Patent Publication Number: US-6715000-B2

Title: Method and device for providing high data rate for a serial peripheral interface

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This Application is a Continuation-in-part of U.S. patent application Ser. No. 09/810,994, filed Mar. 16, 2001; furthermore, this application claims the benefit of, and priority to, provisional application Ser. No. 60/288,915, filed May 4, 2001; and both applications are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a serial peripheral interface for use in microcontroller-based products. More particularly, the present invention relates to a method and device for providing a high data rate serial peripheral interface using virtual special function registers and direct memory access techniques. 
     BACKGROUND OF THE INVENTION 
     The demand for higher performance, microcontroller-based products for use in communication and processing applications continues to increase rapidly. As a result, manufacturers are requiring the components and devices within these microcontroller-based products to be continually improved to meet the design requirements of a myriad of emerging audio, video and imaging applications. 
     These microcontroller-based products use various types of processors, for example, general purpose microprocessors for controlling the logic of various digital devices, such as clock radios, microwave ovens, digital video recorders and the like, and special purpose microprocessors, such as math coprocessors for mathematical computations, or digital signal processors used in manipulating various types of information, including sound, imaging and video information. For the transmitting and receiving of data between various devices and components, microprocessors and other devices utilize various types of serial interfaces. One such type of interface definition typically used is the serial peripheral interface (SPI). In addition, for the temporary storage of data, for example, to permit the microprocessors to manipulate the data before transferring the data through the SPI to another device, the microprocessors generally utilize one or more buffers. These buffers are configured with the SPI&#39;s to enable the processors to transmit and receive data to and from the buffers as needed in an application. 
     In many SPI applications, due to the burst nature of the data communications and the limited hardware resources available, e.g., resulting from the high costs for dedicated transfer/receive buffers and control logic, the data to be transferred or received needs to be stored in the memory devices. Such approaches thus require undesirable amounts of overhead. 
     Upon operation of the SPI, the CPU communicates with the memory devices for data exchange, which may be achieved by firmware controls and the like. However, when the data rate is of a prime concern, this technique dramatically affects the microcontroller operation, resulting in undesirable performance. 
     For example, an SPI may provide for a data structure that organizes the memory in a circular FIFO buffer configuration having various pointers, such as a CPU transmit buffer write location, a CPU receive buffer write location, an SPI transmit shift operation location, and an SPI receive shift operation location. As a result, to maintain SPI operations, a significant number of clock cycles must be available. For example, to maintain the above pointers, at least 100 clock cycles are necessary. To handle any interrupt requests for each SPI transmit/receive interrupt, at least 150 clock cycles are required. Further, to maintain the circular buffer structure, including the circular buffer head/tail wrap around, at least 30 more clock cycles are necessary. Finally, any data move instructions either to or from the memory devices would require approximately 30 or more clock cycles. As a result, over 300 clock cycles are required. Based on a clock rate of 4 MHz, the translates into a data rate of only 13 Kbytes per second, an undesirably low data rate. 
     Accordingly, a need exists for an improved, high performance scheme for a serial peripheral interface to provide a data rate which does not require high overhead. 
     SUMMARY OF THE INVENTION 
     The serial peripheral interface and high performance data transmission and receiving scheme according to the present invention addresses many of the shortcomings of the prior art. In accordance with various aspects of the present invention, an improved high performance scheme is provided with a serial peripheral interface (SPI) to enable microcontroller-based products and other components and devices to achieve a higher serial transmit and receive data rate. In accordance with an exemplary embodiment, an exemplary technique utilizes a CPU and an SPI having a circular FIFO structure, configured with a single port memory device. To prevent the memory traffic associated with any SPI accesses from conflicting with other CPU memory accesses, the technique utilizes cycle stealing direct memory access techniques for SPI data transfers with the memory. 
     In accordance with an exemplary embodiment, during a CPU read/write sequence, data is read/written from/to the memory through a virtual special function register (SFR). Once the virtual SFR access is detected, all accesses are redirected to the circular FIFO buffer memory, with no additional pipelining necessary. The CPU pointers can suitably increment as appropriately controlled by hardware. In addition, once an SPI transmit/receive request is made, data communication can be established between the transmit/receive buffer and the memory. To avoid structural hazard, the transmit/receive request can be suitably pipelined until the next available clock phase, for example, within one instruction cycle. As a result, for a 4 Mhz clock rate, the technique can enable a significantly higher data transfer rate, e.g., at 250 Kbytes per second, an improvement of almost twenty times the prior art data rates. 
     In accordance with another aspect of the present invention, the high performance technique avoids the firmware overhead with minimum hardware control cost. For example, compared to the hardware approach using deeper buffer structures, e.g., with FIFO buffers implemented using flip-flop devices, the exemplary techniques utilize memory, e.g., dynamic or static random access memory (DRAM or SRAM) with direct memory access (DMA). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and: 
     FIG. 1 illustrates a block diagram of an exemplary microcontroller system which can implement the techniques and devices of the present invention; 
     FIG. 2A illustrates an exemplary memory stack in accordance with an exemplary embodiment of the present invention; 
     FIG. 2B illustrates a block diagram of an exemplary data transfer technique in accordance with an exemplary embodiment of the present invention; 
     FIG. 3 illustrates an exemplary block diagram of an SPI in accordance with an exemplary embodiment of the present invention; 
     FIG. 4A illustrates an exemplary CPU/SPI transmit technique in accordance an exemplary embodiment of with the present invention; and 
     FIG. 4B illustrates an exemplary CPU/SPI receive technique in accordance with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     The present invention may be described herein in terms of various functional components and various processing steps. It should be appreciated that such functional components may be realized by any number of hardware or structural components configured to perform the specified functions. For example, the present invention may employ various integrated components, e.g., buffers, voltage and current references, I/O devices, memory components and the like, comprised of various electrical devices, e.g., resistors, transistors, capacitors, diodes or other devices, whose values may be suitably configured for various intended purposes. In addition, the present invention may be practiced in any microcontroller-based application. Such general applications that may be appreciated by those skilled in the art in light of the present disclosure are not described in detail herein. However for purposes of illustration only, exemplary embodiments of the present invention will be described herein in connection with a SPI interface for a master device, such as a microprocessor, or a slave device, such as an A/D converter. Further, it should be noted that while various components may be suitably coupled or connected to other components within exemplary circuits, such connections and couplings can be realized by direct connection between components, or by connection through other components and devices located therebetween. 
     In accordance with an exemplary embodiment of the present invention, a first microcontroller may communicate with a second microcontroller. The microcontrollers may include interface modules such as a serial peripheral interface device, wherein one of the serial peripheral interface devices operates as a “master device” to drive the synchronization of the data transfer between the master device and the other “slave device”. The master device may be, for example, a personal computer. The slave device may be, for example, memory, a digital-to-analog converter, or an analog-to-digital converter. Analog-to-digital devices may, for example, measure humidity or temperature and convert that reading into a digital formal which can be provided back to the master device for processing. 
     For example, with reference to FIG. 1, an exemplary microcontroller  100  is illustrated. However, it should be noted that the exemplary embodiments of the present invention may be suitably implemented in any microcontroller configuration. In accordance with an exemplary embodiment, microcontroller  100  is suitably configured in communication with microcontroller  101  via communication path  150 . Microcontroller  100  suitably comprises a central processing unit (CPU) core  102  configured for the processing of data, and a Bus Interface (“BusIF”)  104  for communication with the various memory or input and output devices. For the storage of data, microcontroller  100  can comprise various types of memory. For example, microcontroller  100  can comprise an internal CPU memory  106  which can be implemented using static random access memory (SRAM) and the like which can provide very low access time, e.g., as low as 10 nanoseconds. In addition, microcontroller  100  can also include data memory  114  which can also comprise SRAM-type memory, and read-only memory (ROM)  116  which can comprise the non-reprogrammable memory for the microcontroller  100 . Still further, microcontroller  100  can also include flash memory for the programming and storage of data, such as a large page of memory  124  comprising, for example, 32 KB of data storage, as well as a smaller configuration of flash memory  126 , comprising, for example, 128 bytes. 
     For the transmitting and receiving of data between various components, microprocessor  100  can also comprise serial peripheral interface (SPI)  108 . SPI  108  may communicate directly with CPU core  102  via BusIF  104  or may communicate directly with the CPU memory  106  via direct memory access (DMA)  112  and BusIF  104 . In other words, SPI  108  can transfer data to and from memory without passing the data through the CPU. Furthermore, SPI  108  can transmit and receive information from other microcontroller devices. For example, SPI  108  may communicate, over communications path  150 , with microprocessor device  101 . SPI  108  may be an integral part of a peripheral device or may be a stand alone interface apart from and in communication with the peripheral device. 
     SPI  108  may comprise various components and may communicate with various signal paths. In an exemplary embodiment, SPI  108  comprises shift registers for receiving and sending data via communication lines such as: Master In Slave Out and Master Out Slave In lines. SPI  108  may further be configured to operate in either a master or slave mode. Further information regarding SPI configurations and operation and master/slave shift register and shift buffer configurations and operation may be obtained in the referenced U.S. patent application, Ser. No. 09/810,994, entitled “Serial Peripheral Interface With High Performance Buffering Scheme”, having common inventors and assignee with this application. 
     In addition, microcontroller  100  can also include various input/output devices. For example, an I/O port device  118  can be provided, as well as a breakpoint device  120 . Further, microcontroller  100  can also include a system clock  125  for providing clock cycles for triggering various functions and sequences during operation. Microcontroller  100  can also include a Power On Reset (POR)  119  for use during ramping up of a power supply. 
     As discussed above, previous attempts for providing data transmission with an SPI comprise firmware solutions that provide very low data rates, or hardware solutions that involve high overhead, e.g., very deep and complex transmit/receive buffer and FIFO buffer configurations, as well as additional control logic, without significant improvement in the data rates to desirable levels. However, in accordance with various aspects of the present invention, an improved high performance scheme is provided with a serial peripheral interface (SPI) to enable microcontroller-based products and other components and devices to achieve a higher serial transmit and receive data rate. 
     In accordance with an exemplary embodiment, an exemplary technique utilizes a CPU and an SPI having a circular FIFO structure, configured with a single port memory device. To prevent the memory traffic associated with any SPI accesses from conflicting with other CPU memory accesses, the technique utilizes cycle stealing direct memory access techniques for SPI data transfers with the memory. The cycle stealing direct memory access (DMA) techniques may be employed such that the DMA module waits until the CPU is not using the bus interface before the DMA module transfers data between the serial peripheral interface and memory. Therefore, the central processing unit may not have to give up cycle time for that communication to occur. Thus, the DMA module can be said to be “stealing time” that the central processing unit is not using on the bus interface. As a result, the central processing unit is not slowed down and, furthermore, instructions are not required from the central processing unit to carry out these steps, with the overall effect being to increase the speed of the microcontroller and the data transmission. 
     In accordance with an exemplary embodiment of the present invention, CPU memory may be apportioned so as to reserve a predetermined portion of the memory addresses for use as buffer memory. With reference to FIG. 2A, although any portion of CPU memory may be used, in an exemplary embodiment, CPU memory  106  has  256  address locations. In this example,  256  memory address locations are represented in FIG. 2A in a linear fashion. A portion of these memory address locations may be designated as buffer memory. These buffer memory address locations may be reserved to temporarily store data being sent and received, and may improve the speed and efficiency of the data transmission and central processing unit processing. The buffer memory may be any size, and reserved out of any portion of CPU memory  106 . However, in an exemplary embodiment, the buffer memory begins at memory address  128  and may be a maximum of  128  consecutive addresses ending at memory address  255 . In FIG. 2A, for example, buffer memory  208  begins at memory address location  128  and ends at memory address location  143 . 
     With reference to FIG. 2B, buffer memory  208  may be utilized in a circular First In First Out (“FIFO”) buffer memory scheme. In one embodiment of the present invention, for example, the CPU memory  106  is physically a linear array, but buffer memory  208  is conceptually arranged end-to-end in a circle. In accordance with this embodiment, the size of buffer memory  208  is flexible and may be programmed by providing a starting address and ending address. Upon providing the starting and ending addresses, hardware in SPI  108  can be configured to cause the designated buffer memory to act as a circular FIFO memory structure with no further software intervention. For example, the hardware keeps track of the pointers and counters for the FIFO buffer memory  208 . Furthermore, hardware configuration in SPI  108  provides, for example, the automated responses to signals and requests from the BusIF and DMA module. Thus, for example, with reference again to FIG. 2A, a pointer progressing through the memory locations would move from  128  to  129 , from  129  to  130 , and so forth to  143 , and then return to memory address location  128 . It should be noted that in order to have a circular FIFO buffer, a minimum of two memory address locations are required in buffer memory  208 . 
     The circular FIFO buffer may be used, in one exemplary embodiment of the present invention, to buffer the transmission of information from CPU  102  to other devices. For example, CPU  102  may transmit data to the SPI which is then stored on the FIFO buffer for later transmission by the SPI to a connected device. Data may be transmitted and received simultaneously. With reference to FIG. 2B, in accordance with an exemplary embodiment, a CPU  102  may be configured to communicate with connected devices via BusIF  104 , virtual special function register (V-SFR)  110 , FIFO buffer  208 , SPI  108 , and communication lines  150 . V-SFR  110  may comprise virtual transmitter register  206  and virtual receiver register  212 . SPI  108  may comprise transmitter buffer  210  and receiver buffer  214 . During a CPU read/write sequence, data can be read/written from/to memory  208  through virtual special function registers  212  and  206 . 
     CPU  102  may communicate data to BusIF  104 , and BusIF  104  may in turn provide the data to a virtual SFR which may store the data in FIFO buffer memory  208 . This data is written to FIFO buffer memory  208  by virtual SFR transmitter  206 . This data may later be removed from FIFO buffer memory  208  by SPI transmitter buffer  210 . The data may then be sent out over communication channel  150 . As SPI transfer buffer  210  sends out each byte over lines  150  to a second device  101 , a byte is received on SPI receive buffer  214  which may then be transferred to FIFO buffer memory  208 , where the byte can be recalled by virtual SFR receiver  212  to be provided to CPU  102  via BusIF  104 . 
     The CPU pointers, CPUtxp  221  and CPUrxp  222 , can suitably increment to indicate the current location for data storage and retrieval on FIFO  208  by CPU  102 . The SPI pointers, SPItxp  223  and SPIrxp  224 , can suitably increment to indicate the current address for retrieval and storage of data from/on FIFO  208  by SPI  108  respectively. As mentioned above, hardware in, for example, SPI  108  keeps track of the pointers. Thus, the CPU and the DMA module are not aware that pointers exist, but act as though the data were written to a SFR. Thus CPU  102  and the DMA module do not have to keep track of where the data was temporarily stored other than a single special VSFR address. 
     In this exemplary embodiment, a transmitter buffer counter, Txcnt, can count the bytes of data that are stored on buffer  208  awaiting transmission to other microprocessors. A receiver buffer counter, Rxcnt, can count the bytes that are stored on buffer  208  awaiting transmission to CPU  102 . These counters are tracked via hardware in SPI  108 . Further information regarding aspects and embodiments of the FIFO buffer and virtual SFR operation may be obtained via reference to parent U.S. patent application Ser. No. 09/810,994. 
     Furthermore, in an exemplary embodiment of the present invention, threshold values are programmed where if the transmit counter is below a predetermined value, e.g. 10 bytes are waiting for transmission from the FIFO buffer, an interrupt (CPU_Txirq) is sent to the CPU alerting the CPU that more data can be written to the FIFO buffer. Similarly, if the receive counter is greater than a predetermined value, e.g. 10 bytes have been received and are waiting to be read by the CPU, an interrupt (CPU_Rxirq) is sent to the CPU alerting the CPU that it needs to read data from the FIFO buffer. 
     In addition, once an SPI transmit/receive request is made, which may come at any phase of an instruction cycle, data communication needs to be established between the transmit/receive buffers  210  and  214  and the memory. To avoid structural hazard, such as any conflicts with the CPU memory access, the transmit/receive request is suitably pipelined until the next available clock phase, for example, within one instruction cycle. Pipelining is a technique that may be used in microcontrollers where the microcontroller begins executing a second instruction before the first has been completed. That is, several instructions may be in the “pipeline” simultaneously, each at a different processing stage. A similar memory technique may be used in which the memory loads the requested memory contents into a small cache composed of SRAM and then immediately begins fetching the next memory contents. This can create a two-stage pipeline, where data may be read from or written to SRAM in one stage, and data may be read from or written to memory in the other stage. 
     Furthermore, cycle stealing techniques can be combined with the pipelining techniques. The cycle stealing techniques may reduce interfering with the CPU&#39;s use of the bus interface, thus speeding up processing. The pipelining techniques can increase the rate that data is transferred. By combining the techniques, data transfer rates can be improved without adversely affecting processor speed. As a result, for a 4 Mhz clock rate, the technique can enable a significantly higher data transfer rate, e.g., at 250 Kbytes per second, an improvement of almost twenty times the prior art data transfer rates. 
     In an exemplary embodiment of the present invention, SPI  108 , in a first device, may communicate data to a second device. In an exemplary embodiment, one of the two devices may be a master device which controls the synchronization of the data transfer to the slave device. Although exemplary embodiments may be described herein with microcontroller  100  as the master device, microcontroller  100  may alternatively be the slave device. When SPI  108  is acting as a master device, it may send a clock signal from a clock device to provide synchronization of the data transfer between the master and slave SPI modules. SPI  108  may be configured with a communication line labeled “master out slave in” (MOSI) carrying the output signal of the master device or the input signal for the slave device. SPI  108  is also configured with another communication line labeled “master in slave out” (MISO) carrying the input signal for the master device or the output signal for the slave device. 
     Furthermore, an interrupt signal may provide the ability to pause or stop the shifting in the slave device to allow the master device to perform other tasks temporarily until it is ready to receive data from the slave device again. It should be noted that every time SPI  108  sends a byte of data, it may receive a byte of data in return. Similarly, every time a piece of data is intentionally received, another piece of data is transmitted regardless of its value. This data may, or may not, be useful data. Further information regarding the SPI transmit and receive buffers, shift registers, and MISO/MOSI operating modes can be obtained by reference to parent U.S. application Ser. No. 09/810,994. 
     With reference to FIG. 3, an exemplary SPI configuration  300  is illustrated. In accordance with this exemplary embodiment, SPI configuration  300  shows the interfaces between the SPI module and various peripheral devices. To understand the various operational sequences of SPI configuration  300 , an exemplary description is provided. However, it should be noted that the following embodiments are merely for illustration purposes, and the invention is not limited to those disclosed in the illustrative examples. 
     CPU core  102  may communicate with various devices in SPI configuration  300  via BusIF  104 . BusIF  104  may communicate with DMA module  112 , SPI module  108 , and CPU SRAM module  106 . CPU SRAM module  106  may also contain FIFO memory buffer  208 . In accordance with an exemplary embodiment of the present invention, DMA module  112  may send several signals to BusIF  104 , comprising a DMA request for time on BusIF  104  (“DMAreq”), a DMA read or write signal (“DMA_R/W”), a DMA address signal (“DMAadr”) for referencing the FIFO memory address, and a DMA data output signal (“DMAdo”) for transmitting the value to be stored in FIFO memory and later transmitted to device  101 . DMA module  112  may also receive several signals from BusIF  104  comprising a DMA acknowledgement signal (“DMAack”) for indicating that BusIF time is available for use, and a DMA data input signal (“DMAdi”) for transmitting the value read from FIFO memory. 
     DMA module  112  may also send several signals to SPI module  108 , such as: a transmit data signal (“Txdata”), a DMA transmit acknowledgement signal (“DMAACKtx”) for acknowledging the ability of the DMA to serve the SPI transmit request, and a DMA receive acknowledgment signal (“DMAACKrx”) for acknowledging the ability of the DMA to serve the SPI receive request. DMA module  112  may also receive several signals from SPI module  108 , such as: (“SPIreqTx”) which is a request signal for initiating transmission of data stored in the FIFO memory, a SPI transmitter pointer signal (“SPItxp”) for indicating where the data can be found in FIFO memory, (“SPIreqRX”) which is a request signal for initiating the sending of data from the SPI to the FIFO memory, a SPI receiver pointer signal (“SPIrxp”) for indicating the correct FIFO address for storing the receive data, and a receive data signal (“RX data”). 
     SPI module  108  may also send signals to BusIF  104  such as: a CPU transmitter pointer and receiver pointer signal (“CPUtxp/CPUrxp”) for indicating the correct FIFO memory locations for the CPU to store/read FIFO stored data and data register chip select signal (“DRCS”) for causing BusIF  104  to write/read to/from the memory location indicated by the address provided to the buffer by SPI  108 . In addition, SPI module  108  may also receive signals from BusIF  104  such as: a virtual SFR address signal (“SFRadd”) for causing data to be stored in FIFO  208 ; a virtual SFR read/write instruction signal (“SFR_rd/wr”), and a virtual SFR data out signal (“sdo”). 
     Communication path  150  facilitates communications between SPI  108  of a first device  100  and a second device  101  which may also include a second SPI  108 . Communication line  150  may further include lines such as a master in slave out line (MISO), a master out slave in line (MOSI), and an SPI-clock line (SCLK). The SCLK signal will either be sent out from a master device to a slave device driving the synchronization and shifting in the slave device, or the SCLK signal will be received from a master device into this device as a slave device. Every serial peripheral interface SPI  108  will operate in either a master or slave mode. In a master mode, the MISO operates as an input/receiver line, and the MOSI line operates as an output/transmitter line. In slave mode, the SPI-MISO line operates as an output or transmission line and the MOSI line operates as an input or receiver line. Furthermore, interrupt signal, SSn  111 , may provide the ability to pause or stop the shifting in the slave device to allow the master device to perform other tasks temporarily until it is ready to receive data from the slave device again. In addition, interrupts  103 , CPU_Txirq and CPU_Rxirq, may be sent from SPI  108  to CPU  102  as discussed above. 
     With reference to an exemplary CPU transmit technique in accordance with an exemplary embodiment of the present invention, as illustrated in FIG. 4A, CPU  102  may send data to FIFO buffer memory and from there to SPI for transmitting. For example, in step  402 , CPU  102  may send data or instructions (sdo), a write signal (SFRwr), and an address (SFRadd) to BusIF  104  which forwards the information to SPI  108 . As far as CPU  102  is concerned, CPU  102  is writing the data or instructions to a SFR. However, the hardware in SPI  108  recognizes the SFRadd as a special VSFR address and sends DRCS and CPUtxp signals to BusIF  104 . The DRCS signal causes BusIF  104  to store the data, still held on BusIF  104 , to a particular location (CPUtxp) in FIFO buffer memory (Steps  404  and  406 ). The SPI hardware maintains address pointers (CPUtxp) referencing the next available memory location for transmitting from the CPU to the FIFO buffer memory. 
     In a further exemplary SPI transmit embodiment, the SPI may keep track of the data accumulating in the FIFO memory. The SPI may detect, in a step  412 , that data is waiting to be transferred when the CPUtx pointer  221  is not equal to the SPItx pointer  223 . In an optional embodiment, the SPI may detect, in a step  412 , that data is waiting to be transferred when a predetermined threshold value has been reached for the difference between these two pointers. In step  414 , the SPI may send a request signal (SPItx) and address information (SPItxp) to the DMA asking that the data be transferred out of the FIFO memory to the SPI for transmission to another device  101 . In step  416 , the DMA sends to the BusIF the request (DMAreq), and address (DMAadd) signals and a “read” (DMA_R) signals. The DMA_R signal instructs BusIF  104  to read the data from the DMAadd. The data transfer request is satisfied when time is available on the bus interface. When the bus interface has available cycles it will read the data from the DMAadd in the FIFO memory in a step  418 . The data signal (DMAdi) and an acknowledgment signal (DMAACK) may then be sent by the bus interface to the direct memory access module (step  420 ), on to the SPI (step  422 ) as Txdata and DMAACKtx, and then out to the destination device via transmit buffer  210  and signal lines  150  (step  424 ). 
     With reference to FIG. 4B, the reverse process may occur when the serial peripheral interface receives data from an associated peripheral device  101 , checks whether FIFO buffer is full, and requests the DMA to store the received data in the FIFO immediately. If FIFO buffer is full, then the data is lost. In an exemplary embodiment of the present invention, in a step  430 , SPI  108  receives a byte of data. SPI sends a request (SPIreqRx) to the DMA to send the data to the FIFO along with the received byte (Rxdata) and SPI receive pointer (SPIRxp) in a step  432 . In step  434 , the DMA module in turn may send a DMA request signal (DMAReq), data signal DMAdo, write command (DMAwrite), and FIFO memory address signal (DMAadr) to the BusIF. In step  435 , the BusIF may then write the data (DMAdo) to memory at the FIFO memory address (DMAadr). The bus interface acknowledges that it is available for and has written the data by sending a DMA acknowledgement signal (DMAACK) in step  436 . The DMA module may then send an acknowledgment signal (DMAACKrx) to the SPI in step  440 . 
     The central processing unit may request to read the data, from what it assumes is a special function register, by sending a read instruction signal (SFRrd) and a virtual SFR address signal (SFRadd) in a step  454 . The SPI module may recognize that request via the unique VSFR address and send a Data Register Chip Select (DRCS) signal and FIFO memory address CPUrxp to the BusIF  104  (step  456 ). The data at that FIFO memory address may be provided through the BusIF  104  to the CPU (step  458 ). 
     At the end of every data read or write event, the pointers to the FIFO buffer memory addresses are updated. This updating may involve incrementing the pointer, maintaining the pointer at the present value in the FIFO buffer, or wrapping the pointer around to the beginning of the FIFO buffer. The SPI module may keep track of pointer (SPIrxp)  224  to the memory location where the received data was stored and the pointer (CPUrxp)  222  to the memory location from which the data is to be read by CPU  102 . When those pointers are not equal, data is available to be sent to the CPU. In one exemplary embodiment, the level at which interrupt signal  103  is sent is adjustable and may be programmed such that no transmission occurs until a threshold is reached, e.g. 4 bytes. This option provides flexibility in how full the buffer can get before interrupting the CPU to start to empty the buffer. 
     The above described technique can include “cycle stealing” features. In accordance with another aspect of the present invention, cycle stealing can be employed such that the direct memory access module waits until the CPU is not using the bus interface before the DMA module transfers data between the serial peripheral interface and memory. This can occur by the DMA sending requests to the BusIF for sending or receiving data to or from the memory and then waiting for an acknowledgment signal. Therefore, the CPU&#39;s use of the bus interface may not be interrupted in the above described process and CPU processing rate may be enhanced while also increasing the data transmission rate. The data transmission rate may be improved due to the use of, for example, FIFO buffer memory and virtual SFR&#39;S. 
     In an exemplary embodiment of the present invention, a virtual SFR technique may be applied. For example, the CPU provides data and instructions to the SPI as if the CPU were writing to a SFR; however, no physical SFR buffer exists, and instead that information is actually being written to memory. Furthermore, the SPI accesses the memory through a DMA module using cycle stealing techniques such that interference with the CPU&#39;s use of the Bus Interface is reduced. This allows the CPU to work unimpeded by this sort of data transferring. In one exemplary embodiment, an SFR access instruction may have a unique virtual address, for example E8. Once the virtual SFR access is detected, e.g., by same cycle detection, all accesses are redirected to the circular FIFO buffer memory  208 , with no additional pipelining necessary. 
     In accordance with another exemplary embodiment, the use of FIFO buffers may be avoided, and instead data may be sent from the CPU via the BusIF to the SPI for transmitting. Although the SPI may contain a shift register and a transmit buffer, this small buffer may not be sufficient to keep up with data bursts. This embodiment may have the disadvantage of requiring the CPU to wait while the SPI transmitters send the information. In another exemplary embodiment, no virtual SFR technique is used, and instead, the CPU may store the data directly to memory. In this embodiment, the CPU may not have to wait for the SPI to transmit; however, this embodiment may have the undesirable affect of requiring the CPU to keep track of the pointers to the memory location where the data was stored. In yet another exemplary embodiment, the CPU may write to a SFR where the data could then be transferred to computer memory. This embodiment can require more processing steps and still require the CPU to maintain pointers to the location where the data was stored in memory. Finally, although DMA cycle stealing techniques may be omitted from various embodiments of the present invention, these embodiments may realize undesirable delay time when the communication between the FIFO buffer and the SPI device interrupts the CPU&#39;s use of the bus interface. 
     In accordance with another aspect of the present invention, the technique avoids the firmware overhead with minimum hardware control cost. For example, compared to the hardware approach using deeper buffer structures, e.g., with FIFO buffers implemented using flip-flop devices, the current techniques utilize memory, e.g., dynamic or static random access memory (DRAM or SRAM) with direct memory access (DMA). In addition, the technique avoids the use of buffers which otherwise are required, in some applications, to store the data that the CPU wants to write/read. 
     The present invention has been described above with reference to an exemplary embodiment. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiment without departing from the scope of the present invention. For example, the various components may be implemented in alternate ways, such as varying or alternating the steps in different orders in writing or reading of data. These alternatives can be suitably selected depending upon the particular application or in consideration of any number of factors associated with the operation of the system. In addition, the techniques described herein may be extended or modified for use with other types of devices, in addition to the microprocessor and slave devices described above. These and other changes or modifications are intended to be included within the scope of the present invention.