areas and large current source arrays which make the sensor rather large. Oscillator thermal sensors are a strong function of the supply voltage and these voltages are very hard to calibrate in wafer and chip fabrication making this technique impractical for real time measurements. 
     Using a single thermal sensor measures the temperature of the IC at and near that particular point. The design of ICs may result in certain hot spots on the IC and there may be a fairly large thermal gradient across the die. Accordingly, it is desirable to position a thermal sensor near the hot-spots. However, it may be difficult to find room in the IC for diode thermal sensors. Moreover, the on-chip hot spots cannot be predicted accurately at the early stages of the design. The hot spots may only be known after the floor plan has been designated and there is a substantial amount of gate level real estate already on the die. 
     Accurate thermal monitoring of the IC, and in particular the hot spots on the IC, is needed to provide information for throttling and fan regulation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the various embodiments will become apparent from the following detailed description in which: 
         FIG. 1  illustrates an example localized IC thermal sensor, according to one embodiment; 
         FIG. 2  illustrates an example circuit used as a miniaturized thermal sensor, according to one embodiment; and 
         FIG. 3  illustrates a functional diagram of an example system utilizing an example localized IC thermal sensor, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example localized IC thermal sensor  100  that includes an array of miniature thermal sensors  110  on an IC die. The miniature sensors  110  may be small enough that they may be spread across the IC at defined intervals (e.g., at the vertices). The use of the array of miniature sensors  110  may provide true chip level thermal information (localized thermal measurements across the die) during processor operation without significant area or power penalty. The true chip level thermal information accordingly enables improved thermal management of the IC (e.g., activation of fans, throttling/activation of the IC). It should be noted that for ease of illustration the die circuitry is not included in  FIG. 1 . 
     The localized thermal sensor  100  may include a centralized thermal sensor  120  (e.g., thermal diode) that may give an absolute (or near absolute) temperature measurement of the IC at and around location of the centralized thermal sensor  120  on the IC. The miniature sensors  110  may be used to calculate a temperature difference (ΔT) and determine relative temperature gradients across the IC. One of the miniature sensors  110  may be located near the centralized thermal sensor  120  to act as a reference sensor  130 . The frequency of the reference sensor  130  may be compared to the frequency of other miniature sensors  110  to calculate a frequency difference (Δf) that can be used to calculate the ΔT. 
     The localized thermal sensor  100  may have its own power grid (e.g., an on-chip regulated supply) for supplying power to the array of miniature sensors  110 . The power grid may be very sparse since the miniature sensors  110  may require minimal current to operate. Moreover, the miniature sensors  110  may be turned on one at a time. The miniature sensors  110  may have a good power-supply rejection ratio (PSRR), so the DC variations and noise on the power grid (V CC ) and/or supply voltage (V SS ) should not affect the temperature measurement. A voltage regulator supplying the grid can be located in a convenient place (e.g., a phase lock loop (PLL) farm) where there may be reference circuits available. 
       FIG. 2  illustrates an example circuit  200  that can be utilized as a miniaturized thermal sensor (e.g.,  110  of  FIG. 1 ). For convenience, hereinafter the circuit  200  will be referred to as thermal sensor  200 . The thermal sensor  200  operates by converting temperature to current. The current is then used as the bias for a ring-oscillator. The overall effect is a sensor which converts temperature to frequency. The frequency may then be converted to a temperature. 
     The thermal sensor  200  utilizes analog circuitry (e.g., transistors) and the properties associated therewith to generate a temperature dependent frequency. The temperature dependent frequency is based on the temperature dependent properties of the analog circuitry (threshold voltage (V t ) and mobility (u)). Utilizing these properties results in a nearly linear or quasi linear relationship between frequency and temperature. The analog circuitry is designed with currents and voltages referenced to V CC , so that there is negligible V SS  dependence, hence a very good PSRR. 
     The miniaturized thermal sensor  200  may include a reference voltage generator  210 , a current generator  230  and a current to frequency converter  250 . The reference voltage generator  210  may be used to generate a reference voltage (V ref ), and the V ref  may be linear with respect to temperature (e.g., proportional to absolute temperature (PTAT)). The current generator  230  may generate a reference current (I ref ) based on the V ref . The current to frequency converter  250  may convert the I ref  to a digital pulse train representing a frequency and the digital pulse train may be fed to a counter (not illustrated) to get a temperature reading. 
     The reference voltage generator  210  may include a first pair of parallel transistors (e.g., PMOS)  212 ,  214 , an amplifier  216 , a resistor  218 , and a second pair of parallel transistors (e.g., NMOS)  220 ,  222 . The amplifier  216  is connected to the transistors  212 ,  214  as input and provides feedback (e.g., a voltage) to the transistors  212 ,  214 . The feedback is used to determine the current in the transistors  212 ,  214  and accordingly the current in the transistors  220 ,  222 . The transistor  220  may be much larger than the transistor  222  so that the drain saturation voltage (V dsat ) of the transistor  222  may be roughly equal to the voltage drop across the resistor  218  and the V dsat  of the transistor  220  may be very small (e.g., gate-source voltage (V gs ) of transistor  220  is approximately equal to it&#39;s threshold voltage (V t )). Accordingly, an output (V ref1 )  224  of the reference voltage generator  210  may be approximately V t , such that V ref1≈ V t . The V t  is a linear function with respect to temperature so that the V ref1    224  generated may be linear with respect to temperature as well. 
     The current generator  230  may include a transistor (e.g., PMOS)  232 , a transistor stack (two transistors (e.g., NMOS)  234 ,  236 ), and an amplifier  238 . The amplifier  238  receives the V ref1    224  (from the reference voltage generator  210 ) and an output (V ret2 )  240  of the transistor stack  234 ,  236  as inputs and provides feedback (e.g., a voltage) to the transistor  232 . The feedback is used by the transistor  232  to provide a current to the transistors  234 ,  236 . The current is such that the V ref2    240  may be equal to the V ref1    224 , such that V ref2≈ V ref1≈ V t . The transistor  234  may be much larger than the transistor  236  so that most of the V dsat  of the transistor stack  234 ,  236  is in the transistor  236 , such that V ref2≈ V dsat . 
     As the inputs to the amplifier  238  may be of high impedance, the currents in transistors  232 ,  234 ,  236  may be substantially equal. The current (I) flowing through the transistor  236  may be determined based on the transductance (K′) of the transistor  236  multiplied by the V dsat  of the transistor  236  squared, such that I=K′*(V dsat ) 2 . Based on the configuration of the current generator  230  (e.g., amplifier feedback, size of transistors) the V dsat  of transistor  236  may be equal to V t , such that I=K′V t   2 . The transductance of the transistor is equal to the mobility (u) of the transistor times the gate oxide capaitance per unit area of the channel (C ox (W/L)), such that K′=u C ox (W/L). The mobility is the only parameter utilized in determining the transductance that is dependent on temperature and this dependence is nearly linear. 
     The current to frequency converter  250  may include a transistor (e.g., PMOS)  252 , a capacitor  254 , a transistor  256 , an amplifier  258  and a triggered pulse generator  260 . The feedback from the amplifier  238  of the current generator  230  is provided to the transistor  252 . The current generated by the transistor  252  (I ref ) may be equal to or a multiple of the current generated by the transistor  232  such that I ref= K′V t   2 . The I ref  is used to charge the capacitor  254 . The amplifier  258  receives the V ref1    224  and the charge stored in the capacitor  254  as inputs. When the capacitor  254  is charged to a value equal to the V ref1 , the amplifier  258  trips and triggers the triggered pulse generator  260  to generate a well-defined pulse. The pulse is feedback to the transistor  256  to discharge the capacitor  254 . 
     The pulse is also fed into a counter (not illustrated) to measure the frequency. The frequency (f) will be determined by the transductance times the V t  divided by the capacitance (C), such that f = K′V t /C. Both K′ and V t  are temperature dependent parameters (in the same direction), hence the sensitivity of the circuit. The miniature temperature sensor  200  may be nearly linear because V t  is a linear function with regard to temperature, while K′ is 1/linear function. When the two functions are multiplied, the results are nearly linear. 
     Using analog circuitry and the transistor properties enables the thermal sensor  200  to be small enough that an array of them can be placed on the IC (see  FIG. 1 ). This enables improved thermal management by enabling localized thermal measurements across the die, without area or power penalty, since the sensor is both small and consumes minimal current. Positioning the thermal sensors  200  in an array across the chip insures that there will be sensors  200  very close to the hot spots. 
     The thermal sensors  200  may be calibrated during wafer probing at low temperature (e.g., −25° C.) and during final chip test at high temperature (e.g., 110° C). Doing this enables the operating range and slope of the sensors to be calibrated. For example, if the operating range was from −20° C. to 100° C. and the frequency generated at the end points of the operating range was 70 MHz and 10 MHz respectfully, the slope for a linear relationship would be −0.5 MHz/1° C. (−60 MHz/120° C.). It may only be necessary to calibrate those sensors which are near hot-spots which are characterized at the end of the design or during testing. 
     The sensor may exhibit non-linearity in the middle of the temperature range (away from the calibration points). The non-linearity may be the greatest near the center of the range and decrease as it approaches the calibration points. The thermal sensor may generate a frequency that when used to generate a temperature based on a linear slope does not result in the actual temperature. For example, at 40° C. the frequency generated may be 35 MHz which would correspond to a temperature of 50° C. using the calibrated linear slope described above (a difference of 10° C. between the actual temperature and the temperature measured using the miniature sensor  200 ). 
     Referring back to  FIG. 1 , the non-linearity errors of the thermal sensors  110  may be addressed by determining the ΔT between a remote thermal sensor  110  (located near a hot spot) and the reference sensor  130  which is located near the centralized thermal sensor  120 . It should be noted that the reference sensor  130  and the remote sensor  110  are the same type of sensors (e.g., miniature thermal sensor  200 ) and may exhibit similar non-linearity errors. It is assumed that the centralized thermal sensor  120  gives a very accurate temperature measurement. Utilizing the ΔT to generate a relative temperature reading at the remote sensors  110  causes the temperature error caused by the non-linearity of the sensors to be smaller over a smaller temperature range. The gradients across the IC will be much smaller than the total temperature range for which the thermal sensor operates. Thus when measuring the gradients between the remote thermal sensor  110  and the reference sensor  130 , the percentage error, caused by non-linearity will be small compared to the non-linearity error between the calibration points mentioned earlier. 
     The non-linearity error will be small when the temperature is close to the calibration point, and larger when the temperature is further away from the calibration temperature. Thus, according to one embodiment, in order to prevent the error in ΔT from corrupting the temperature readings provided for the remote sensors  110  the output of the remote sensors  110  may be controlled between the relative temperature and the measured temperature. For example, at temperatures near the calibration points (e.g., −25 to 25° C., 75 to 110° C.) the absolute frequency/temperature reading of the remote sensor  110  may be used, while in the middle of the temperature range (e.g., 25-75° C.), the relative reading may be utilized. The central thermal diode/sensor  120  may be used to control whether the absolute or relative temperature measurement is used. 
     According to one embodiment, the thermal sensors  110  are characterized as a monotonic (albeit non-linear) curve between direct temperature and frequency. The frequency generated by the sensors  110  is converted to a temperature based on this monotonic curve. Accordingly, no further corrections would need to be made to account for non-linearities. The central thermal diode/sensor  120  may not be required in this embodiment. 
     The localized IC thermal sensor (analog thermal sensor array) may be used in any type of IC to monitor temperature of the IC at hot spots. The analog thermal sensor array may be utilized in computer systems. The computer systems may include one or more processors (ICs) to operate the device. The processors may have a single core or multiple cores. The processors may include on die memory, may utilize off die memory, or some combination thereof. The analog thermal sensor array may be used to monitor the temperature of any ICs in the system to ensure the ICs are operated within an appropriate temperature range. 
       FIG. 3  illustrates an example functional diagram of a system  300  utilizing a localized IC thermal sensor (e.g.,  100 ). The system  300  includes a processor (IC)  310  to perform operations, a power supply  320  to provide power to the processor  310 , a cooling system  330 , memory  340 , a user interface  345  and a communications interface  350 . The processor  310  may include active circuitry  360  (core), temperature control circuitry  370  and power throttle circuitry  380 . The temperature control circuitry  370  may determine the temperature of the IC  310  at various points (e.g., hot spots) utilizing an analog thermal sensor array  390  (illustrated in the background). The temperature control circuitry  370  may control activation of the cooling system  330  and the power throttling circuitry  340  based on the analog thermal sensor array  390 . 
     It should be noted that the circuit  200  disclosed in  FIG. 2  has been described with respect to being utilized as a miniaturized thermal sensor (e.g.,  110 ), wherein an array of the miniaturized thermal sensors  110  make up a localized IC thermal sensor (e.g.,  100 ). However, the circuit is not limited to being used as a thermal sensor, being implemented in an array, or being utilized at the IC level to provide a localized IC thermal sensor. Rather, the circuit could be used to measure other parameters could be implemented as a stand alone, and could be utilized at a board or system level without departing from the scope. 
     Although the various embodiments have been illustrated by reference to specific embodiments, it will be apparent that various changes and modifications may be made. Reference to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
     Different implementations may feature different combinations of hardware, firmware, and/or software. It may be possible to implement, for example, some or all components of various embodiments in software and/or firmware as well as hardware, as known in the art. Embodiments may be implemented in numerous types of hardware, software and firmware known in the art, for example, integrated circuits, including ASICs and other types known in the art, printed circuit broads, components, etc. 
     The various embodiments are intended to be protected broadly within the spirit and scope of the appended claims.