Abstract:
According to one embodiment of the present invention, there is provided a power management system for use in a computer system having a memory system incorporating a non-volatile memory and a controller which presents the logical characteristics of a disc storage device to a host, the power management system comprising means for monitoring the operational activity levels within at least some of the components of the controller and arranged, in response to the monitored levels, to vary the power consumed by selected components of the controller.

Description:
BACKGROUND OF THE INVENTION  
       Cross Reference to Related Application  
         [0001]    This application claims the benefit of the priority date of my earlier filed British Application No. 0123421.0, entitled “Power Management System”, filed on Sep. 28, 2001.  
           [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a power management system for managing power used and energy consumed in a computer system, particularly a portable computer system where there is often a limit to the peak power that can be supplied and where the energy is usually provided by batteries which have a shorter life if required to deliver increased energy and particularly to a flash disc device which is a memory system having a controller which presents the logical characteristics of a disc storage device to a host, but, which uses a non-volatile semiconductor memory device as its physical storage medium.  
           [0004]    2. Description of the Prior Art  
           [0005]    Minimizing peak power (where power is energy per unit time) and minimizing energy consumption are sometimes conflicting aims. To minimize the peak power drawn by a Flash Storage System may require that the Flash Storage System takes longer to perform its operations, which can lead to higher energy dissipation since the system is active over a longer period, though at a lower power over this period.  
           [0006]    The standard Flash Controller includes a number of hardware blocks. These blocks include a Host Interface Block, a Flash Interface Block and a Microprocessor Block, which are connected to memories via a System Bus. Each of these hardware blocks consumes energy within the Flash Controller. The Host Interface and Flash interface blocks also consume energy on external interfaces. To minimize the energy consumption of the whole computer system requires the minimization of energy consumption within the Flash Controller itself, within the Flash memory, and on the Flash and Host Interfaces.  
           [0007]    Thus, a need arises to obviate or mitigate at least one of the aforementioned problems.  
         SUMMARY OF THE INVENTION  
         [0008]    According to a first aspect of the invention there is provided a power management system for use in a computer system having a memory system incorporating a non-volatile memory and a controller which presents the logical characteristics of a disc storage device to a host, the power management system comprising means for monitoring the operational activity levels within at least some of the components of the controller and arranged, in response to the monitored levels, to vary the power consumed by selected components of the controller.  
           [0009]    Preferably the power management system further comprises at least one power management algorithm which is implemented within firmware of the power management system.  
           [0010]    In another of its aspects the present invention comprises a non-volatile memory system having a controller incorporating a plurality of components and which presents the logical characteristics of a disc storage device to a host, where the controller incorporates a power management system having means for monitoring the operational activity levels within at least some of the components of the controller, said means being arranged, in response to the monitored levels, to vary the power consumed by selected components of the controller.  
           [0011]    The power management system may be embodied in a discrete system manager or in a distributed manner through components of the controller.  
           [0012]    Preferably the power management system generates the main clock signals for the controller and determines which are active and the frequency of such active clock signals.  
           [0013]    The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments which make reference to several figures of the drawing. 
       
    
    
     IN THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates a computer system incorporating a power management system in accordance with the present invention;  
         [0015]    [0015]FIG. 2 illustrates the power management system of FIG. 1 in greater detail; and  
         [0016]    [0016]FIG. 3 illustrates an example of the way in which the controller  16  of FIG. 1 switches between different power levels during the execution of a Write Sector command from a host. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    As is shown in FIG. 1, a computer system  10  comprises a flash memory  14 , a flash controller  16  and host system  12 . The controller  16  comprises a host interface block  16 , a microprocessor  24 , a flash interface block  22 , an SRAM  28 , a ROM  30 , all of which are connected to a memory access control structure or system bus  21  in a manner which is well known in the art and which enables the memory system  16 ,  14 , to present to the host system  12  the logical characteristics of a disc storage device.  
         [0018]    Controller  16  of a system  10  additionally incorporates a power management system in the form of a system manager  20 .  
         [0019]    Having a discrete System Manager Block  20  simplifies the design and explanation of the power management features, however, the features to be described could equally be distributed and incorporated into other blocks within the hardware of controller  16 . The term ‘system manager’ is intended to embrace both the distributed and the discrete arrangements.  
         [0020]    The System Manager  20  is concerned with the control of reset, timing and interrupt signals within the controller  16 , and the control logic for these signals may also be incorporated within the System Manager, however, this is not necessary for achieving power management.  
         [0021]    [0021]FIG. 2 shows the structure of the system manager  20 . The system manager  20  comprises a system manager bus interface and control logic block  30  which connects the system manager  20  to the system bus  21  via bus interface  22 . The system manager  20  also includes an event monitor block  32 , a microprocessor throttle control block  34 , system clock control block  36 , a phase locked loop (PLL) block  38 , clock oscillator block  40  power-down controller (PDC) level  1  block  42 , PDC level  2  block  44 , PDC level  3  block  46  and PDC level block  48 .  
         [0022]    The System Manager  20  generates clock signals for the rest of the controller  16 . Though one clock is shown, multiple clocks for different parts of the controller  16  may be generated. Whether a clock is enabled, and its frequency, are determined by power management features. The system Manager  20  also generates other control signals to control activity within the Flash Controller  16 . One signal enables the microprocessor  24 . Firmware reads and writes to memory-mapped registers within the system manager  20  across the interface  30  via the system bus  21 .  
         [0023]    The Event Monitor  32  takes signals from a number of event sources  29  within the Flash Controller  16 . These event sources  29  indicate when significant system events have occurred within other blocks of the controller  16  such as the Flash and Host Interface Blocks  22 ,  26  or the Microprocessor  24 . For example, the flash and host interface blocks  22 ,  26  typically indicate when certain operations have completed via these signals. Events that are used to generate processor interrupts or wake-up from a power-down state are listed in Table 3. Synchronous events require the system clock to be running and so will not be generated when the controller is in power-down level  2  or higher. The processor  24  typically indicates when a special event has occurred, such as a request to enter a Debug or Test Mode. The event monitor  32  and system manger bus interface and control logic block  30  communicate via four channels which carry the signals EVT_CLEAR  31 A, EVT_LEVEL  31 B, EVT_STATUS  31 C, and EVT_WAKEUP  31 D.  
         [0024]    The Event Monitor  32  feeds the existing level of these events to the Bus Interface and control logic block  30  on the EVT_LEVEL signal, which is composed of one bit per event. The event source signals can be de-asserted by the source at any time. In some cases, when the controller  16  is in a low-power state, it may not be able to respond to the EVT_LEVEL signal immediately, and could miss an event. Thus, the event monitor  32  provides a second copy of the events called EVT_STATUS, that cannot be de-asserted by the source of the event, but can be set (even when the rest of the system is in a low-power state). An event in EVT_STATUS can only be de-asserted by the Bus Control Logic Block  30  asserting the appropriate bit on the EVT_CLEAR signal or a System Reset.  
         [0025]    During certain low-power modes, the Event Monitor  32  may be the only active part of the system manager  20 . If required, it outputs a signal  31 E to the rest of the System Manager  20  called WAKEUP, which causes the rest of the system manager  20  to exit from a low-power mode. The WAKEUP signal is asserted when an event is asserted on EVT_STATUS and the corresponding event bit is asserted on EVT_WAKEUP. Thus, the GBus Interface block  30  can control which events cause the manger  20  to wake-up. firmware via the Bus Interface  30  block reads the values of both EVT_LEVEL and EVT_STATUS and asserts EVT_CLEAR and EVT_WAKEUP.  
         [0026]    Included within the System Manager  20  are blocks  36 ,  38 ,  40  for generating the main clock signals for the controller  16  and for enabling other blocks within the controller  16  such as the Microprocessor  24 .  
         [0027]    The clock generation chain consists of clock oscillator module  40 , that generates the fundamental clock for the controller  16  (named OSC_CLK). The frequency of this clock is normally determined by external components such as a Quartz Crystal or a Resistor-Capacitor charging/discharging circuit. The next stage in the clock generation is the PLL (Phase Locked Loop) block  38  which takes the fundamental clock frequency OSC_CLK and multiplies it by a factor to generate the signal PLL_CLK. Finally, this goes into the system clock control block  36 , which controls the distribution of the clock to the rest of the controller  16 .  
         [0028]    The System Manager  20  has four power-down modes that are used to control which clock signals are active within the controller  16 . Successive Levels of Power-Down mode turn off more functionality within the controller  16  to save power. The term Power-Down Mode  0  is used to describe normal system operation when all parts of the controller  16  are active. Table 1 illustrates how functionality of the controller  16  is progressively turned off to save power with successive Power-Down Modes.  
         [0029]    In Power-Down Mode  1  which is determined by block  42 , the Microprocessor  24  is disabled in a controlled fashion, so that other blocks within the controller  16  such as the Flash and Host Interface Blocks  22 ,  26  can continue to access memory  14  and perform their functions. The enable signal to the processor  24  is turned off using the MP_ENABLE signal from block  42  that feeds into the Processor Throttling Block  34 .  
         [0030]    In Power-Down Mode  2  which is determined by block  44 , the System Clock  36  is disabled using the CLK_ENABLE signal from block  44 . Functions within the controller  16  that rely on the system clock being enabled are disabled and their power dissipation reduced or eliminated. A result of this is that the main system bus  21  will be disabled so that communication between blocks within the controller  16  across the bus  21  will be disabled.  
         [0031]    In Power-Down Mode  3  which is determined by block  46 , the PLL  38  is disabled using the PLL_ENABLE signal from block  46 . PLL  38  may take a certain time to synchronize with the OSC_CLK signal from oscillator  40 , so a synchronization delay is usually required when the controller is powering up from Power-Down Mode  3  to Power-Down Mode  2 .  
         [0032]    In Power-Down mode  4  which is determined by block  48 , the clock oscillator  40  is disabled using the OSC_ENABLE signal from block  48 . The clock oscillator  40  may take a certain time to start oscillating again depending on the nature of the external components used to determine the clock frequency, so a delay is required when the controller is powering up from Power-Down Mode  4  to Power-Down Mode  3 .  
         [0033]    As regards sequencing of power-down and power-up each PDC  42 ,  44 ,  46 ,  48  receives a request for entry to a power-down mode on a PDOWN signal or entry to a power-up mode on the PUP signal. For example, the first PDC  42  will power-down the part of the controller  16  that it controls and then if this is not the target Power-Down Mode (as indicated on the PMODE signal issued by block  30 ), it will assert its PDOWN signal to the next PDC  44  so that it should power-down and so on.  
         [0034]    When the Event Monitor  32  asserts the WAKEUP signal  31 E, the PDC&#39;s sequentially from the PDC of the target Power-Down Mode will wake-up the part of the controller  16  that it is responsible for. If a delay is required before this part of the controller  16  is ready then the PDC&#39;s ensure that this delay is met. The length of the delay may be configured Firmware writing to registers within the Bus Interface and Control Logic  30 . The value of these registers is passed onto the appropriate PDC, which alters the Power-Up delay to reflect the register value. These signals indicating the length of the delay are not illustrated. Finally the PDC asserts its Power-Up output PUP which causes the next PDC in the chain to wake-up in a similar fashion.  
         [0035]    Initially entry to a power-down mode is made by firmware writing to a register within the System Manager Bus Interface and Control Logic block  30 , which indicates the desired Power-Down Mode. This causes the PMODE signal to indicate the target power-down level, and the PDOWN 0  signal to be asserted which initiates the entry into the Power-Down Mode.  
         [0036]    The modular structure of one PDC  42 ,  44 ,  46 ,  48  for each section of the clock generation and processor control  34 ,  36 ,  38 ,  40  allows the structure to be easily adapted for different clock generation structures. In addition, this structure is robust in that it guarantees that the controller  16  is powered-down and powered-up in an orderly manner, so that, for example, the processor  24  is not powered-up before the system clock signal is enabled.  
         [0037]    A second power-management feature of the manager  20  is the ability to change the clock frequency by changing the multiplication factor that relates the PLL  38  input frequency to its output frequency. Lowering the Clock frequency lowers the power dissipation within the controller  16 , but, also can reduce the data transfer performance of the controller  16 .  
         [0038]    To vary the PLL multiplication factor firmware writes to a register within the Bus Interface and Control Logic block  30  which sets the value of PLL_FACTOR that indicates the PLL Multiplication Factor. In some cases, Firmware may want the value of PLL_FACTOR to be reset to a certain value when a system event occurs. For example, the Firmware sets a low clock frequency to reduce power, but it then wants to process an interrupt quickly, the firmware may not want to continue to run at the low clock frequency. However, it takes a certain amount of time to write to the register that determines PLL_FACTOR. To overcome this problem, the firmware can set a flag within the Bus Interface and Control Logic block  30  which will cause the block  30  to reset the PLL_FACTOR when certain events occur as indicated by the Event Monitor  32 .  
         [0039]    The Microprocessor  24  is often the main source of power dissipation in the Controller  16  as it consumes power itself and also is the main source of memory access requests within the Controller  16 . To allow the power consumption of the Microprocessor  24  to be controlled the System Manager  20  includes a Microprocessor Throttle Block  34 .  
         [0040]    The Throttle Block  34  controls how often the Microprocessor  24  is enabled. The fewer clock cycles that the Microprocessor  24  is enabled for, the lower the power it consumes. The mechanism used to achieve disablement of the Microprocessor  24  can vary. For example, the output of throttle block  34  can be used directly to disable the Microprocessor  24  or to switch off the clock signal within controller  16  to the Microprocessor  24 . Alternatively, the output of the throttle block  34  can be used to deny access of the Microprocessor  24  to the Main-System Bus  21 , thus preventing it from fetching instructions and causing it to halt.  
         [0041]    The Throttle Block  34  takes two inputs: one is MP_ENABLE from the PDC  42  for the Power-Down Mode  1 . The MP_ENABLE signal is used to completely disable the Microprocessor  24  when Power-Down Mode  1  is entered. The other input to the Throttle Block  34  is THROTTLE which consists of three values M,S and B which determine the proportion of time that the Microprocessor  24  is enabled. The value of THROTTLE can be changed by the firmware writing to registers within the Bus Interface and Control Logic Block  30 .  
         [0042]    The values M (Mark) and S (Space) determine the ratio of clock cycles for which the Microprocessor  24  is enabled and disabled. The B value determines the minimum number of clock cycles in a row the Microprocessor  24  will be enabled or disabled for. This allows the Microprocessor  24  to gain access to the Memory  14  for a minimum number of clock cycles, since there would be overhead and inefficiencies when enabling and disabling the Microprocessor  24  for too few clocks cycles.  
         [0043]    Thus, the values M, S and B indicate that the Microprocessor  24  is enabled for M*B clock cycles from every (M+S)*B clock cycles. The hardware interleaves M blocks of cycles with S blocks of cycles in an optimum way to minimize long sequences of cycles with the Microprocessor  24  disabled, which could reduce the Microprocessor  24  responsiveness to events such as interrupts.  
         [0044]    The pattern set by M, S, and B is as follows. The pattern starts with a block of B clock cycles with the Microprocessor  24  being enabled and then B clock cycles this with the Microprocessor  24  being disabled. The alternation of blocks of the Microprocessor  24  being enabled and disabled repeats up to the minimum value from M and S. If M=S then the pattern now repeats, otherwise, if M&gt;S, then the Microprocessor  24  is enabled for M−S blocks of B clock cycles and then the pattern repeats, but, if M&lt;S then the Microprocessor  24  is disabled for S−M blocks of B clock cycles and then the pattern repeats.  
         [0045]    The hardware always ensures that the first block within a pattern of enabling and disabling the Microprocessor  24  has the Microprocessor  24  enabled regardless of the value of M, thus setting M=0 is equivalent to M=1 in order to prevent the Microprocessor  24  being never enabled. The hardware also interprets the value of B=0 as the maximum block size allowed by the hardware.  
         [0046]    Some examples of the patterns possible of the hardware enabling and disabling the Microprocessor  24  for different values of M, S and B are shown in Table 2.  
         [0047]    Other schemes for defining the ratio of cycles for which the microprocessor  24  is enabled and disabled are possible.  
         [0048]    As with the PLL Multiplication Factor feature discussed previously, it is useful to allow Firmware to allow the Microprocessor Throttle  34  to reset the time for which the Microprocessor  24  is enabled to its maximum value when certain system events occurs, to allow for fast reaction to controller events. Firmware can write to a register within the Bus Interface and Control Logic block  30  to enable this feature.  
         [0049]    It will be appreciated that the Flash Interface Block  22  is non standard in that it incorporates features to operate with a range of different main system clock frequencies, since the main clock frequency of the controller  16  may be changed to reduce power consumption by operation of blocks  36 ,  38  of the system manager  20 .  
         [0050]    If the main clock frequency of the flash controller is changed then this will affect the timing of signals generated by the Flash Interface Block  22 . If the clock frequency is increased then the timing of signals on the flash interface may become too quick for the Flash memory  14 . If the clock frequency is decreased then the transfer rate of data to and from the Flash memory  14  will be reduced.  
         [0051]    If the Flash Interface  22  is a major source of power dissipation, then it may be advantageous to reduce the transfer rate on the interface to reduce peak power consumption, but this reduces the data transfer rate to and from the Flash memory  14 .  
         [0052]    Accordingly, to support these power management modes, the Flash Interface Block  22  is designed to allow the timing of signals to and from the Flash memory  14  to be changed relative to the main controller system clock. Two features in the Flash Interface Block  22  are incorporated to support this. The first feature is a frequency divider circuit that is placed on the main clock that supplies the basic timing reference for the signals on the Flash interface  22 . This allows the speed of the Flash Interface to be reduced to reduce peak power consumption, without affecting the frequency of the main clock. The second feature is the timing of signals in the Flash interface  22  can be controlled on a clock cycle by clock cycle basis. When the main clock frequency is decreased; this allows the timing of signals to be made quicker by reducing the number of clock cycles for which a signal on the Flash Interface is asserted or de-asserted.  
         [0053]    Finally, the Flash Interface Block  22  is designed so that it can use the power management features within the Flash memory  14  which requires that Flash memory select signal to be taken to a voltage close to that of the power supply rail to engage a low power mode.  
         [0054]    It will further be understood that the host interface block  26  is different from the flash interface block  22 , in the most actions on the host interface block  26  are initiated and timed by the host  12  and not by the controller  15 . Many host interface protocols allow the Flash Storage System  14 ,  16  to indicate at system power-up what host interface timing will be used, but do not allow this timing to be changed later.  
         [0055]    Though in most Host interface protocols, the data transfer rate to and from the Flash Storage System  14 ,  16  is determined by the host  12 , most host interface transfer protocols allow the Flash Storage System  14 ,  16  to indicate when it is ready to accept the transfer of data or of a command. The Flash Controller  16  uses this feature of the host interface  26  to control the rate of data and command transfer and thus minimize peak power though this reduces system performance. To support this, the Host Interface block  26  needs to be flexible in when it asserts signals that say if it is ready to accept a command or do a data transfer. If features are incorporated to let hardware automatically set these signals, then the automatic setting of these signals should configurable, so that flags can be set under direct Firmware control if necessary for power management.  
         [0056]    If the protocol allows for the basic timing of a data or command transfer to be slowed down by asserting signals on the interface  26  during the transfer then these should also be settable by Firmware to allow the transfer rate to be reduced.  
         [0057]    Reference has been made to firmware that has to utilize the power management features within the flash controller hardware to minimize power consumption with minimal impact or performance. This will now be explained.  
         [0058]    If at a point within the firmware, the firmware has to stop and wait for an event monitored by the Event Monitor  32 , then the firmware enables the system manager  20  to wake-up on this event and then enter a power-down mode. The Power-Down Mode powered-down to is determined by activity in other parts of the controller  16 . For example, if the Host or Flash interface  26 ,  22  need the clock to be running to transfer data then Power-Down Level  1  is the maximum level that can be entered. Higher power down levels can be entered if no such constraint exists, but may be limited by the time taken for the Oscillator  40  and PLL  38  to power-up.  
         [0059]    Other events that are required to interrupt the processor  24  also trigger the system manager  20  to wake-up. If the system manager  20  is woken-up by an interrupt event, it then responds to the interrupt and then the Firmware returns to the Power-down mode selected, if the system event being waited for has not occurred.  
         [0060]    Examples of doing this are events such as waiting for the host  12  to issue a command or transfer data, or waiting for Flash memory  14  to complete an operation.  
         [0061]    Events within the controller  16  that are not monitored by the Event Monitor  32  cannot cause the system manager  20  to wake-up. In these cases the Microprocessor  24  has to wait, polling a register until the event occurs. In this mode, the Microprocessor  24  must be active, but, need not run at full speed.  
         [0062]    Accordingly using the Microprocessor Throttling mechanism of block  34  can reduce power consumption in this case by reducing the frequency of polling the register. Also, when reducing the clock frequency will not affect the performance of other parts of the controller  16  then the PLL multiplication factor of block  38  can be reduced. When the event being polled for has occurred, the firmware can return the controller  16  to its normal operating frequency.  
         [0063]    When firmware determines that it needs to limit power consumption on the Host Interface Block  26 , then it can reduce the power being consumed by indicating to the Host  12  that it is busy, even when it has actually finished an operation or is ready to accept data. During this time, the controller  16  can perform other operations, or the controller  16  can enter a Power-Down Mode for a period of time to lower power consumption and trigger the system manager  20  to wake-up after a specified time by using an event triggered by a timer within the Flash Controller  16 . At this point, the controller  16  can release busy and continue operation. As an alternative to asserting busy, the controller  16 , if the host interface transfer protocol permits, can slow sown the host transfer timing. This allows the host  12  to continue data transfer but at a reduced rate.  
         [0064]    When the controller cycle time is changed to reduce power, Firmware may choose to adjust the timing of the Flash interface  22  to use fewer clock cycles to ensure that the transfer rate to memory  14  is maintained.  
         [0065]    If Firmware wants to reduce power consumption specifically on the Flash interface  22  then it can lengthen the timing of commands on the Flash Interface  22 , though this will reduce the transfer rate to memory  14 . One example, of lengthening Flash Commands is when polling the status of the memory  14 . Firmware can lengthen the timing of the polling command, and then set the Flash Interface  22  to trigger an event when the Polling command has finished. Firmware can then go to sleep for the duration of the polling command.  
         [0066]    When Firmware enters sections of code that requires the Microprocessor  24  to be active for a long period of time, then peak power consumption can be reduced by using the Microprocessor Throttle Mechanism of block  34  and the PLL Multiplication Factor of block  38 .  
         [0067]    An example of the way in which the controller switches between different power levels during the execution of a Write Sector command from a host is given with reference to FIG. 3. The relative levels of the different power levels are for illustration only. In this example, the controller does not need to respond rapidly to host commands, and the startup times of clock oscillator  40  and phase locked loop  38  during wake-up from power-down levels  4  and  3  are not important. If fast response to a host command is required, it might not be possible to switch to power-down level  4  when the host interface is in the idle state.  
         [0068]    At time A the host writes a command to the controller, which generates event  4  shown in Table 3 and causes the controller to wake-up through levels  3 ,  2  and  1  before the processor starts executing in level  0 .  
         [0069]    The processor clears the host command event and sets up the DMA hardware to allow the host to transfer data to the controller. Once the DMA is set up, the controller is put into power-down level  1  at time B. It is not possible to enter a higher power down level as the MDA transfer requires that the system clock is running.  
         [0070]    When the host transfers the required data at time C, event  5  is generated and wakes up the controller to power-down level  0 . The processor now sets up the Flash Interface Control to transfer the data to Flash memory and then reverts to power-down level  1  at time D. Again, a higher power-down mode cannot be used because the transfer to Flash memory requires that the system clock is running.,  
         [0071]    When the data transfer to Flash memory completes at time E, event  7  is generated. The controller again wakes up to power down level  0 . The processor checks that the transfer was successful, starts the Flash programming operation and then enters power-down level  2  at time F, which halts the system clock.  
         [0072]    At time G, the Flash programming operation completes and the Flash busy line makes a low to high transition, which generates event  6 . The controller wakes up through power-down level  1  to power-down level  0 . The processor checks that the programming operation was successful, sets up the response to the host and powers-down to level  4  at time H.  
         [0073]    Although the present invention has been described in terms of specific embodiments it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modification as fall within the true spirit and scope of the invention.  
                                             TABLE 1                           Power Down Modes                Power Down Mode:   0   1   2   3   4                       Microprocessor   Y   N   N   N   N           Enabled           System Clock   Y   Y   N   N   N           Enabled           PLL Enabled   Y   Y   Y   N   N           Oscillator Enabled   Y   Y   Y   Y   N                      
 
         [0074]    [0074]                                 TABLE 2                           Pattern of Microprocessor Access with varying M, S                        Pattern of Enabling and                   Disabling Microprocessor                   M = microprocessor enabled       M   S   B   S = microprocessor disabled               1   1   1   MS       1   1   3   MMMSSS       1   3   3   MMMSSSSSSSSS       4   2   1   MSMSMM       0   2   2   MMSSSS       2   0   2   MMMM                    
         [0075]    [0075]                       TABLE 3                           Synchronous/           Event   Asynchronous   Description                   0   Async   Host Reset 0.               Triggered when the host reset goes from logic               1 to logic 0.       1   Async   Host Reset 1               Triggered when the host reset goes from logic               0 to logic 1       2   Async   Host Software Reset 0               Host Software reset bit change from 1 to 0               triggers this event.       3   Async   Host Software Reset 1               Host Software reset bit change from 0 to 1               triggers this event.       4   Async   Host Command               The host interface block generates this interrupt               when a new host command is received.       5   Async   Host DMA Completion               The host interface block generates this interrupt               when a Host DMA Transfer is complete.       6   Async   Flash Not Busy               Generated when a BUSY signal from Flash               memory goes high.       7   Sync   Flash Interface Control Operation Complete               Generated when completes the current sequence               of operations.       8   Sync   Timer Interrupt               Generated when the internal timer reaches zero.       9   Async   Host Activity               Generated when activity on Host Interface.