US 7142477 B1
A memory interface system and method are provided for transferring data between a memory controller and an array of storage elements. The storage elements are preferably SRAM elements, and the memory interface is preferably one having separate address bus paths and separate data bus paths. One address bus path is reserved for receiving read addresses and the other address bus path is reserved for receiving write addresses. One of the data bus paths is reserved for receiving read data from the array, and the other data bus path is reserved for receiving data written to the array. While bifurcating the address and data bus paths within the interface is transparent to the memory controller, the separate paths afford addressing phases of a read and write address operation to be partially overlapped, as well as the data transfer phases. This will essentially reduce the cycle time between a read and write memory access, and proves useful when maximizing the data throughput across the data bus when implementing double data rate (QDR) mechanisms.
1. A memory interface system for addressing an array of storage elements, comprising:
a first latch coupled to receive a read address and a write address;
a first path and a second path coupled to an output of the latch for receiving respective said read address and said write address;
a storage device coupled to the second path; and
a multiplexer having a first input coupled to the first path and a second input coupled to the second path for sending the read address into the array before sending the write address stored by the storage device into the array.
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8. A method for accessing an array of storage elements, comprising:
storing upon an input to a multiplexer a write address sent over a write address path;
sending upon another input to the multiplexer a read address sent over a read address path in parallel with the write address path;
sensing read data from the array of storage elements sent across a read data path read data accessed by the read address; and
while sensing read data, sending write data across a write data path to be written to the array at the write address.
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1. Field of the Invention
This invention relates to semiconductor memory and, more particularly, to a memory interface that transparently separates the read and write address and data buses to achieve a faster sequential read and write cycle time.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
Most computing systems consist of four functional blocks: a microprocessor (or CPU), memory, input/output (I/O) interface, and an I/O device. The CPU is interconnected to the memory and the I/O interface via an address bus and a data bus. The address bus provides a path in which the CPU can select certain storage locations in which data is stored. The data bus provides a path over which data is transferred between the CPU, memory, and the I/O interface. Most microprocessors handle bits in 16, 32, or 64 bit groups. Thus, the address bus and data bus are normally 16, 32, or 64 bits wide.
The mechanism by which a CPU or I/O interface accesses memory depends on the type of memory being accessed. There are numerous types of memories available in the marketplace. For example, data can be stored in magnetic memory, such as a hard disk drive, or stored in memory elements upon an integrated circuit, sometimes referred to as “semiconductor memory.” Semiconductor memory is typically arranged closer to the CPU or execution unit than a hard disk drive and, therefore, can be accessed much faster than magnetic memory.
Common to semiconductor memory is an array of storage elements. Depending on the type of semiconductor memory, each storage element can have a significantly different architecture and function. For example, a storage element can be volatile or non-volatile. Types of volatile memory include memory that must be periodically refreshed (DRAMs) or memory that will lose its programmed state if power is removed (SRAMs).
The differences between SRAMs and DRAMs are fairly significant. Each storage element of SRAM includes latch and pass transistors. Conversely, each cell of DRAM involves simply one transistor and a capacitive storage element. While DRAMs are significantly denser than SRAMs, DRAMs require additional support circuitry to coordinate the accesses of each element, along with the need to periodically refresh that element.
SRAMs typically implement complementary bit lines and bit line signals and enjoy the benefits of faster access times than DRAMs. SRAMs are oftentimes used as the primary cache of the CPU, whereas DRAMs are generally used as the main semiconductor memory. SRAM has a faster access time than DRAM since performance of a read operation simply involves asserting an address, asserting a chip select line, and a read/write enable signal. The requested data will then appear sometime thereafter upon the data lines.
As used herein, the term “access time” for a read operation, for example, is the time between when an address is placed on the address bus and the addressed data appears on the data bus. Access time often gauges the speed of the memory, which is the time from assertion of a valid address to valid data (read operation), or to completion of the write into the array (write operation).
Even with fast access time associated with SRAM, one memory access cannot be rapidly followed by another memory access. The time from one memory access to the next is often referred to as the “cycle time.” For SRAM, cycle time is generally equal to the access time. Therefore, in an SRAM, a write operation must wait until the read operation has completed before the write operation can begin. This is due primarily to the address bus and data bus needing to be free of the previous operation before new addresses and data are presented on those respective buses. The problem of having a cycle time constraint on conventional SRAM is made more profound with the advent of newer double data rate (DDR) SRAM.
DDR memory allows data to be captured at a rate of twice the frequency of the clock signal sent along the control bus. This is accomplished by utilizing a 2n prefetch architecture, where the internal data bus of the DDR memory is twice the “n” width of the external data bus to allow data capture of twice per system clock cycle. A special form of DDR, when implementing both read and write accesses during the same cycle is referred to as quad data rate (QDR) SRAM. Under QDR transfer mechanisms, the internal data bus allows external data to be captured at four times per system clock cycle. Details of the difference between single data rate (SDR) and DDR, and the ramifications for QDR memories can be found in “General DDR SDRAM Functionality,” Micron Technology, 2001 (herein incorporated by reference).
While both SDR and QDR memory devices generally include the same array of storage elements, the input/output memory interface is considerably different. For example, QDR utilizes a differential pair of system clock signals to formulate the triggering rising and falling clock edges, and data strobe signals are needed to drive the data signal to and from the QDR-accessed memory banks. The differential system clock signals and the data strobe signals can allow accesses to occur and data to be strobed every one-half cycle of the system clock. Data throughput can, therefore, be substantially increased at a 2× factor.
While it would be beneficial to implement QDR SRAM with faster access time than DRAM, it would also be beneficial to reduce the cycle time between accesses. A desired solution should be one that can implement QDR SRAM accesses, but with subsequent accesses occurring partially concurrent with the previous accesses in order to reduce the cycle time, and therefore take full advantage of a high-speed system clock implemented in a QDR methodology.
The problems outlined above are solved by an improved semiconductor memory and, more specifically, a QDR SRAM that can output data in multiples of each every clock cycle of the system clock. The system clock, synchronous with the data strobe, is sent with the control bus between the functional elements of the CPU, memory, and I/O interface. By transferring data to and from the memory each one-half clock cycle of the system clock, data throughput across the data bus is substantially increased. To take full advantage of this increased throughput, however, cycle times of a read operation, followed by a write operation (i.e., read access followed by a write access) is reduced. The reduced cycle time is preferably achieved using separate address and data buses internal to a memory interface of the SRAM. While the memory and data buses are bifurcated within the memory interface, the data and address buses are not bifurcated outside of the memory interface and, specifically, are not bifurcated between the interface-fitted memory and a CPU or I/O interface. Therefore, outside of the memory interface, neither the address bus nor the data bus appear as bifurcated and, in fact, are not bifurcated. Only internal to the interface, and transparent to a user and system designer, is the address bus and data bus bifurcated.
Bifurcation of the address and data buses are achieved by forming two paths in the interface. A first path of the address bus can receive a read address, and a second path can receive a write address. The write address can be stored in one or more registers along the second path. While the read address can be multiplexed into the array first, the write address is nonetheless prefetched and stored at or near the input of the multiplexer in readiness for the subsequent write operation. The output of the multiplexer is connected directly into the array and, therefore, all prefetching involved with decoding and making the address available has previously taken place, so that the write address can be immediately presented to the array once the data is read from the array during the previous read operation. Separating the read and write address paths into a bifurcated address bus allows faster addressing and, specifically, overlapping a portion of the write address with the read address to lower the cycle time.
In addition to the bifurcation of the address bus, the data bus is also bifurcated internal to the memory interface. As data is being read from the array using sense amplifiers, the data can be stored in buffers within a read data path, while write data is sent to the array across a write data path. The data to be written to the array can occur across the write data path partially and contemporaneous with data being read from the array across the read data path. Similar to the read address and write address paths, the read data and write data paths allow concurrent movement of data and partial overlap of the read and write operations to lower the memory cycle time. The data paths are separated because the address paths are separated and, therefore, partially concurrent read/write addressing follows hand-in-hand with partially concurrent read and write data.
According to one embodiment, a memory interface system is provided. The memory interface includes a first latch coupled to receive a read address and write address. The first path and the second path are coupled to an output of the latch for receiving the read address and the write address along those respective paths. A storage device, such as a register, is coupled to the second path. A multiplexer having a first input and a second input is coupled to the respective first and second paths for sending the read address into the array before sending the write address. The array is preferably an SRAM device, and preferably the first and second paths each comprise a predecoder and a buffer. The write address is sent through the predecoder and the buffer upon the second path from the storage device, and held within a second latch whose output is directly coupled to the second input upon the first input of the multiplexer receiving the read address. The first input and the second input of the multiplexer are preferably coupled to receive the write address before a read enable signal is received upon a select pin of the multiplexer.
In addition to the memory interface system for addressing an array of storage elements, the memory interface system also includes a system for sending read data from and write data to the array of storage elements. The system which sends read data from and write data to the array includes a first data path and a second data path. The first path includes a sense amplifier and a buffer. The sense amplifier is coupled to sense read data from the array and the buffer is coupled to the sense amplifier for storing the read data. The second path includes a storage device and a write driver. The storage device is coupled to store write data sent to the array contemporaneous with the sense amplifier coupled to sense read data. The write driver is further coupled to drive the stored write data contemporaneous with the output buffer storing the read data.
According to yet another embodiment, a method is provided for accessing (i.e., addressing, reading, and writing data) an array of storage elements. The method includes storing upon an input to a multiplexer a write address sent over a write address path. Upon another input of the multiplexer, a read address is sent over a read address path in parallel with the write address path. Then, read data is sensed from the array of storage elements sent across a read data path. While sensing the read data, write data is sent across a write data path to be written to the array at the write address.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:
While the invention is susceptible to various modifications and alternative forms, specific embodiments hereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, are intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
Turning now to the drawings,
Communication between CPU 12, I/O interface 18, and memory controller 16 occur across a local bus or, possibly, a mezzanine or peripheral bus such as a PCI bus associated with I/O interface 18. The bus architecture generally involves a control bus, a data bus, and an address bus of varying widths. The functional units are typically synchronized to a master clock, oftentimes referred to as a system clock sent along the control bus. The memory controller can either by synchronized to the system clock on the control bus or synchronized to its own controller clock. Synchronizing the controller clock and the system clock at the same frequency will cause synchronous accesses to the memory and, to take advantage of the synchronous accesses, synchronous memory systems are employed. Popular synchronous memory includes SDRAM and, specifically, SRAM.
Memory controller 16 is shown connected to a memory interface 20 hereof. Memory controller 16 sends the conventional control, address, data, and system clock signals into memory interface 20 and, therefore, it is transparent to the user what memory interface 20 does to those signals by bifurcating the control and address signals as will be described herein below. The clock signal (CLK) can be used to form a data strobe, and can be formed into a complementary pair of clock signals from which a QDR operation can ensue. If so, if the system clock (CLK) transitions at, for example, 150 MHz, then the data transfer rate across the data bus might be 300 MHz if memory 14 is synchronous to CLK. Details of memory interface 20 operation and architecture are described in two embodiments, beginning with
Turning now to
Latch 24 involves either one omnibus latch or a plurality of latches coupled to receive the buffered address bits which are sampled, for example, on the rising edges of the system clock. If QDR is employed, then the address bus is sampled on both the rising and falling edges of the system clock. Latch 24 is preferably a gated latch and, more specifically, involves a transparent mode of operation. When enabled, the outputs from latch 24 will follow the inputs, and the output will hold the last value placed into the input when disabled. The inputs are gated by the clock signal (CLK). For example, beginning at the rising edge of the clock signal, the read address bit values on the address bus are sampled and passed onto latch 24 output, and during after the falling edge of the clock signal, those outputs are held in conformance with the transparent mode. However, beginning with the falling edge of the clock signal, write addresses are sampled and thereafter held after the rising edge of the clock signal.
Output from latch 24 is thereby channeled based on the phase of the clocking signal to either a read address path or a write address path. Within the write address path is a storage device 26. Preferably, the storage device includes one or more registers that sample and hold write address bit values on the rising edges of an internal clock, referred to as IN_WR_CLK. More than one register may be needed depending on the depth of the write pipeline. In the example of
The bit values sent over the read and write address paths are held on the input of multiplexer 28 or within storage device 26 (subsequently held at the input of multiplexer 28). The impedance to multiplexer 28 prevents substantial loss of voltage values so that whatever set of address bits are placed on those inputs, the values will remain for at least one cycle of the clocking signal.
A read enable (RD_EN) and a write enable (WR_EN) signal synchronized to the rising and falling edges of the clocking signal determine whether the read address bit values or the write address bit values are forwarded to a predecoder 30, buffer 32, and finally to the word-line drivers 34 of memory array 14. Thus, connected to the select input of multiplexer 28 is the read enable and write enable signal. Decoder 30 essentially functions to convert, for example, a binary coded decimal (BCD) input of 2n states to an n decimal value. The n values then feed directly to each word line of driver 34 after those values have been appropriately buffered and voltaged by buffer 32.
Whatever read or write address is placed into the word-line drivers 34, is also placed into a column select circuit 36. Although not shown for brevity and clarity in the drawing, the column select circuitry 36 responds to the incoming address, similar to driver 34, by selecting a particular pair of bit lines (bl and blb). The pair of bit lines are oftentimes referred to as complementary bit lines, with bl representing the true voltage value and blb representing the inverted voltage value. In this fashion, the address signals can address the particular storage element within array 14. Column select circuit 36 is shown placed within a data path by which incoming date (during a write operation) or outgoing data (during a read operation) traverse.
Outgoing data can be sensed by sense amplifier 38. Sense amplifier 38 essentially compares the differential voltage values on the complementary bit lines or data lines (dl and dlb). The sensed voltage difference can then be output on the complementary data input/output lines (dio and diob) onto input/output buffer 40. If, instead of a read operation, a write operation forces data to be written to array 14 that is originally sent into buffer 40, and then driven onto the column select circuit 36 via a write driver circuit 42. The operation of the read/write addressing and read/write data paths of interface 20 in
Latch 73 is also a transparent latch, similar to latch 64 in
In addition to the more elongated read and write address paths, a more elongated read and write data paths are provided. Similar to column select circuit 36, sense amplifier 38, and write driver 42 of circuit 20, circuit 60 illustrates identical function and structure for column select circuit 76, sense amplifier 78, and write driver 82. However, instead of the input/output buffer 40 being bidirectional, a substitute output-only buffer 80 is provided in the read data path. In the separate write data path in which write driver 82 is provided, registers and another buffer 84 are provided. In this fashion, read data can be stored in the output buffer 80, while write data is written into the column select circuit 76 via the write data bus, on which write driver 82 and registers 84 are coupled. This allows the read data to traverse the read data path while the write data traverses the write data path at the same time as the read data.
Circuit 60 allows the read address information to be stored and placed closer to the memory array 14 in readiness for a write enable signal than circuit 20. This ensures that the write address information is placed through the time-delayed buffer 72 b as the read address traverses the read address path. Therefore, portions of the read and write address operations can overlap one another. Moreover, circuit 60 allows the read data to be output and stored along the read data path at the same time as write data is initiated onto the column select circuit. Again, circuit 60 allows overlap of data read and write operations.
At state B (see reference 94 in
Once state D has begun, then whatever is sent along the read address path is forwarded to the word-line driver 34 to begin the read access of memory array 14. Data read from array 14 will be sensed by sense amplifier 78 and placed on output buffer 80 as the complementary data lines (dl/dlb) are released from their voltage values. This will allow the next step in the write operation to occur by driving the write driver 82 within the separate write data bus path from the read data bus path on which output buffer 80 exists. This overlap 100 is made possible due to the separate read and write data bus paths. The read access and release of the complementary data lines, sense amplifier, and maintaining the read data in the output buffer, while driving the write driver 82 are shown by states 102 and 104 (
Next, at state E, it is recognized that the read operation has completed and essentially the write address exists at the input of multiplexer 68, while the write data exists at the input of the column select circuit 76. Thus, when state E occurs by asserting the second internal clock at the input of multiplexer 68, knowing that write enable has been previously asserted at the beginning of the write cycle, the write address will be sent to the memory array to initiate a write access for the current write address. Thus, the write enable signal occurs previous to the second internal clock transitioning, to allow immediate write access once that second internal clock transitions to a high logic value. State E is shown as reference 106 in
Thereafter, at state F, the next write address is clocked into storage device 66 to initiate the write address at the output of storage device 66. States F and G are similar to states B and C, which sends the next write address to the latch and holds within the latch to await the second internal clock transition, as shown by state 108 (
Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims are to be interpreted to embrace all such variations and modifications. In addition, the foregoing description is illustrative of only exemplary embodiments. The disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. The specification and drawings are to be regarded in an illustrative rather than a restrictive sense.