(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.

FIGURE 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.

FIGURE 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 mernory 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 resymchronizes 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 ovherein the count (three, four, five, etc. ) represents the ratio of system clock frequency to a peripheral clock<BR> 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 DRAWING (S) FIGURE 1 is schematic illustrating a prior art simplified microcontroller with system bus.

FIGURE 2 is a schematic illustrating a prior art simplified microcontroller with individual bus lines.

FIGURE 3 is a schematic illustrating one embodiment of the invention in a microcontroller.

FIGURE 4 is a timing diagram illustrating operation of the invention at two different clock frequencies.

FIGURE 5 is a schematic illustrating one embodiment of bridge circuit from FIGURE 3.

FIGURE 6 is a schematic illustrating one embodiment of bridge circuit from FIGURE 3.

FIGURE 7 is a flow diagram illustrating a method for driving multiple peripherals with different clock frequencies.

DETAILED DESCRIPTION OF THE INVENTION FIGURE 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.

FIGURE 4 is a timing diagram illustrating operation of the invention at two different clock frequencies. Clock 400, from, for example, clock 324 of FIGURE 3, is twice the rate of clock 405, from, for example, clock 326 of FIGURE 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 FIGURE 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 309-2 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.

FIGURE 5 is a schematic illustrating one embodiment of bridge circuit 316 from FIGURE 3. FIGURE 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 FIGURE 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.

FIGURE 6 is a schematic illustrating one embodiment of bridge circuit 316 from FIGURE 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, 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.

FIGURE 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.