Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-224402, filed on Aug. 2, 2005, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clock supply circuit and method. 
     2. Description of the Related Art 
     To an LSI such as a microcontroller, a quartz oscillator or the like is connected to supply clock signals. While the quartz oscillator has merits such that it is cheap, it needs a small mounting area, and the like, it requires a certain time period for its oscillation to be stable. Also, in some cases, an oscillation circuit may be provided inside an LSI. In such cases, similarly, a certain time period is required for its oscillation to be stable. 
     A time for oscillation to be stable is determined depending on characteristics of an oscillator and an LSI, a temperature, a voltage, resistance or load on a board, and the like. During this oscillation stabilization waiting time, the operation of an LSI is stopped by measuring a certain time period using a counter mounted inside a microcontroller and supplying an initialization signal to the LSI during the certain time period, or by externally supplying a reset signal that includes a sufficient oscillation stabilization waiting time, or the like. The reset signal is negated at the time when the oscillation is considered to be stable, and the LSI starts operating. 
       FIG. 3  is a diagram showing a configuration example of an LSI including a clock supply circuit, and  FIG. 4  is a timing chart showing an operation example thereof. 
     When a quartz oscillator  301  is connected to an oscillation cell (oscillation circuit)  302  inside an LSI  300 , an oscillation signal X oscillates. The oscillation cell  302  has a transistor, and inputs the oscillation signal X and outputs a clock signal CKIN. The clock signal CKIN is a binary signal representing the oscillation signal X by a high level or a low level depending on a threshold voltage of the transistor. 
     A two-divider  303  is constituted of a flip-flop and divides the clock signal CKIN in two to output a clock signal DIV 2 . The frequency of the clock signal DIV 2  is ½ of the frequency of the clock signal CKIN. The two-divider  303  is provided for making a duty ratio for the clock signal DIV 2  to be 50%. 
     An oscillation stabilization waiting counter  306  is constituted of a plurality of D-type flip-flops, which counts the number of pulses of the clock signal DIV 2  and turns a count completion signal CRDY to a high level and outputs it when the number of pulses exceeds a predetermined value. A power supply voltage monitoring circuit  305  monitors stability of a power supply voltage after a start-up by turning on of power, and turns a reset signal PRST to a low level and outputs it when the power supply voltage becomes stable. A reset signal ERST is an external reset signal that is supplied externally. 
     A reset generating circuit  307  inputs the reset signals PRST, ERST and the count completion signal CRDY, and outputs a system reset signal RST 1  and a clock enable signal CLKEN. The clock enable signal CLKEN turns to a high level after the reset signals PRST and ERST turn to a low level and the count completion signal CRDY turns to a high level. 
     An AND (logical product) circuit  304  outputs a logical product signal of the clock signal DIV 2  and the clock enable signal CLKEN as a system clock signal CK 1 . Specifically, when the clock enable signal CLKEN is at a low level, the system clock signal CK 1  is at a low level. When the clock enable signal CLKEN is at a high level, the system clock signal CK 1  is the same as the clock signal DIV 2 . After the clock enable signal CLKEN turns to a high level and supply of the system clock signal CK 1  is started, the system reset signal RST 1  turns from a high level to a low level. The low level of the system reset signal RST 1  indicates that the system clock signal CK 1  is usable. 
     Responding to turning on of the power, the power supply voltage increases, and the quartz oscillator  301  starts oscillating. At this time, the oscillation signal X starts oscillating with a small amplitude at first, which gradually becomes a large stable amplitude. A waveform of the oscillation signal X with a large amplitude becomes the clock signal CKIN having a normal pulse width, but a waveform of the oscillation signal X with a small amplitude may become the clock signal CKIN having a short pulse width. The two-divider  303  divides the clock signal CKIN in two and outputs the clock signal DIV 2 . At this time, if the clock signal CKIN has a sufficiently large pulse width, the clock signal DIV 2  toggles at a rising edge of a pulse of the clock signal CKIN. In other words, by synchronizing with rising of the clock signal CKIN, the clock signal DIV 2  logically inverts between a high level and a low level. However, when the clock signal CKIN has a short pulse width, the two-divider  303  may fail to operate, which makes the clock signal DIV 2  inconstant. 
     At the time when the power is turned on, the power supply voltage monitoring circuit  305  asserts (turns to a high level) the reset signal PRST. The oscillation stabilization waiting counter  306  counts the number of pulses of the clock signal DIV 2 , and turns the count completion signal CRDY to a high level and outputs it when the number of pulses reaches a predetermined value. Note that when the clock signal DIV 2  becomes inconstant, the output signal CRDY of the counter  306  does not necessarily become accurate. Therefore, the predetermined value needs to be sufficiently long. 
     When the reset signal PRST is asserted (turned to a high level), a reset generating circuit  307  asserts (turns to a high level) the system reset signal RST 1 . When the count completion signal CRDY turns to a high level, the reset generating circuit  307  asserts (turns to a high level) the clock enable signal CLKEN, and starts supplying the system clock signal CK 1 . Also, thereafter, the reset generating circuit  307  negates (turns to a low level) the system reset signal RST 1 . An oscillation stabilization waiting time  401  from this turning on of the power until the negating of the system reset signal RST 1  needs to be a few milliseconds to a few tens of milliseconds. 
     In addition, Patent Document 1 listed below describes a clock supply circuit having a PLL output stabilization detecting circuit which detects, when a PLL-type frequency multiplying circuit returns from a clock supply halt state in which it is halted and clock supply is stopped, whether a multiplication clock signal outputted from the PLL-type frequency multiplying circuit is stable or not, and transmits the multiplication clock signal as a system clock signal to an integrated circuit when it detects that the multiplication clock signal is stable. 
     [Patent document 1] Japanese Patent Application Laid-open No. 2001-313547 
     The oscillation stabilization waiting time  401  needs to be determined by anticipating the worst value, which is a time that cannot be neglected as a start-up time for the LSI  300 . When the quartz oscillator  301  is used, it requires a few milliseconds to a few tens of milliseconds. Therefore, from turning on of the power until the LSI  300  becomes operable, at least a few milliseconds to a few tens of milliseconds are needed. 
     A program mounted on the LSI such as a microcontroller generally performs initialization of a RAM, development of a program from a low speed ROM to a high speed RAM, and the like immediately after a start-up thereof. After these initialization operations are completed, a main program starts operating. Since also these initialization operations are necessary at the time when turning on the power, a longer time is needed until the main program becomes able to start operating. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a clock supply circuit and method capable of supplying a clock signal with a short oscillation stabilization waiting time. 
     According to an aspect of the present invention, there is provided a clock supply circuit having: a filter removing from a first clock signal pulses having a shorter pulse width than a threshold value and passing pulses having a longer pulse width than the threshold value to thereby output a second clock signal; and a divider dividing the second clock signal to thereby output a third clock signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration example of an LSI including a clock supply circuit according to an embodiment of the present invention; 
         FIG. 2  is a timing chart showing an operation example of the LSI of  FIG. 1 ; 
         FIG. 3  is a diagram showing a configuration example of an LSI including a clock supply circuit; and 
         FIG. 4  is a timing chart showing an operation example of the LSI of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a diagram showing a configuration example of an LSI including a clock supply circuit according to an embodiment of the present invention, and  FIG. 2  is a timing chart showing an operation example thereof. 
     When a quartz oscillator  101  is connected to an oscillation cell (oscillation circuit)  102  inside an LSI  100 , an oscillation signal X oscillates. The oscillation cell  102  has a transistor, and inputs the oscillation signal X and outputs a clock signal CKIN. The clock signal CKIN is a binary signal representing the oscillation signal X by a high level or a low level depending on a threshold voltage of the transistor. 
     The two-divider  103  is constituted of a D-type flip-flop, and divides the clock signal CKIN in two to output a clock signal DIV 2 . The frequency of the clock signal DIV 2  is ½ of the frequency of the clock signal CKIN. The two-divider  103  is provided for making a duty ratio for the clock signal DIV 2  to be 50%. The two-divider  103  may be omitted. In such cases, the clock signal DIV 2  is the same as the clock signal CKIN. Also, the two-divider  103  may input a clock signal CKINF instead of the clock signal CKIN. 
     The output of the oscillation cell  102  is connected to an analog filter  111 . The analog filter  111  removes from the clock signal CKIN pulses having a shorter pulse width than a threshold value and passes pulses having a longer pulse width than the threshold value to thereby output the clock signal CKINF. Specifically, the analog filter  111  removes pulses having a short pulse width which cannot enable a divider  112  and a flip-flop constituting an oscillation stabilization waiting counter  106  in the subsequent stage to operate. Here, for example, if the shortest pulse width which enables the flip-flop to operate is 1 nanosecond, the analog filter  111  only removes pulses having a pulse width shorter than 1 nanosecond. 
     The divider  112  divides the clock signal CKINF to generate a divided clock signal CK 2 . The dividing ratio of the divider  112  is obtained from (the highest operating frequency of the LSI  100 )/(the highest frequency of the clock signal CKINF). The divider  112  performs dividing by a necessary dividing ratio thereof. For example, it is assumed that the highest operating frequency of the LSI  100  (for example a first internal circuit  114  and a second internal circuit  115 ) is 100 MHz. From the clock signal CKINF, pulses having a shorter pulse width than 1 nanosecond are removed by the analog filter  111 . Therefore, the shortest cycle of the clock signal CKINF is 1 nanosecond×2=2 nanoseconds, so that the highest frequency of the clock signal CKINF is 500 MHz. Thus, the dividing ratio of the divider  112  is 100 MHz/500 MHz=⅕, which means that the divider  112  should divide at least by 5. In this case, the divided clock signal CK 2  is 100 MHz. In the case of ripple carry type divider, which is one of preferable examples, divisions by 2 n  can be obtained, so that dividing by 8 is the optimum dividing ratio. 
     Note that during an initial stage of oscillation, the clock signal CKINF has an unstable cycle and has a high frequency. For example, the clock signal CKINF has a highest frequency of approximately 500 MHz at the initial stage of oscillation-and then becomes stable with a frequency of 200 MHz at a stable stage of oscillation thereafter. 
     The oscillation stabilization waiting counter  106  is constituted of a plurality of D-type flip-flops, which counts the number of pulses of the clock signal CKINF and turns a first count completion signal PRDY to a high level and outputs it when the number of pulses exceeds a first predetermined value (2 4 =16 for example) and turns a second count completion signal CRDY to a high level and outputs it when the number of pulses exceeds a second predetermined value (2 17 =131072 for example). A power supply voltage monitoring circuit  105  monitors stability of a power supply voltage after a start-up by turning on of power, and turns a reset signal PRST to a low level and outputs it when the power supply voltage becomes stable. A reset signal ERST is an external reset signal that is supplied externally. 
     A reset generating circuit  107  inputs the reset signals PRST, ERST and the count completion signals PRDY, CRDY, and outputs a system reset signal RST 1 , an early reset signal RST 2 , a clock enable signal CLKEN and a clock select signal CLKSL. The early reset signal RST 2  turns from a high level to a low level after the reset signals PRST and ERST turn to a low level and the first count completion signal PRDY turns to a high level. 
     The clock enable signal CLKEN and the clock select signal CLKSL turn from a low level to a high level after the reset signals PRST and ERST turn to a low level and the second count completion signal CRDY turns to a high level. 
     A AND (logical product) circuit  104  outputs a logical product signal of the clock signal DIV 2  and the clock enable signal CLKEN as a system clock signal CK 1 . Specifically, when the clock enable signal CLKEN is at a low level, the system clock signal CK 1  becomes a low level. When the clock enable signal CLKEN is at a high level, the system clock signal CK 1  becomes the same as the clock signal DIV 2 . After the clock enable signal CLKEN turns to a high level and supply of the system clock signal CK 1  is started, the system reset signal RST 1  turns from a high level to a low level. The first internal circuit  114  inputs the system clock signal CK 1  and the system reset signal RST 1  and operates. The low level of the system reset signal (enable signal) RST 1  indicates that the system clock signal CK 1  is usable. 
     A selector  113  selects the system clock signal CK 1  or the divided clock signal CK 2  depending on the clock select signal CLKSL and outputs it as a clock signal CK 3 . When the clock select signal CLKSL is at a low level, the divided clock signal CK 2  is outputted as the clock signal CK 3 . When the clock select signal CLKSL is at a high level, the system clock signal CK 1  is outputted as the clock signal CK 3 . In other words, the selector  113  selects the divided clock signal CK 2  until the oscillation becomes stable, and selects the system clock signal CK 1  after the oscillation became stable. The selector  113  selects and outputs the divided clock signal CK 2  until the count value of a counter reaches a first count value, and selects and outputs the system clock signal CK 1  after the first count value is reached. The second internal circuit  115  inputs the clock signal CK 3  and the early reset signal RST 2  and operates. The low level of the early reset signal (enable signal) RST 2  indicates that the clock signal CK 3  (divided clock signal CK 2 ) is usable. 
     Responding to turning on of the power, the quartz oscillator  101  starts oscillating. At this time, the oscillation signal X starts oscillating with a small amplitude at first, which gradually becomes a large stable amplitude. A waveform of the oscillation signal X with a large amplitude becomes the clock signal CKIN having a normal pulse width, but a waveform of the oscillation signal X with a small amplitude may become the clock signal CKIN having a short pulse width. The analog filter  111  removes this pulse having a short pulse width to thereby generate the clock signal CKINF. 
     The divider  112  divides the clock signal CKINF by 8 for example to generate the divided clock signal CK 2 . Thus, the divided clock signal CK 2  becomes a pulse having a sufficiently long cycle for the LSI  100  to operate though this cycle is not constant. Prior to the operation of the system of the LSI  100 , necessary initialization operations and the like operate with this divided clock signal CK 2 . 
     At the time when the power is turned on, the power supply voltage monitoring circuit  105  asserts (turns to a high level) the reset signal PRST. Thus, the reset generating circuit  107  asserts (turns to a high level) the system reset signal RST 1  and the early reset signal RST 2 . When the reset signal PRST is negated. (turned to a low level), the early reset signal RST 2  is also negated (turned to a low level). 
     The oscillation stabilization waiting counter  106  counts the number of pulses of the clock signal CKINF, and outputs the first count completion signal PRDY when the number of pulses reaches the first predetermined value and outputs the second count completion signal CRDY when the number of pulses reaches the second predetermined value. Due to generation of the first count completion signal PRDY, the reset generating circuit  107  negates (turns to a low level) the early reset signal RST 2 . Also, due to generation of the second count completion signal CRDY, the reset generating circuit  107  asserts (turns to a high level) the clock enable signal CLKEN, and starts supplying the system clock signal CK 1 . Also, when the clock select signal CLKSL turns to a high level, the selector  113  selects the system clock signal CK 1  and outputs it. Thereafter, the reset generating circuit  107  negates (turns to a low level) the system reset signal RST 1 . 
     As described above, at the initial stage of oscillation, the pulse width of the clock signal CKIN may become short. In this embodiment, since pulses having a short pulse width are removed by the analog filter  111 , the clock signal CKINF has a pulse having a sufficiently long pulse width. Accordingly, operation failure of the divider  112  and the oscillation stabilization waiting counter  106  is prevented, and thus the stable divided clock signal CK 2  and count completion signals PRDY, CRDY can be generated. Since the stable clock signal DIV 2  can be generated and also the count completion signals PRDY, CRDY become accurate, the first and second predetermined values to be counted by the counter  106  are not needed to be longer than necessary. Accordingly, the reset signals RST 1  and RST 2  can be negated (turned to a low level) early, and the clock signals CK 1  and CK 2  can be made usable early. 
     A period in which the early reset signal RST 2  is at a low level and the system reset signal RST 1  is at a high level is an initial period of oscillation, and during this period, the second internal circuit  115  uses the divided clock signal CK 2  as the clock signal CK 3 . At this moment, the clock signal CKINF has a high frequency (500 MHz for example). When the dividing ratio of the divider  112  is ⅕, the clock signals CK 2  and CK 3  become 100 MHz. The second internal circuit  115  can use the clock signal CK 3  of 100 MHz. 
     When the LSI  100  is a microcontroller or the like, a program mounted on the LSI  100  performs initialization of a RAM, development of a program from a low speed ROM to a high speed RAM, and the like immediately after a start-up thereof. The second internal circuit  115  can perform these initialization operations during the above-described initial period of oscillation and allows a main program to operate thereafter. Accordingly, the second internal circuit  115  can start the initialization operations early and finish them early. 
     A period in which the system reset signal RST 1  is at a low level after the initial period of oscillation is a stable period of oscillation, in which the second internal circuit  115  uses the system clock signal CK 1  as the clock signal CK 3 . At this time, the clock signal CKINF has a low frequency (200 MHz for example). Since the dividing ratio of the two-divider  103  is ½, the clock signals CK 1  and CK 3  become 100 MHz. The second internal circuit  115  uses the clock signal CK 3  of 100 MHz and is able to perform processing of a main program for example. 
     In the case of  FIG. 4 , an oscillation stabilization waiting time  401  from turning on of the power until the system reset signal RST 1  becomes a low level needs to be a long time (a few milliseconds to a few tens of milliseconds). 
     In this embodiment, the first internal circuit  114  that is not required to perform early processing uses the system clock signal CK 1  and the system reset signal RST 1 , and the second internal circuit  115  that is required to perform early processing can use the clock signal CK 3  and the early reset signal RST 2 . The first internal circuit  114  can use the system clock signal CK 1  after a period  201  is passed, which is from turning on of the power until the system reset signal RST 1  turns to a low level. 
     The second internal circuit  115  can use the clock signal CK 3  after a short period  202  is passed, which is from turning on of the power until the early reset signal RST 2  turns to a low level. According to this embodiment, the clock signal CK 3  (CK 2 ) with the short oscillation stabilization waiting time  202  can be supplied. The clock signal CK 3  (CK 2 ) can become usable earlier than the clock signal CK 1 . The clock signal CK 3  uses the divided clock signal CK 2  in the initial period of oscillation and uses the system clock signal CK 1  in the stable period of oscillation. The dividing ratio of the divider  103  is ½, and the dividing ratio of the divider  113  is ⅕ or ⅛. Since the dividing ratio of the divider  112  is smaller than the dividing ratio of the divider  103 , the system clock signal CK 1  has a higher frequency than that of the divided clock signal CK 2 . 
     As above, according to this embodiment, the analog filter  111  is inserted, the formed clock signal CKINF from which pulses having a short width are removed is generated, and a divider  112  constituted of one or more flip-flops is provided for dividing this formed clock signal CKINF. The clock signal CK 2  outputted by the divider  112  is supplied to the second internal circuit  115 , thereby enabling it to operate. 
     The analog filter  111  only passes pulses having a width equal to or wider than a pulse that enables the flip-flop constituting the divider  112  to operate. Thus, the flip-flop of the divider  112  operates properly. 
     The divider  112  divides the formed clock signal CKINF so that it becomes a frequency equal to or lower than the highest frequency to enable the second internal circuit  115  to operate. The dividing ratio required for the divider  112  is (the highest frequency to enable the second internal circuit  115  to operate)/(the highest frequency of the formed clock signal CKINF). Here, the shortest cycle of the formed clock signal CKINF becomes two times the pulse width that can pass through the analog filter  111 , so that the highest frequency of the formed clock signal CKINF is 1/(the shortest pulse width that can pass through the analog filter  111 ×2). When the shortest pulse that can pass through the analog filter  111  is 1 nanosecond, the highest frequency of the clock signal CKINF is 500 MHz. 
     In this embodiment, for predicting stabilization of clock oscillation, the counter  106  is provided. When the counter  106  reaches a certain value, it is assumed that the clock oscillation became stable. While the oscillation is not stable, the clock signal CK 2  outputted by the divider  112  is supplied to the second internal circuit  115 , and after the oscillation became stable, the clock signal CK 1  that does not pass through the divider  112  is supplied to the second internal circuit  115 . 
     While the oscillation is not stable, not necessarily all the functions in the LSI  100  should operate, where only the second internal circuit  115  needs to operate, which performs transferring from a low speed memory to a high speed memory, initializing memories and the like for example. Accordingly, only to the second internal circuit  115  needed for these operations, the clock signal CK 2  outputted by the divider  112  is allowed to be supplied. Also, at the same time, the second internal circuit  115  to which the clock signal CK 2  outputted by the divider  112  is supplied and a circuit other than the second internal circuit  115 , namely the first internal circuit  114 , use the different reset signals (initialization signals) RST 2  and RST 1 , respectively. For the first internal circuit  114 , the reset signal RST 1  is negated (turned to a low level) after the oscillation became stable. 
     As above, according to this embodiment, it is possible to generate the clock signal CK 2  capable of operating safely even before the oscillation of a clock becomes stable, and the start-up time of a system can be largely reduced by performing operations not depending on the cycle of a clock in advance. Also, it is possible to effectively utilize the oscillation stabilization waiting time of a clock signal that is supplied externally or internally. 
     A clock signal with a short oscillation stabilization waiting time can be supplied. Accordingly, an LSI or the like which operates based on a clock signal can start initialization operations early and finish them early. 
     The present embodiments are to be considered in all respects as illustrative and no restrictive, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.

Technology Category: 3