Patent Publication Number: US-6215705-B1

Title: Simultaneous program, program-verify scheme

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to electrically erasable and programmable non-volatile memories. More particularly, this invention relates to a method and circuit that speeds up the programming and program verification operations of a memory device by simultaneously programming in one bank and verifying the programming in another bank of a memory device. 
     Electronic systems typically include processors and memory. The memory is used to store instructions and data. In some systems, such as cellular phones, non-volatile memory is needed to ensure that the stored data is not lost even when the system is turned off. One non-volatile memory family is Read Only Memory (ROM), Programmable ROM (PROM), and Erasable-Programmable ROM (EPROM), with varying degrees of flexibility of use. ROM memories have high density, low power consumption, and high performance, but they are not in-system updateable. On the other hand is the volatile memory family of Random Access Memory (RAM), Dynamic RAM (DRAM), and battery-backed Static RAM (SRAM). The RAM family, however, is in-system updateable and has high performance, but it is volatile. DRAM stores temporary data, and SRAM integrates a battery to retain stored data when system power is removed. SRAM is considerably more expensive than DRAM. Electrically-Erasable-Programmable ROM (EEPROM) is a special kind of ROM that is in-system modifiable on a byte-by-byte basis, like RAM, but it is also non-volatile, like ROM. 
     Flash memory is one type of inherently nonvolatile memory, with no refresh or battery requirements, which can be read, erased, or programmed in units of memory such a sectors or banks. It is a variation of EEPROM which, unlike flash memory, is erased and rewritten at the byte level, which is slower than flash memory updating. Flash memory is often used to hold control code such as BIOS in personal computer. When BIOS needs to be changed, the flash memory can be updated in block (rather than byte) sizes, making it easy to update. Flash memory is used in digital cellular phones, digital cameras, LAN switches, PC cards for notebook computers, digital set-up boxes, embedded controllers, and other devices. Flash memory is in-system updateable. Its simpler cell architecture (only one transistor) gives it significant density advantages over both EEPROM and SRAM, and compares favorably with densities achieved by ROM and DRAM on analogous manufacturing processes. Finally, flash memory is the only approach to satisfy the desired characteristics of nonvolatility, upgradeability, high density, and low cost. 
     One problem with prior flash memories is that they do not provide sufficient random access. For example, prior flash memory devices typically do not allow a processor to perform a program-verify operation while a program operation is underway in the memory device. Typically, the processor periodically polls a status register of the flash memory device to detect the end of the program operation before initiating a program-verify operation of the memory device. That is, a processor should wait until a program cycle is complete before initiating a program-verify cycle, because both program and program-verify operations cannot be performed simultaneously. 
     While prior art memory systems allow simultaneous read and write operations, they suffer from the problem that they can only do asynchronous (not clocked) memory read operation in combination with another embedded memory operation, such as embedded memory program or embedded memory erase operations. Embedded memory operations are command-sequence-mode operations which are carried out by writing a specific address and data sequence into a sequence register. Because asynchronous memory read operation is not a command-sequence-mode (embedded) operation, rather it is a direct-mode-operation like a default operation, prior art memory systems are not capable of simultaneously performing two command-sequence-mode (embedded) operations such as memory erase, memory write, and memory program-verify operations. Therefore, there is a need for an efficient flash memory device that allows simultaneous embedded program and program-verify operations in the same cycle. 
     BRIEF SUMMARY 
     By way of introduction only, the present invention provides simultaneous program and program-verify operations for a non-volatile memory device. In one preferred embodiment, a memory device is divided into two or more banks. Each bank includes a number of sectors. Each sector includes a set of memory cells. Each bank has a decoder that selectively receives an address from an input address buffer or from an internal address sequencer controlled by an internal state machine. When one bank receives a program command, the internal state machine takes control and starts the program operation, while the other bank can be accessed for program-verify operation. 
     One preferred embodiment of the present invention is to split the memory into upper bank and lower bank. The data to be programmed is also split into even-addressed data words and odd-addressed data words. The even-addressed data words and the odd-addressed data words are programmed alternatively into upper bank and lower bank, respectively. After programming a word of data into either upper bank or lower bank, the processor initiates the program-verify operation of the same word of data, while simultaneously programming the next word of data into the other bank. This process is repeated over time until the last word of data is programmed and program-verified. Therefore, there is always a word of data being-verified while another word of data is being programmed, during the same cycle. 
     Additional objects, features and advantages of the various aspects of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a block diagram of a memory device. 
     FIG. 2 is a block diagram of a memory unit according to one embodiment; 
     FIG. 3 is a flow chart for describing a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a non-volatile memory device  100  that enables simultaneous program and program-verify operations according to one embodiment. Memory device  100  includes an address input (Addr)  102 , a data input/output (Data)  104 , power inputs (Vcc, Vss, and Vpp)  106 , and control inputs  108 . The control inputs  108  include Reset (RESET#), Write Enable (WE#), Chip Enable (CE#), Clock (CLK), and Output Enable (OE#). The Chip Enable signal activates the chip&#39;s control logic and input buffers (not shown). When Chip Enable is not asserted, the memory device operates in standby mode. Output Enable is used to gate the outputs of the device through I/O buffers during read cycles. Write Enable is used to enable the write functions of the memory device. In one embodiment, all of the components of FIG. 1 are contained on a single integrated circuit chip. 
     Memory device  100  is configured into an upper bank  110  and a lower bank  112  that are arrays (or sectors) of flash memory cells. However, other non-volatile memories are also within the scope of the present invention. The memory banks  110  and  112  are arranged in arrays of memory cells with pre-determined numbers of rows and columns. The address decode logic for upper bank  110  includes X decode  114  and Y decode  116 . The address decode logic for lower bank  112  includes X decode  118  and Y decode  120 . 
     State machine and control logic  122  provides the control for read, program, program-verify, and erase operations. Many of the selection lines used to select between upper bank and lower bank are controlled by state machine and control logic  122 . Alternatively, some sector decoders may be provided whose output can be used to select between banks of memory cells. A more detailed description of the memory device  100  in FIG. 1 can be found in U.S. Pat. No. 5,867,430, Bank Architecture For A Non-volatile Memory Enabling Simultaneous Reading and Writing, and U.S. Pat. No. 5,847,998, Non-volatile Memory Array That Enables Simultaneous Read and Write Operations. These patents are assigned to the assignee of the present invention and are incorporated herein by reference. 
     Memory device  100  is programmed using an embedded programming command sequence and is program-verified using an embedded program-verify command sequence. The embedded sequences allow a processor to initiate a program or program-verify sequence and perform other tasks while the program and program-verify operations are being carried out. The embedded program and program-verify sequences are controlled by state machine and control logic  122 , which uses a command register (not shown) to manage the commencing of either command sequence. The program and program-verify operations are accessed via the command register, which controls an internal state machine that manages device operations. Commands are written to the command register via the data input  102  to memory device  100 . 
     While one bank is being programmed, the other bank can be accessed for a program-verify operation. For example, during a program or program-verify operation of a data word in upper bank, state machine and control logic  122  would cause multiplexer  124  to select the upper bank address for communication to decoders  114  and  166 . Similarly, during a program or program-verify operation of a data word in lower bank, state machine and control logic  122  would cause multiplexer  126  to select the lower bank address for communication to decoders  118  and  120 . 
     Now, a detailed description of one embodiment of the present invention is given in reference to FIG.  2  and FIG.  3 . FIG.2 illustrates how a word of data is programmed or written in the memory banks. As mentioned before, the even-addressed data words  201  are written into the upper bank  202 , while the odd-addressed data words  203  are written into the lower bank  204 . FIG. 3 shows a flowchart  300  showing the detailed process of simultaneous program and program-verify according to one embodiment of this invention. The left portion of FIG. 3 illustrates the sequence of acts for programming and program-verifying the even-addressed data words  201  into the upper bank  202  in FIG.  2 . The right portion of FIG. 3 illustrates the sequence of acts for programming and program-verifying the odd-addressed data words  203  into the lower bank  204  in FIG.  2 . Initially, in act  301 , the first even-addressed word of data, which is located at address A0 (Hex), is programmed into the upper bank  202  in FIG.  2 . At the completion of this programming act, the two acts of  302  and  305  are initiated simultaneously. In act  302  and  303 , while the programming of the first even-addressed data word in the upper bank is verified for correctness, the first odd-addressed data word, which is located at address A1 (Hex), is programmed into lower bank in act  305 . At the completion of act  303 , if the programming operation of the first even-addressed data word into the upper bank was successfully performed, as determined by “Y” in act  303 , then the second even-addressed data word, which is located at A2(Hex), is written into the upper bank in act  304 . However, if the programming operation of the first even-addressed data word into the lower bank was not successfully performed, as determined by “N” in act  303 , then the same data has to be programmed again into the upper bank. The sequence of acts  302 ,  303 , and  304  are repeated until the last even-addressed data word is successfully programmed and program-verified. In the meantime, after the programming the first odd-addressed data word into the lower bank in act  305  is complete, its program-verify operation is delayed, through acts  306  and  307 , until the program-verify operation of the fist even-addressed data word in the upper bank is complete. Then, the program-verify operation of the first odd-addressed data word in the lower bank is carried out in act  308  and  309 , simultaneously with the program operation of the second even-addressed data word into the upper bank in act  304 . If the programming operation of the first odd-addressed data word into the lower bank was successfully performed, , as determined by “Y” in act  309 , then the programming of the second odd-addressed data word, which is located at address A3(Hex), is delayed, through acts  310  and  311 , until programming of second even-addressed data word into upper bank is complete. However, if the programming operation of the first odd-addressed data word in lower bank was not successfully performed, as determined by “N” in act  309 , then the same data has to be programmed again into the lower bank, but only after making sure, through acts  313  and  314 , that there is no programming operation going on in the upper bank. The sequence of acts  305 ,  306 ,  307 ,  308 ,  309 ,  310 ,  311 ,  312 ,  313 , and  314  are repeated until the last odd-addressed data word is successfully programmed and program-verified. 
     Therefore, while one memory bank is being programmed with a word of data, the other memory bank is simultaneously being program-verified. Consequently, the programming operation is made faster and more reliable.