Abstract:
An integrated circuit includes secure logic that requires protection. Secure assurance logic protects the secure logic. The secure assurance logic includes a plurality of protection modules that monitor the occurrence of insecure conditions. Each protection module monitors a different type of insecure condition. Each protection module asserts an alarm signal when an associated insecure condition is detected. The alarm signals asserted by the plurality of protection modules are stored.

Description:
RELATED APPLICATION 
     The subject matter of the present patent application is related to the subject matter set out by the same inventor (Mark Leonard Buer) in a copending patent application Ser. No. 09/231,175, filed on the same date as this patent application, for POWER-ON-RESET LOGIC WITH SECURE POWER DOWN CAPABILITY, assigned to the same assignee. 
    
    
     BACKGROUND 
     The present invention concerns security protection within an integrated circuit design and pertains particularly to integration of security modules on an integrated circuit. 
     For some processing applications, it is essential to operate in a secure environment so that operations cannot be probed or altered. In the prior art, various methods have been used to provide for a secure processing environment. 
     For example, a mechanical chassis can be used to house processing equipment. This mechanical chassis can include tamper switches and other elements to detect and protect against tampering and alterations. Unfortunately, such a mechanical chassis can add a significant amount of expense to a product. 
     Alternatively, in order to restrict access to particular integrated circuits, the integrated circuits can be covered with epoxy or other chemical materials to hinder access. Unfortunately, often this can be easily defeated and so provides only a nominal amount of protection. 
     Another method to provide for a secure processing environment is to implement the system on a single integrated circuit. A portion of the integrated circuit, for example, can be used to perform secure operations. However, there may still be attempts to defeat this arrangement. 
     For example, an attacker may attempt to expose information about a security key or information about a security system by applying radiation or alpha particles in the proper location. The excess radiation or alpha particles can result in a single event upset (SEU). The single event upset can affect the data integrity of a secure operation. If the single event upset occurs in an operation related to a security key or data encrypted with the security key, this may weaken the effectiveness of the protection within the integrated circuit and perhaps provide an avenue to break the security system. 
     Other types of attacks can be perpetrated as well. While there have been various types of circuitry added to protect an integrated circuit, these have usually been added on an ad hoc basis. There has been no integrated effort to protect integrated circuits. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiment of the present invention, an integrated circuit includes secure logic that requires protection. Secure assurance logic protects the secure logic. The secure assurance logic includes a plurality of protection modules that monitor the occurrence of insecure conditions. Each protection module monitors a different type of insecure condition. Each protection module asserts an alarm signal when an associated insecure condition is detected. The alarm signals asserted by the plurality of protection modules are stored. 
     In the preferred embodiment, once an alarm signal is asserted, the alarm signal is received by a first register. A second register is used for masking the alarm signals. The masking performed by the second register is used to prevent selected alarm signals from being propagated. This allows certain alarms to be blocked during testing of the secure assurance logic. A third register stores the alarm signals that have been asserted but have not been masked by the second register. The integrated circuit can be reset when an alarm signal is detected. 
     The plurality of protection monitors include, for example one or more of the following: a high frequency monitor that detects when a monitored clock exceeds a predetermined frequency, a low frequency monitor that detects when a monitored clock is less than a predetermined frequency, a single event detector monitor that monitors single event upsets within the integrated circuit, a reset monitor that monitors an amount of times the integrated circuit is reset, and a voltage detector that monitors for invalid voltage levels. 
     In addition, the secure assurance logic generally includes a power-on-reset circuit for resetting the integrated circuit to a known state upon power-up of the integrated circuit. 
     The above-described integration of the protection modules into secure assurance logic requires that someone attacking the security features of the integrated circuit must simultaneously defeat more than one security component. This increases the complexity of the attack required to successfully circumvent the security features. The integrated solution described herein can be used to protect integrated circuits that implement firmware that must access two independent address spaces. The programmable features of secure assurance logic also allows register values to be changed separately such that there is no insecure period of overlap in the operation of the integrated circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram that shows an integrated circuit within which secure assurance logic is used to guard protected logic in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a simplified block diagram that shows secure assurance logic that links together electrical protection within an integrated circuit in accordance with a preferred embodiment of the present invention. 
     FIG. 3 is a simplified block diagram that shows additional secure assurance logic that links together electrical protection within an integrated circuit in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a simplified block diagram that shows structure of a status register shown in FIG. 1 in accordance with a preferred embodiment of the present invention. 
     FIG. 5 is a simplified block diagram of power-on-reset logic in accordance with a preferred embodiment of the present invention. 
     FIG. 6 is a simplified block diagram of a mode of a power-on-reset cell. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified block diagram of an integrated circuit  11  that includes a processor  123 , protected logic  112  and other logic  140 . Secure assurance logic  12  links together electrical protection within integrated circuit  11 . The integration of all secure assurance logic  12  components into a single modular protected block allows processor  123  to control and observe the protection modules for testing without affecting the security of integrated circuit  11 . By coupling multiple protection modules together, the security of the system is improved since the protection modules now work together. Additionally reset states for the protection modules can be controlled such that power down for IDDq testing does not defeat the security of the integrated circuit. 
     FIG. 2 is a simplified block diagram of secure assurance logic  12 . A monitored clock signal (MON_CLK) is placed on a monitor clock signal line  41 . Components within secure assurance logic  12  monitor and protect the monitored clock. A mask register  29  is used to allow masking of particular security assurance features so that individual security features (i.e., alarm signals) can be turned off for certain operations via processor  123 . This also allows each of the features to be tested. Since there are duplicated security features in integrated circuit  11 , the assurance logic can be tested at startup without causing a problem and without allowing an opening for an attacker. 
     Secure assurance logic  12  includes a high frequency monitor  21 , a low frequency monitor  22 , a single event upset (SEU) detect monitor  24 , a reset monitor  25 , and an over/under voltage monitor  26 . 
     Alarms from the monitoring modules are integrated using a raw register  28 , mask register  29  and a status register  30 . Processor  123 , using a bus  45  can read and write information to raw register  28 , mask register  29  and status register  30 . 
     Raw alarm sources are trapped asynchronously in raw register  28 . The alarms are output on alarm lines  54 . The alarms are masked by values in mask register  29  prior to being forwarded to status register  30 . The masked alarms are forwarded to status registers  30  on lines  55 . The asynchronous trap is required since a valid clock source cannot be guaranteed at the time of the alarm. The asynchronous trap is double synchronized to the clock domain of processor  123  and then sampled by status register  30 . Once a bit in the status register is set, the corresponding asynchronous trap is cleared. An alarm set in status register  30  results in a reset signal being placed on a line  57 . 
     FIG. 4 shows an example of logic used within status register  30  for double synchronization of an alarm bit. A first delay (D) flip-flop  91  and a second D flip-flop  92  are connected in series. The D input of D flip-flop  91  is connected to VDD through a line  94 . The clock input of D flip-flop  91  receives from mask register  29  a masked alarm bit on a line  95 . A clock input  97  of D flip-flop  92  receives a system clock (CLK) signal. The Q output of D flip-flop  92  places a status bit on line  98 . The system clock (CLK) signal is used to clock all the synchronization blocks and register of secure assurance logic  12 . A reset signal on a line  40 , from synchronization block  31  (shown in FIG.  2 ), is used to reset D flip-flop  92 . A logic NOR gate  93  is used to generate a reset for D flip-flop  91 . 
     As shown in FIG. 2, a power-on-reset cell  27 , a software reset placed on a line  42  and a reset (PINRST_L) line  43  all are used to reset the monitoring system. The reset signals are synchronized by a synchronization block  31 , a synchronization block  32  and a synchronization block  33 , connected as shown. A logic OR gate  34  and a logic OR gate  35  combine the reset signals to form the reset (RSROUT_L) on line  58 . A hold reset block  36  holds the resets for a predetermined number of clock cycles and generates a reset delete (RSTDEL_L) signal on a reset delete line  59 . 
     A soft reset synchronizer  64  (shown in FIG. 3) generates the software reset placed on line  42 . Control register  63  (shown in FIG. 3) stores two control bits for soft reset synchronizer  64  that are forwarded to soft reset synchronizer  64  on a line  79  and a line  80 . 
     Low frequency monitor  22  uses a reference clock that is generated by an internal ring oscillator (ROSC)  23  to make comparisons to the monitored clock signal (MON_CLK) on monitor clock signal line  41 . Ring oscillator  23  must be used since low frequency monitor  22  needs to be able to detect when the monitored clock signal has stopped. Low frequency monitor  22  places a low frequency alarm on line  47 . 
     Since the monitored clock is sampled, low frequency monitor  22  may produce a false error at any input clock frequency that violates the Nyquist rate. The monitored clock must be less than eight times the frequency of the reference clock generated by the ring oscillator. An eight-bit low frequency value stored in a register  62  (shown in FIG. 3) is used to configure low frequency monitor  22 . 
     High frequency monitor  21  is used to ensure the monitored clock signal (MON_CLK) on monitor clock signal line  41  is below the maximum frequency of the device. Since high frequency monitor  21  uses the critical path of the device to determine if the clock frequency is too fast, the result is an operating point detection that includes monitoring the voltage, process, and temperature. 
     The high frequency limit is not fixed, but is determined by the capability of the device. In a system where there is a best case processed device at zero degrees with nominal voltage, the frequency of the device is allowed to be higher than when operation occurs at high temperature and lower voltage. Since the operating point is programmable, high frequency monitor  21  can be used to adjust a phased locked loop (PLL) frequency for the current operating environment. High frequency monitor  21  places a high frequency alarm on a line  46 . 
     An eight-bit high strike value and an eight-bit high frequency value stored in a register  61  (shown in FIG. 3) are used to configure high frequency monitor  21 . 
     Reset monitor  25  counts the number of resets (reset out signal (RSROUT_L) on reset out line  58 ) that are issued. Once the reset limit has been reached, reset monitor  25  issues an alarm on a line  51 . Reset monitor  25  can be cleared using a clear strike bit in a control register  63  (shown in FIG.  3 ). Software is used to clear reset monitor  25  after completing the boot processing for the integrated circuit and waiting a random period of time. 
     The alarm from reset monitor  25  is used to issue a device reset (if unmasked in mask register  29 ). After reset, further manual resets (PINRST_L) issued on a line  43  are ignored until this alarm has been cleared in status register  30 . Control register  63  (shown in FIG. 3) stores a clear bit for reset monitor  25  that is forwarded to reset monitor  25  on a line  78 . 
     Over and under volt age detectors  26  are used to protect against a voltage level that is not valid for the process technology. The minimum operating core volt age and the maximum operating core voltage for the process are us ed as boundaries for the voltage detection. 
     The voltage ranges specified for the activation ranges of the detectors are based on the process technology in which the integrated circuit is implemented. Below the under voltage activation range, an under voltage error will always be detected. Above the power-on-reset (POR) activation range, power-on-reset cell  27  will always be inactive (not asserted). Above the over voltage detection (OVD) activation range, the over-voltage detector will generate an alarm. Below the over-voltage detector activation range, the over-voltage detector will not generate an alarm. 
     In many process technologies the under voltage activation range and the POR activation range overlap. In this case, the under voltage detector is not implemented (i.e., it is tied off in design). Whenever, the under voltage detector is not implemented, the POR activation range should be set based on the VDD−10% requirement instead of VDD minimum to ensure that the under voltage violations will cause a POR reset. 
     In the preferred embodiment the under voltage detection portion of over and under voltage detectors  26  monitors the core VDD voltage supply to ensure that the voltage level never drops below the minimum required core voltage for the given process. An error must not be detected when VDD is within the valid operating range of the device which is core VDD+/−10%. 
     In the preferred embodiment the under voltage detection portion of over and under voltage detectors  26  has a power down (PD) input  81  that can be used for IDDq testing. The output state of the device should not indicate an error when powered down regardless of the core VDD value. The under voltage design also rejects noise on the power supply line. The under voltage detection portion of over and under voltage detectors  26  is capable of being routed with the digital logic and does not cause errors due to digital switching noise. 
     The over voltage detection portion of over and under voltage detectors  26  monitors the core VDD supply to ensure that the voltage level never rises above the maximum allowed core voltage for the given process. Additionally, an error must not be detected when VDD is within the valid operating range of the device which is core VDD+/−10%. 
     The over voltage detection portion of over and under voltage detectors  26  also has a power down (PD) input  82  that can be used for IDDq testing. The output state does not indicate an error when powered down regardless of the core VDD value. The over voltage detection portion of over and under voltage detectors  26  is constructed in such a way to reject noise on the power supply line. 
     Single event upset (SEU) detect monitor  24  tracks the logic operation and detects single or multiple bit errors within 256 clock periods. In the preferred embodiment the detection logic is fully digital logic implemented within integrated circuit  11 . When a single event upset is detected, single event upset detect monitor  24  places an alarm on alarm line  49  or alarm line  50 . Control register  63  stores two error bits (r_err and p_err) for single event upset detect monitor  24  that are forwarded to single event upset detect monitor  24  on a line  76  and a line  77 . 
     FIG. 1 shows use of single event upset detectors scattered throughout integrated circuit  11 . Some of the single event upset detectors are placed near protected logic  112  (e.g., cryptographic logic) that is being protected. Additional single event upset detectors are placed as far away from the protected logic as possible to be used for comparison. The number of single event upset detectors used is determined by the overall gate count of the integrated circuit  11 . A good rule of thumb is about one counter for every 15 k-20 k gates of cell based logic (do not include RAM or ROM in this calculation) in the design per logic block within integrated circuit  11 . 
     The single event upset detectors are represented in FIG. 1 by a single event upset detector  114 , a single event upset detector  115 , a single event upset detector  116 , a single event upset detector  117 , a single event upset detector  118 , a single event upset detector  119 , a single event upset detector  120 , a single event upset detector  121 , and a single even upset detector  122 . FIG. 1 is not drawn to scale. The single event upset detectors are scattered throughout secure integrated circuit  11  in order to detect events that occur even in seemingly non-critical areas of secure integrated circuit  11 . 
     Single event upset detect monitor  24  (shown in FIG. 2) within secure assurance logic  12  collects single event upset detection information from single event upset detector  114  via a data path  124 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  115  via a data path  125 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  116  via a data path  126 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  117  via a data path  127 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  118  via a data path  128 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  119  via a data path  129 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event up set detection information from single event upset detector  120  via a data path  130 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  121  via a data path  131 . Single event upset detect monitor  24  within secure assurance logic  12  collects single event upset detection information from single event upset detector  122  via a data path  132 . 
     Each of single event upset detectors  114  through  122  uses digital logic to detect single event upsets. For example bit registers, each composed of a flip-flop, within single event upset detectors are monitored for a state transition due to single event upset . The flip-flops are utilized in a predetermined pattern and then are monitored for errors that occur during operation. An error in the state transition can indicate, for example, a single event upset caused by radiation, alpha particles or some other operation error. 
     A power-on-reset cell  27  provides a known state for the device when power is applied. Initializing to a known state is critical to a secure integrated circuit. Power-on-reset cell  27  uses the application of power to generate a reset output that is not released until the power has stabilized. 
     The integrity of power-on-reset cell  27  is imperative for the security of integrated circuit  11 . Current test strategies require that all elements of an integrated circuit should be powered down into a low power state for IDDq testing. Since the initial state of any element on the device is only trusted when power-on-reset cell  27  is active, there is an external pin connected to a power down line  44  used to power down power-on-reset cell  27 . The external pin cannot be trusted by secure assurance logic  12  such that it would bypass the power-on-reset cell  27  and result in an unknown state after power up. 
     Ring oscillator (ROSC)  23  provides the reference clock for low frequency monitor  22 . In addition ring oscillator  23  provides an oscillator clock (OSC CLK) on an output line  48 . The oscillator clock is used for active zeroization. Ring oscillator  23  is fully tested via scan test modes by breaking the ring oscillator chain and inserting flip-flops that make the chain observable and controllable. 
     The frequency value of the ring oscillator  23  can be tuned after layout using (OSC_CFG) input pins to tune which tap point is used for the end of the ring oscillator  23 . These configuration inputs must be fixed for the final layout of the device. Control register  63  (shown in FIG. 3) stores a disable bit for ring oscillator  23  that is forwarded to ring oscillator  23  on a line  75 . 
     Pin reset line (PINRST_L)  43  is the external reset from the input pad of the integrated circuit. Since the pin reset (PINRST_L) is controlled outside the integrated circuit, it cannot be trusted. The pin reset is assumed to be asynchronous by the integrated circuit. The external reset must be asserted for at least one clock cycle to ensure that synchronization logic  32  captures the reset. Synchronization logic  32  provides double synchronization for meta-stability. The pin reset (external reset) will not be propagated to the core as a reset for two clock periods when asserted. 
     Secure assurance logic  12  can be used to implement active zeroization in a security device. Secure assurance logic  12  supplies a guaranteed valid clock at all times by coupling the high and low frequency monitor. Whenever a high or low frequency error is detected, enabled and trapped, the switch clock (SWTICLK) signal is active on line  47 . The SWTICLK signal can be used to multiplex the OSC_CLK signal on output line  48  with the system clock (prior to the clock tree). Since a device reset is issued based on the trapped violation, there is no requirement that this switch over be glitch free. Since the current clock may indeed be stopped, in the preferred embodiment this connection is made directly with a multiplexer. Once the switch over has occurred to the oscillator clock signal on output line  48 , processor  123  (or other hardware logic) can make decisions about active zeroization based on the status register bits within status register  30  or the alarm output. 
     FIG. 5 is a simplified block diagram which explains operation power-on-reset logic. A power-on-reset signal on a line  153  is generated by a power-on-reset cell  154  (equivalent to power-on-reset cell  27  shown in FIG.  1 ), a delay (D) flip-flop  156  (equivalent to sync  33  shown in FIG. 2) or a low frequency monitor  157  (equivalent to low frequency monitor  22  shown in FIG.  2 ). The reset signals are collected by a logic OR gate  155 . A power down (PD) signal is placed on a line  151 . D flip-flop  156  is clocked by the system clock (CLK) placed on a line  152 . Low frequency monitor  157  monitors the frequency of the system clock (CLK). 
     When power is applied to integrated circuit  11 , power-on reset cell  154  is powered down and integrated circuit  11  remains in a constant state (no reset is issued). 
     If an attacker were to attempt to bypass power-on-reset cell  154  by asserting the power down (PD) signal on line  151 , D flip-flop  156  assures that the system clock (CLK) cannot be used without causing a reset of the integrated circuit. The reset causes integrated circuit  11  to be initialized to a known state. Low frequency monitor  157  prevents the system clock (CLK) from being stopped. That is, if the power down (PD) signal, generated by a power down pin connected to line  151 , is asserted before integrated circuit  11  goes into a test mode (where low frequency monitor  157  is disable), then integrated circuit  11  will go into a reset state if the system clock (CLK) is stopped. The result is that power-on-reset logic  154  cannot be bypassed effectively. 
     FIG. 6 is a simplified block diagram that models an implementation of power-on-reset cell  154 . Power-on-reset cell  154  is implemented as a custom cell for a particular process technology. Power-on-reset cell  154  is constructed in such a way to reject noise on the power supply line. 
     A reset (POR_L) output  158  of power-on-reset cell  154  must stay low for a minimum of 20 microseconds after power has stabilized. The stabilized value of the power rail must be a voltage level that is greater than the implemented process technology minimum operating voltage. 
     Power down (PD) input  151  to power-on-reset cell  154  must not change the state of reset output  158 . If power down input  151  is held high while power is applied, reset output  158  never releases the reset. If power down input  151  is asserted high after the power has been applied and the reset pulse has completed (greater than 20 microseconds after power is applied) then reset output  158  must remain high and not issue a reset. 
     FIG. 6 shows power-on-reset cell  154  modeled as an RC network consisting of a resistor  164  and a capacitor  165  connected to ground through line  166 . VDD is placed on an input line  161 . A buffer  162  forwards VDD to a switch  163  controlled by power down input  151 . When power down input  151  is inactive (low) then capacitor  165  is allowed to charge. When the power down input  151  is asserted (high) capacitor  165  is not allowed to charge. Capacitor  165  is used to drive reset output  158  through a Schmitt trigger  167 . Thus once capacitor  165  reaches the threshold (a time constant equal to 20 microseconds), the reset output  158  is released. While in FIG. 6, power-on-reset cell  154  is modeled as an RC network, actual implementation of the power-on-reset cell is dependent upon the technology utilized. 
     The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.