Abstract:
An electronic system is provided that includes a plurality of power consuming electronic circuits and a power management system that interfaces a power supply to the plurality of power consuming electronic circuits. The power management system includes a power level detect circuit that includes a voltage level detector circuit that receives an analog voltage level signal indicative of a level of voltage provided from the power supply. The power level detect circuitry also includes digital encoding circuitry that encodes the analog voltage level signal as a digital powered level signal also indicative of the level of the power supply voltage. Each of the power consuming electronic circuit includes configuration circuitry to receive the digital power level signal and to configure operation of that particular power consuming electronic circuit responsive to the digital power level signal.

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. 120 from application Ser. No. 08/451,206, filed May 26, 1995, now U.S. Pat. No. 5,805,923, issued Sep. 8, 1998. This application is also a divisional of U.S. Pat. No. 5,805,923, issued Sep. 8, 1998, under 35 U.S.C. 121. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power management systems, and more particularly, to a configurable power management system. 
     2. Description of the Related Art 
     Previous power management systems for use with integrated circuit (IC) chips have been limited in their ability to be configured. Thus, there is a need for a power management system which is configurable. 
     SUMMARY OF THE INVENTION 
     The present invention also provides a power level detect circuit for use in a power management system. An analog voltage-level detector interface has a programmable override function for providing a digitally encoded voltage level as an output which is used for global configuration. An input receives an analog enable signal to turn on a DC-current source of an external voltage-level detector and a read strobe. A voltage-level detector input is sampled. 
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B represent a block diagram illustrating a power management system in accordance with the present invention. 
     FIG. 2 is a block diagram illustrating a system which incorporates the power management system shown in FIG.  1 . 
     FIGS. 3A and 3B represent a schematic diagram illustrating the configuration unit shown in FIG.  1 . 
     FIG. 4 is a schematic diagram illustrating the external oscillator interface shown in FIG.  1 . 
     FIG. 5A is a schematic diagram illustrating the powergood qualification block shown in FIG.  1 . 
     FIG. 5B is a state diagram illustrating the operation of the powergood qualification schematic shown in FIG.  5 A. 
     FIG. 6 is a schematic diagram illustrating the pad clock and self test block shown in FIG.  1 . 
     FIGS. 7A and 7B are schematic diagrams illustrating the clock enable block shown in FIG.  1 . 
     FIGS. 8 and 9 are schematic diagrams illustrating the power level detect block shown in FIG.  1 . 
     FIG. 10 is a schematic diagram illustrating the internal source clock generation block shown in FIG.  1 . 
     FIG. 11 is a schematic diagram illustrating the power-save mode change detection block shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is illustrated a power management system  30  in accordance with the present invention. Referring to FIG. 2, the power management system  30  is ideal for being implemented in the system  32 . The system  32  is described in the data sheet entitled “Elentari Optimized 32-bit 486-class Controller With On-chip Peripherals for Embedded Systems”, authored by National Semiconductor Corporation of Santa Clara, Calif., and is incorporated herein by reference. The system  32  includes a CPU  34 , a DMA controller  36 , a DRAM memory controller  38 , a PCMCIA controller  40 , a bus interface unit (BIU)  42 , an ECP parallel port  44 , an LCD controller  46 , as well as other components. Although the power management system  30  is ideal for incorporation into the system  32 , it should be well understood that such incorporation is not a requirement of the present invention and that the teachings of the present invention may be applied to smaller (or larger) stand-alone applications. Also a document entitled “Elentari Core Internal Bus Spec” authored by National Semiconductor Corporation of Santa Clara, Calif., is also incorporated herein by reference. Finally, a document entitled “Internal Peripheral Bus Signals” authored by National Semiconductor Corporation of Santa Clara, Calif., is also incorporated herein by reference. 
     The power management system  30  includes a number of power saving mechanisms that can be combined to achieve comprehensive power savings under a variety of system conditions. First of all, the core processor  34  power consumption can be controlled by varying the processor/system clock frequency. The internal CPU clock can be divided by 4, 8, 16, 32 or 64. In addition, in idle mode, the internal processor clock will be disabled. Finally, if an crystal oscillator circuit or external oscillator is being used, it can be disabled. For maximum power savings, all internal clocks can be disabled (even the real-time clock oscillator). 
     Some peripherals, notably the timer  48  and the PCMCIA interface  40  can be switched between a fixed frequency (external oscillator/2) and the CPU clock. When the CPU clock is being divided, this can reduce their power consumption. Note that the clocks for other on-board peripherals can be individually or globally controlled. By setting bits in the power management control registers (discussed below), the internal clocks to the DMA controller  36 , the ECP port  44 , the three-wire interface  50 , the timer  48 , the LCD controller  46 , the DRAM controller  38 , the PCMCIA controller  40  and the UART  52  can be disabled. In addition, the power management system  30  can programmed the of use CMOS level I/Os or TTL level I/O settings in the system  32 . Finally, the external SYSCLK can be disabled via a bit in the Power Management Control Register. 
     The power management system  30  includes several modes of operation which are listed here in decreasing power consumption order (i.e., full power to least power). In the Normal Mode all clocks are at fill speed, with the Timer, PCMCIA, SYSCLK connected to cpu_dock or external OSC/2, and the UART, DRAM refresh logic, and LCD Controller connected to OSC dock, and the RTC connected to RTC_osc. In the Power Save Mode, first, the CPU clock is divided by 4, 8, 16, 32 or 64, with the Timer, PCMCIA, SYSCLK connected to external OSC/2, the UART, DRAM logic, and LCD Controller connected to OSC, and the RTC connected to RTC_osc. Second, the CPU clock is divided by 4, 8, 16, 32 or 64 with the Timer, PCMCIA, SYSCLK connected to cpu_clk, UART, DRAM logic, LCD Controller connected to OSC, and RTC connected to RTC_osc. In the Peripheral Power Down Mode the individual Peripherals can be disabled. In the Idle Mode the CPU clock is disabled with all peripherals unaffected, and RTC connected to RTC_osc. In the Crystal Oscillator Circuit Disable/Power Down Mode, first, if a crystal oscillator circuit is being used to drive the system  32 , this mode will disable the oscillator circuit (NOTE: after being turned back on, it will take approximately 1 msec for the external crystal to stabilize). Second, if an external oscillator is being used, Power Down mode will disable all the system  32  clocks except for the RTC_osc. (NOTE: This is much faster upon recovery, as there is no stabilization delay). It should be noted that the RTC oscillator is always enabled even during power down mode; it can be disabled if desired. 
     The following indicates what peripherals are connected to which clocks and how those clock can be disabled/enabled. The CPU  34 : 1) Uses cpu_clk (Full speed clock=OSC_CLK/2); 2) cpu_clk can be divided by 4, 8, 16, 32 or 64; 3) In Idle mode, the clock is disabled. The Timer  48 , PCMCIA  40 , SYSCLK: 1) uses cpu_clk (full speed or divided by 4 8, 16, 32 or 64); 2) or can use external OSC_CLK/2 (when cpu_clk is divided); 3) can be individually disabled. The UART  52 , DRAM refresh logic, LCD Controller  46 : 1) Connected to OSC; 2) can be individually disabled. 
     The ECP  44  and the Three-wire Serial Interface  50 : 1) Connected to OSC_CLK/2; 2) can be individually disabled. The DMA Controller  36  and Bus Interface Unit  42 : 1) Uses cpu_clk (full speed or divided). The DRAM Controller  38 : 1) Must use OSC_CLK for DRAM refresh cycles; 2) Sequencer can selectably use cpu_clk or 2*cpu_clk; 3) For state machine logic, must use cpu_clk. The Real-Time Clock: 1) Uses RTC_ose—typically always enabled, but it can be disabled through the RTC interface. The Global Peripheral Clock Disable/Enable: 1) Controls DMA Controller, ECP, Three-wire Interface, and UART. 
     The power management system  30  includes several power management modes. Power saving features include the following. In Idle Mode the internal clock to the CPU  34  will be disabled. All enabled peripheral blocks will continue to operate. Any interrupt or reset will re-enable the internal clock to the CPU  34 . It should be noted that when the CPU  34  is in Idle Mode, the instruction cache cannot snoop. Normally, the cache will snoop the addresses to see if a cache address is being updated. If so, it flushes the cache. Therefore, the user&#39;s can take the appropriate action when the CPU  34  is idled. Also, when the CPU  34  is in Idle Mode, the BIU  42  is designed to mimic the CPU  34  during DMA interchanges between memory and peripherals. By responding to DRQs and generating DACKs, HOLDs and HOLDAs signals as required, the BIU  42  eliminates the need to reactivate the CPU  34  during such transfers as screen updates from memory to the LCD controller  46 . This gives the designer added flexibility in conserving power while maintaining basic system functions. 
     A Power-save Mode reduces the internal CPU  34 /system clock&#39;s frequency by dividing the internal CPU clock by 4, 8, 16, 32 or 64 (Refer to Power Management Register  1  for more information) The internal clocks for the UART  52 , DRAM refresh logic, LCD Controller  46  and RTC will be unaffected in this mode. The Timer, PCMCIA and SYSCLK all have selectable clock sources between a fixed frequency, which is the external oscillator/2 and cpu_clk. Only when a cpu_clk source is selected will these clocks be affected by Power-save mode. 
     The Crystal Oscillator Circuit Disable function disables the feedback output of the crystal oscillator circuit (i.e. forces OSCX 2  low). Normally, the feedback output is used to provide a high-gain feedback to an external crystal to start, stabilize, and maintain a reference oscillation from the crystal. If the feedback is disabled the oscillation will stop. After the feedback output is re-enabled, it takes approximately 1 msec for the external crystal to start and stabilize. On-chip, there is a lowpass filter and counter to insure that none of the start-up and stabilize oscillations are allowed to pass into the rest of the chip. If an external TTL or CMOS oscillator is used then the feedback output can be disabled to save power. Also, the low-pass filter and counter can be bypassed by setting bit  7  of Power Management Configuration Register  4 . This latter action may be useful when an external TTL or CMOS oscillator is used. 
     In the Power Down Mode all of the internal system  32  clocks except the RTC oscillator will be disabled. If a crystal is used to generate the CPU clock, the CPU Oscillator Circuit Disable feature may be used to turn off the clock instead of this mode. If an external oscillator drives CPUX 1 , then this mode should be used to turn off the system  32  internal clocks. It is important that power be applied to and removed from the LCD display in proper sequence, otherwise damage can result. To prevent damage to the LCD panels, the external DC power supplied to the LCD Display (VEE) should be disabled before the LCD Controller&#39;s clock is disabled. 
     The power-up sequence is as follows: 1) Configure the LCD control registers; 2) Apply VDD (5V or 3V) to the display; 3) Enable the LCD clock from the power management registers—this must be done within 20 msec of applying VDD; 4) Enable the LCD controller. 5) Within 20 msec. max after applying the LCD clock, apply VEE (22V/−26V) to the display. The power-down sequence is as follows: 1) Remove VEE from the display; 2) Disable the LCD controller; 3) Within 20 msec. of removing VEE, disable the LCD clock; 4) Within 20 msec. of removing the LCD clock, remove VDD from the display. The LCD clock should never be disabled when the LCD is enabled. 
     The internal clocks for various internal peripherals may be individually enabled/disabled via bits of Power Management Registers  2  and  3  (discussed below). A peripheral&#39;s internal clock should only be disabled if that internal peripheral is not to be used. 
     With respect to global enable/disable of peripheral clocks, when bit  7  of Power Management Register  2  is set to a one, the internal clocks to the DMA Controller  36 , ECP  44 , Three-Wire Interface  50 , and UART logic  52  will all be disabled. When that bit is a zero, the individual peripheral clock enable/disable bits will determine if the individual peripheral clocks are enabled or not. The DRAM  38  and LCD Controllers  46 , PCMCIA  40 , BIU  42  and Timer  48  are not affected by global clock enabling/disabling. 
     The system  32  I/Os are power supply-level configurable. The power management system  30  controls voltage sensing and setting for I/O supply-level configuration. The power management system  30  has the capability to set the operating voltage through firmware (Bit  5  of Power Management Register  4 ). 
     As mentioned above, the power management system  30  includes several Power Management Configuration Registers. The Configuration Register Unit (CRU)  54  contains the configuration registers for controlling the operation of the power management system  30 . The CRU  54  also contains a peripheral bus interface for reading and writing of the configuration registers. 
     Referring to FIG. 3, Power Management Register One  56  is a read/write register and has an I/O map address of EF 90 h. The bit assignments are as follows. Bit  7  is reserved. Bit  6  is the Idle Mode selection bit IDLE. A “1” sets the chip in Idle Mode (cpu_clk disabled). All resets and interrupts force this bit to a “0”. Bit  6  is the oscillator disable bit COSCD-PU (used with crystal oscillator). A “1” disables the CPU oscillator. All resets and interrupts force this bit to a “0”. Bit 4 is a Power-down Mode selection bit PDM (used with external OSC). A “1” sets the chip to Power-down Mode. All resets and interrupts force this bit to a “0”. Bit  3  is Power-save Mode selection bit PSVM (divides cpu_clk). A “1” sets the chip to the Power-save Mode. All resets force this bit to a “0”. Bits  2 - 0  are Power-save Mode clock division bits SVB[ 2 : 0 ]. All resets force these bits to a “0”. Table A illustrates the operation of these bits. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE A 
               
               
                   
                   
               
               
                   
                 SVB[2] 
                 SVB[1] 
                 SVB[0] 
                 Divide By 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 0 
                 0 
                 1 
               
               
                   
                 0 
                 0 
                 1 
                 4 
               
               
                   
                 0 
                 1 
                 0 
                 8 
               
               
                   
                 0 
                 1 
                 1 
                 16 
               
               
                   
                 1 
                 0 
                 0 
                 32 
               
               
                   
                 1 
                 0 
                 1 
                 64 
               
               
                   
                 1 
                 1 
                 X 
                 reserved 
               
               
                   
                   
               
             
          
         
       
     
     Power Management Register Two  58  is a read/write register and has an I/O map address of EF 91 h. The bit assignments are as follows. Bit  7  is a Global peripheral clock disabling selection bit GDIS. A “1” causes global peripheral clock disabling. All resets force this bit to a “0”. Bit  6  is ECP clock disable selection bit ECP. A “1” disables the ECP clock. All resets force this bit to a “0”. Bit  5  is an LCD clock disable selection bit LCD. A “1” disables the LCD clock. All resets force this bit to a “0”. The LCD Controller  46  is not affected by global clock enabling/disabling (GDIS, bit  7 ). Bit  4  is a DMA clock disabling selection bit DMA. A “1” disables the DMA clock. All resets force this bit to a “0”. Bit  3  is a timer block clock disabling selection bit TIMR. A “1” disables the Timer Clock. All resets force this bit to a “0”. The timer is not affected by global clock enabling/disabling (GDIS, bit  7 ). Bit  2  is a three-wire block clock disabling selection bit TWIR. A “1” disables the Three-wire Clock. All resets force this bit -to a “0”. Bit  1  is a DRAM block clock disabling selection bit DRAM. A “1” disables the DRAM Clock. All resets force this bit to a “0”. The DRAM controller  38  is not affected by global clock enabling/disabling (GDIS. bit  7 ). Bit  0  is a UART block clock disabling bit UART. A “1” disables the UART Clock. All resets force this bit to a “0”. 
     Power Management Register Three  60  is a read/write register and has an I/O map address of EF92h. The bit assignments are as follows. Bit  7  is reserved. Bit  6  is an External Driver Configuration bit DRVCON for system bus and DRAM interface I/Os. This bit only has an affect when the interface SETV bit is set to a “1”. A “1” guarantees CMOS level output voltages/drive. A “0” guarantees TTL level output voltage/drive (low noise I/O configuration). Bit  5  is a PCMCIA Clock reference Selection bit PCS. A “1” corresponds to Cpu_clk clock reference (affected by Power Save Mode), and a “0” corresponds to standard clock reference (not affected by Power Save Mode). All resets force this bit to a “0”. Bit  4  is a PCMCIA block clock disabling selection bit PCMCIA. A “1” disables the PCMCIA clock. All resets force this bit to a “0”. Bit  3  is a Timer Clock reference Selection bit TCS. A “1” corresponds to Cpu_clk clock reference (affected by Power Save Mode), and a “0” corresponds to standard clock reference (not affected by Power Save Mode). All resets force this bit to a “0”. Bit 2 is a SYSCLK clock disabling selection bit SYSCLK. A “1” disables the SYSCLK. Only PWRGOOD reset forces this bit to a “0”. Bit  1  is a SYSCLK reference Selection bit SCS. A “1” corresponds to Cpu_clk clock reference (affected by Power Save Mode), and a “0” corresponds to standard clock reference (not affected by Power Save Mode). Only PWRGOOD reset forces this bit to a “0”. Bit  0  is a DRAM sequencer clock frequency mode bit SEQU. A “1” sets the same frequency as the Cpu_clk clock reference, and a “0” doubles the frequency of the Cpu_clk clock reference. Only PWRGOOD reset forces this bit to a “0”. 
     Power Management Register Four  62  is a read/write register and has an I/O map address of EF93h. The bit assignments are as follows. Bit  7  is an external clock source description bit CAN_OSC. A “1” corresponds to a CMOS or TTL oscillator, and a “0” corresponds to a crystal oscillator. Only PWRGOOD reset forces this bit to a “0”. Bit  6  is reserved. Bit  5  is a software setting of Operating Voltage bit SETV. A “1” sets 5V operating voltage, and a “0” sets 3.3V operating voltage (default). Only PWRGOOD reset forces this bit to a “0”. Bits  4 - 0  are reserved. 
     The power management system 30 includes seven other major partitions. Referring to FIG. 4, the External Oscillator Interface (EOI)  64  contains the circuitry that interfaces with an external oscillator. The external oscillator may be a crystal or a can. The circuitry is responsible for controlling the feedback loop of the analog interface to the external crystal. When the feedback look is enabled, the external crystal is forced to oscillate, when disabled the external crystal can not oscillate. If a can oscillator is used the feedback control does not affect the operation of the external oscillator, and thus an oscillation will pass into the external interface circuitry whether or not the feedback look is enabled. Feedback disabling may be reset to an enabled state by any one of the following ways: powering-down and back-up, sending out a watchdog reset (the watchdog timer is driven by a separate clock coming from the real-time clock), and a maskable or non-maskable interrupt. The feedback loop is enabled by programming a one in the COSCD bit in configuration register one  56  to a “1”. Circuitry is used to guarantee that the clock is disabled after a falling edge (Oscillator Disable Mode). 
     The EOI  64  also contains a clock stabilization filter for masking out spurious crystal frequencies during its start-up following the enabling of the feedback loop. The filter is used when crystals are the source of oscillations; otherwise, when an external can oscillator is used (programmed as the clock source), the filter is bypassed. The bypassing is controlled by programming the CAN_OSC bit in configuration register four  62  to a “1”. Circuitry is used to guarantee that clock will come up after filtering, starting with a rising transition, without any logic-generated spurious glitches. 
     The EOI  64  contains a circuit that masks the incoming clock from the rest of the power management block as well as the rest of the chip (independent of the previously described functionality). The circuit allows an external frequency to come into the part but stay isolated within the EOI  54 . The clock masking is enabled by programming a one in the PDM bit in configuration register one  56  to a “1”. General clock masking may be reset to an enabled state by any one of the following ways: powering-down and back-up, sending out a watchdog reset (the watchdog timer is driven by a separate clock coming from the real-time clock), and a maskable or non-maskable interrupt. Circuitry is used to guarantee that the clock masking is disabled after a falling edge and starts back up with a rising transition, without any logic-generated spurious glitches (Power Down Mode). 
     Referring to FIGS. 5A and 5B, the Powergood Qualification (PQ) block  66  contains a detection mechanism for sensing a minimum PWRGOOD disable pulse. The detector will detect a PWRGOOD disable pulse independent of whether or not a clock is present. When a PWRGOOD disable occurs, a power recycle signal is immediately generated and held. The PQ block contains a state machine that guarantees that a power recycle indication is held for at least 2 clock cycles. This minimum duration of time is adequate to insure that the power recycle can be processed in other design blocks, such as for example, in asserting a power-up reset. The signal powergood_int is an asynchronous reset that forces the state machine back to state 00 when asserted. 
     Referring to FIG. 6, the Pad Clock and Self Test (PCST) block  68  provides control of the Pad_clk which is an output buffer to the external world. In standard operation the PCST block will provide one of the following three configurable conditions. Type 1) a clock whose frequency is constantly one-half that of the external oscillator; Type 2) a clock whose frequency is “generally” one-half that of the external oscillator but is forced equal to a programmable fraction of this frequency when in power-save mode; or, Type 3) disabled low. The Pad_clk may be brought in and out of disabling into the previous modes without glitching similar to methods used in the CEB. The PCST has two non-standard operating modes which are the In-circuit emulator mode and the test mode. During In-circuit emulator mode operation as determined by the Icemode signal being active, the Pad_clk&#39;s output is designed to be closely in-phase with the clock generated for the embedded CPU. In test mode as determined by the Test signal being active the PCST is configured to allow observability of internal states of the power management block and force known logic levels on the Pad_clk port. When in test mode and the Test_lvl_en signal deasserted, the clock branches and internal source clocks are selectably muxed out to Pad_clk. The selection of which clock is driven out is controlled by programming of the lower nibble of configuration register four  62 . When the Test_lvl_en signal is active logic level of Pad_clk is equal to the logic level of Test_hi_lowz. 
     Referring to FIGS. 7A and 7B, the Clock Enable Block (CEB)  70  block contains the clock branch generators for each of the clocks coming out of the power management block driving internal sequential devices. The clock enabling/disabling circuitry is guaranteed to be glitch-free. That is, clocks are disabled after a falling edge on the internal source clocks, are held low during disabling, and are re-enabled after falling edge of the source clock (enabling during the low part of the source clock), and will subsequently begin the first rise with the next rising transition of the internal source clock, without any logic-generated spurious glitches. Special system-level considerations are taken into account for Idle mode assertion. Idle is configurable stoppage of the Clock going to the embedded CPU  34 . The main consideration is to not stop the Cpu_core_clk when the embedded CPU  34  is actively performing a bus cycle. The process involved in stopping the embedded CPU  34  is to provide an Idlreq to system control logic and for the system control logic to send back an Idlack which is sampled by the CEB  70  to generate a glitch-free disable. In a similar fashion Idle is disabled by deasserting Idfreq and receiving back a deasserted Idlack by the system control logic. The clock will be restarted glitch-free. (IDLE Mode) The source clocks for the CEB  70  are of type 1, type 2, and a 2×frequency version of type 2. Most of the clocks are generated from one or the other of these source clocks, however, the Timer_clk and Pcmcia_clk may be configured to have either Type 1 or Type 2 clocks. (Global and Individual Peripheral Disable Mode). 
     Referring to FIG. 8, the Power-Level Detect (PLD)  72  acts as an analog voltage-level detector interface with programmable override. It provides the digitally encoded voltage level as an output which is used for global configuration. An analog enable, D 3 VEN from configuration register three  60 , is available to turn on the DC-current sources of an external voltage-level detector and a read strobe. CHK 3 V from configuration register four  62 , is also available to sample the voltage-level detector input, Pup 3 V. After the detector input has been sampled, both the CHK 3 V and D 3 VEN can be deasserted. It is important to assert D 3 VEN before CHK 3 V and deassert in the reverse order so that a correct operational state of Pup 3 v is captured. The default output of the PLD  72  after a hard reset is one, on port Three. The analog interface functions may be bypassed, and thus, the output signal THREE may be driven under configuration control by the SETV input directly from configuration register three  60 . 
     Referring to FIG. 9, many of the system  32  I/Os are power supply-level configurable. As discussed above, the power management system  30  controls the voltage sensing and setting for the I/Os. The power management system  30  described has the capability to set the operating voltage configuration level through firmware and through voltage-level sensing. The interface to an analog voltage sensing circuit is included in the power management block and is controlled by bits  4  and  6  of configuration register four  62 . Bit  4  enables the voltage detector. Since analog circuitry generally consumes DC current when active, the enable switch is used to switch the current on or off. The circuit is, therefore, only enabled when voltage detection is needed to reduce power consumption. Bit  6  is used to latch and hold the level of the voltage detector. In this embodiment, the voltage detector needs to detect either a 3.3 V or 5 V supply level. Thus a 1-bit A/D is used and the output configuration level latched is either a Logic 1 or 0. However, higher order A/Ds may be used if finer levels of voltage-level detection are needed. The power configuration level is stored in configuration register four  62  bit  5 . The level may be overridden by firmware. This interface voltage detection scheme has been defined to discern voltages above and below a target detection trip point of for example, 4.0 V. Thus, this particular application will have a different configuration level at 5.0 V(+/−10%) then at 3.3(+/−10%). 
     Referring to FIG. 10, the Internal Source Clock Generation (ISCG) block  74  generates the internal source clocks. It contains a 7-bit synchronous counter with a synchronous load to a count of one and an asynchronous clear. The block generates the type 2 and 2×frequency version of the type 2 internal source clocks. In standard operation (i.e., NOT power-save mode), the type 2 clock is generated by a feedback of the cpu_clk_z source clock output through a two-input mux driving (pre_cpu_clk) back into the D_input of the cpu_clk_z-generating flip flop which is sampled by a clock referenced to the external oscillator clock called osc_qualified (i.e., this is a divide-by-2 function); and in standard mode the 2x type 2 clock is essentially a buffered-and-muxed version of osc_qualified. When in power-save mode both the 1x and 2x Type 2 clocks are created starting through the Synchronous counter to two separate divide-by-2 final clock generators. For example, in divide-by-4 clock division the 2x clock will originate from YO of the counter (i.e., a/2 of osc_qualified) through the final divide-by-2 clock generator resulting in a divided-by-4 2x clock. Similarly, the 1×clock will originate from Y 1  of the counter. Y 1  of the counter is a divide-by-4 of osc_qualified, which is equal to a divide-by-2 of the “standard” cpu_clk. The standard cpu_clk is the 1x clock reference frequency. In other words, Y 1  of the counter goes through the final divide-by-2 clock generator resulting in a divided-by-4 1x clock. In divide-by-8 mode, counter ports Y 1  and Y 2  are used respectively, and so on up to divide-by-64. When transitioning to, from, or within the power-save modes the transitions are designed to be glitch free. All changes in frequency are made after the first osc_qualified rising edge sample of an active load_ 1  input pulse which is generated by the PSVMCD immediately after a falling edge on cpu_clk. The new values of the svb_d 1 _ 5 _sync and psvm_d 1   —5 _sync inputs on the same rising edge of osc_qualified. This process is done so that the total number of periods of the 2x clock is always double the 1×clock over any amount of changes in clock division. This is a critical feature necessary for correct operation of the system. Note that the changes in clock division occur when both the 1x and 2x type 2 clocks are low. Also, note that a 1 is synchronously loaded in the Synchronous counter during a change in frequency. This keeps the 1x and 2x type clocks phase relationship the same through changes in clock division which is also critical to correct system operation. (Power Save Mode). 
     Referring to FIG. 11, the Power-Save Mode Change Detection (PSVMCD) block  76  is used to sample changes in the Power Save Mode control configuration registers, SVB[2:0] and PSVM. Two banks of flip-flops sampled off of opposite edges of an internal source clock of type 2 are compared (i.e. clocks cpu_clk and cpu_clk_z). When there is a difference between the two, an intermediate indicator is asserted called equality_z. If power-save mode is asserted in either or both of the sampling banks, i.e. psvm_dl_ 5  or psvm_d 1 , then a psvm_change indicator is asserted. This indication is then sampled by cpu_clk_z which is referenced to the falling edge of the system clock and generates a synchronous pulse, referred to as load_ 1 , until the next rising edge of an internally qualified reference to the external oscillator clock, i.e. osc_qualified, which is at least 2×the frequency of the system clock. So in summary, the load_ 1  pulse is generated by the PSVMCD  76  after the falling edge of the system clock and the load_ 1  pulse is again deasserted prior to the rising edge of the next system clock. The PSVMCD  76  is used to create and drive the load_ 1  pulse into the ISCG at a specific time point in the period of the current system clock and provide synchronized power save mode control signals, i.e. svb_d 1 _ 5 _sync and psvm_d 1 _ 5 _sync that change and become valid with the falling edge (deassertion edge) of the load_ 1  pulse which as described in the ISCG is after the first rising edge of osc_qualified immediately after a falling edge on cpu_clk. (The one exception to this is where the SVB[2:0] bus is changing and the PSVM is deasserted. In this case a load_ 1  pulse will not be created.) This process guarantees that there will be no clock glitches generated in the ISCG when changing the level of clock division. 
     It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.