Patent Abstract:
The invention is a system for selecting a peripheral, the peripheral receiving a first clock frequency. The invention comprises the following. A processing circuit receives a second clock frequency, where the first and second clock frequencies are different. The processing circuit is configured to transmit a select signal. A bridge circuit is coupled to the processing circuit and the peripheral, and is configured to receive the select signal and transmit a peripheral select signal to the peripheral. The bridge circuit is further configured to receive the second clock frequency but not the first clock frequency. A counter is coupled to the bridge circuit and is configured to process a count, the count being a predetermined number and based on the value of the first frequency.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to French Application Serial Number 04 00740, filed Jan. 27, 2004, the entirety of which is incorporated by reference herein. 
   BACKGROUND 
   (1) Field of the Invention 
   The invention relates to clock frequencies in peripheral devices and more particularly to running a peripheral device (slave) at a clock frequency different than that of a microprocessor (master), without using clock resynchronization. 
   (2) Prior Art 
   A microcontroller generally includes a microprocessor, memory, a peripheral module that provides communication, for example Universal Asynchronous Receiver/Transmitter (UART), SPI, and USB, and an interrupt controller. Peripherals are generally configured to exchange data with microprocessors through internal buses. A bus typically conveys data, address and control signals. One method of handling data buses for multiple peripherals is by multiplexing the signal. 
     FIG. 1  is schematic illustrating a prior art simplified microcontroller with system bus. Microcontroller  100  includes microprocessor  102  coupled to peripherals  104  and  106 . The address bus, write data bus and read/write signal are combined for simplicity in system bus  108 . System bus  108  includes multiplexer  110  that selects between data from peripherals  104  and  106 . 
     FIG. 2  is a schematic illustrating a prior art simplified microcontroller with individual bus lines. Microcontroller  200  includes microprocessor  202  connected to memory  204 . Address decoder  206  receives and decodes addresses from microprocessor  202  for memory  204  and peripherals  208 - 1  and  208 - 2 . External bus interface  209  connects to microprocessor  202  through system address bus  210 , system read/write  212  and system data bus  214 . External bus interface  209  enables microcontroller  200  to interface to external components (not shown). 
   Address decoder  206  receives and decodes an address from system address bus  210 , and issues a select signal on bridge select line  215  to bridge circuit  216 , which then selects between peripheral  208 - 1  and  208 - 2 . Bridge select logic  216  coordinates bus signals from system address bus  210 , system read/write bus  212 , and system data bus  214  with peripherals  208 . Bridge circuit  216  also translates the protocol of the system bus into protocol for the peripheral bus. Bridge circuit  216  interfaces with peripheral  208 - 1  through peripheral bus  218 - 1  and with peripheral  208 - 2  through peripheral bus  218 - 2 . Communication between microprocessor  202 , bridge circuit  216  and peripheral  208 - 1  is coordinated by a clock signal from clock source  220  (also received by memory  204 , address decoder  206 , and external bus interface  209 ). 
   Peripheral  208 - 2  receives a clock signal from clock source  222 , which differs from clock source  220 . Bridge circuit  216  receives clock source  222  and resynchronizes signals between microprocessor  202 , which operates at the timing of clock source  220 , and peripheral  208 - 2 . In order to resynchronize, microcontroller  200  duplicates some parts of the bus (particularly the address bus), which in turn requires more power. 
   What is needed is a method and system for resynchronizing signals between peripherals at different clock frequencies that uses fewer components and reduce power consumption. 
   SUMMARY OF THE INVENTION 
   The invention consists of determining a count value for a circuit wherein the count (three, four, five, etc.) represents the ratio of system clock frequency to a peripheral clock frequency (2-1, 3-1, 4-1, etc.). The count allows the system to run a peripheral at a clock frequency different from the system clock frequency without resynchronizing the peripheral clock frequency. 
   The invention is a system for selecting a peripheral, the peripheral receiving a first clock frequency. The invention comprises the following. A processing circuit receives a second clock frequency, where the first and second clock frequencies are different. The processing circuit is configured to transmit a select signal. A bridge circuit is coupled to the processing circuit and the peripheral, and is configured to receive the select signal and transmit a peripheral select signal to the peripheral. The bridge circuit is further configured to receive the second clock frequency but not the first clock frequency. A counter is coupled to the bridge circuit and is configured to process a count, the count being a predetermined number and based on the value of the first frequency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is schematic illustrating a prior art simplified microcontroller with system bus. 
       FIG. 2  is a schematic illustrating a prior art simplified microcontroller with individual bus lines. 
       FIG. 3  is a schematic illustrating one embodiment of the invention in a microcontroller. 
       FIG. 4  is a timing diagram illustrating operation of the invention at two different clock frequencies. 
       FIG. 5  is a schematic illustrating one embodiment of bridge circuit from  FIG. 3 . 
       FIG. 6  is a schematic illustrating one embodiment of bridge circuit from  FIG. 3 . 
       FIG. 7  is a flow diagram illustrating a method for driving multiple peripherals with different clock frequencies. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a schematic illustrating one embodiment of the invention in a microcontroller. Microcontroller  300  includes a processing circuit, or microprocessor  302 , connected to memory  304 . Address decoder  306  receives and decodes addresses from microprocessor  302  for memory  304  and peripherals  308 - 1  and  308 - 2 . External bus interface  309  connects to microprocessor  302  through system address bus  310 , system read/write  312  and system data bus  314 . External bus interface  309  enables microcontroller  300  to interface to external components (not shown). 
   Address decoder  306  receives and decodes an address from system address bus  310 , and issues a select signal on bridge select line  315  to bridge circuit  316 , which then selects between peripheral  308 - 1  and  308 - 2 . Bridge circuit  316  interfaces with peripherals  308  through peripheral read/write bus  318 , peripheral address bus  320 , and peripheral data bus  322 . Bridge circuit  316 , microprocessor  302  and peripheral  308 - 1  receive a clock signal from clock source  324 . A clock signal to memory  304 , address decoder  306  and external bus  309  is not illustrated for simplicity. 
   Peripheral  308 - 2  receives a clock signal from clock source  326 , which differs from clock source  324 . Bridge circuit  316  does not need to receive clock source  326  in order to resynchronize signals between microprocessor  302 , which operates at the timing of clock source  324 , and peripheral  308 - 2 . In one embodiment, the ratio of clock frequency from clock source  324  to clock source  326  is known and stored in or available to bridge circuit  316 . 
     FIG. 4  is a timing diagram illustrating operation of the invention at two different clock frequencies. Clock  400 , from, for example, clock  324  of  FIG. 3 , is twice the rate of clock  405 , from, for example, clock  326  of  FIG. 3 . Although a ratio of 2-1 is used in this example, the ratio is only dependent on the clock speeds at which the system and peripherals operate. 
   Microprocessor  302  from  FIG. 3  sends an address corresponding to peripheral  308 - 1  along system address bus  310 . As soon as bridge circuit  316  receives an address, for example address  410  (corresponding to peripheral  308 - 1 ), bridge circuit  316  transmits peripheral select signal  415  to peripheral  308 - 1 . For example, in the Advanced Microcontroller Bus Architecture (AMBA), proposed by ARM, the select signal is asserted for two clock cycles while the setup time for the peripheral is one clock cycle. AMBA is used as an example only to illustrate operation of the invention, not as a limiting feature. Because peripheral  308 - 1  and microprocessor  302  share the same clock frequency from clock source  324 , one clock cycle of clock  400  passes before read data  420  is available from peripheral  308 - 1 . 
   Next, microprocessor  302  sends an address corresponding to peripheral  308 - 2  along system address bus  310 . As soon as bridge circuit  316  receives the address, for example address  425  (corresponding to peripheral  308 - 2 ), bridge circuit  316  transmits peripheral select signal  430  to peripheral  308 - 2 . Because the ratio of clock  400  to clock  405  is 2-1, peripheral select signal  430  lasts four cycles of clock  400  and two cycles of clock  405 . 
   The time at which read data  435  becomes available from peripheral  308 - 2  depends on how the rising and falling edges of clock  405  compares to the rising and falling edges of clock  400 . For example, with clock  405 - 1 , read data  435 - 1  becomes available one and a half clock cycles after peripheral select signal  430  is asserted. Alternatively, read data could become available at one-half of a clock cycle after peripheral select signal  430  is asserted. 
   With respect to clock  405 - 2 , read data  435 - 2  becomes available one clock cycle after peripheral select signal  430  is asserted, on the rising edge of clock  405 - 2 . 
   The duration of the peripheral select depends on the peripheral selected. The bridge circuit determines the number of clock cycles, based on a known ratio, and counts based on its input clock. The ratio, or number of clock cycles that need to pass, may be hardwired into the bridge circuit or provided through an external source. 
     FIG. 5  is a schematic illustrating one embodiment of bridge circuit  316  from  FIG. 3 .  FIG. 5  simplifies the bridge circuit circuitry by illustrating only the components related to the invention. Bridge circuit  500  includes address decoder  510  that receives an address from system address bus  310  and a bridge select signal from bridge select line  315 . Address decoder  510  decodes the address and bridge select, and sends a select signal to state machine  520 . State machine  520  receives, for example, a signal indicating that peripheral  308 - 1  should be asserted. State machine  520  asserts a set signal on D flip-flop (DFF)  530 , which asserts the peripheral select signal for peripheral  308 - 1 . State machine  520  also sends an enable signal to counter  540 . In this example counter  540  is an incrementing counter, though a decrementing counter could be substituted with other appropriate changes. The first clock cycle after the counter is enabled, the counter reads zero. At this point, peripheral  308 - 1  has made read data  420  (see  FIG. 4 ) available. Comparator  550  compares the value from counter  540  (a zero at this time) with a count from multiplexer  560 , which provides a default value of one. Since the numbers differ, comparator  550  takes no action. After the next clock cycle, read data  420  has been available one clock cycle and counter increments its value to one. Comparator  550  identifies that the value from counter  540  (one) equals the count from multiplexer  560  (one), therefore comparator  550  sends a reset signal to counter  540  and informs state machine  520 . The reset signal sent to counter  540  stops the count and resets the value to zero. State machine  520  resets DFF  530 , which deasserts peripheral  308 - 1 , and sends a ready signal to microprocessor  302  indicating that bridge circuit  500  is ready to proceed. 
   In the next example, state machine  520  receives a signal indicating that peripheral  308 - 2  should be asserted. State machine  520  asserts a set signal on D flip-flop (DFF)  570 , which asserts the peripheral select signal for peripheral  308 - 2 . State machine  520  also sends an enable signal to counter  540 . DFF  570  also asserts a select signal at multiplexer  560 . Multiplexer  560  transmits a value of 3 to comparator  550 . One of ordinary skill in the art recognizes that the count transmitted by multiplexer  560  depends on the architecture used and the ratio between the clock frequencies. In this example, multiplexer  560  provides a count of one and three, consistent with AMBA architecture and a 2-1 ratio. The count will vary with different ratios and different architectures. In this example, as long as DFF  570  is asserted, multiplexer  560  will provide a count of three. 
   Four clock cycles after state machine  520  sets DFF  570 , counter  540  increments to a value of three and comparator  550  resets counter  540  and informs state machine  520  of the completion. State machine  520  then resets DFF  570 , which in turn deasserts multiplexer  560 . State machine  520  also transmits a ready signal to microprocessor  302 . Peripheral select  430  was asserted for four clock cycles, based on clock  400 , or two clock cycles based on clock  405 . 
     FIG. 6  is a schematic illustrating one embodiment of bridge circuit  316  from  FIG. 3 . Bridge circuit  600  includes the components of bridge circuit  500  and the components operate in the same manner. Additionally, address decoder  510  receives a system bus read/write signal and transmits a select signal to configuration register  610 . Rather than multiplexer  560  receiving a fixed count when asserted by DFF  570 , multiplexer  560  receives a count from configuration circuit  610 . 
   Address decoder  510  sends a select signal to multiplexer  620 , which then selects the ratio for the count from system data bus  630 . Multiplexer  620  transmits the ratio to DFF  640 , which in turn transmits the ratio to multiplexers  560  and  620 . Once address decoder  510  deselects multiplexer  620 , multiplexer  620  selects the count from DFF  640 , and the count is recycled between multiplexer  620  and DFF  640 . Configuration register  610  provides a configurable count for peripherals with different or varying clock frequencies. 
     FIG. 7  is a flow diagram illustrating a method for driving multiple peripherals with different clock frequencies. In block  700 , receive a first clock cycle in a processing circuit. In block  705 , transmit a select signal from the processing circuit. In block  708 , transmit an address signal from the processing circuit. In block  710 , receive the select and address signals at a bridge circuit, the bridge circuit coupled to the processing circuit, a first peripheral and a second peripheral. In block  715 , determine whether the address signal is associated with the first or second peripheral. In block  720 , transmit a first peripheral select signal to the first peripheral. In block  725 , process a first count in a counter, the counter coupled to the bridge circuit, the first count associated with the first peripheral. In block  730 , track passage of clock cycles based on the first clock frequency. In block  735 , compare the first count with the passage of clock cycles based on the first clock frequency. In block  740 , assert the first peripheral for a period of time based on the first count and the first clock frequency. In block  745 , transmit a second peripheral select signal to the second peripheral. In block  750 , process a second count in the counter, the second count associated with the second peripheral, wherein the second count differs from the first count and is predetermined. In block  755 , compare the second count with the passage of clock cycles based on the first clock frequency. In block  760 , assert the second peripheral for a period of time based on the second count and the first clock frequency, wherein a ratio of the period of time for which the first peripheral is asserted to the period of time for which the second peripheral is asserted is equal to a ratio of the first clock frequency to the second clock frequency. 
   The advantages of the invention include reduced circuitry and therefore reduced power requirements, by allowing frequency differences between peripherals or between a peripheral and the system, without resynchronizing. 
   One of ordinary skill in the art will recognize that multiple masters may be used without straying from the invention. As any person skilled in the art will recognize from the previous description and from the figures and claims that modifications and changes can be made to the invention without departing from the scope of the invention defined in the following claims.

Technology Classification (CPC): 8