Abstract:
In the present invention an EEPROM flash memory is operated using the I/O pins of an EPROM. A novel circuit is used that allows a plurality of voltages to be applied at different times to a single pin designated as CEB (chip enable bar) that permits reading and writing of the flash memory chip. The plurality of voltages can range from a positive voltage, to a ground voltage and to a negative voltage. When a positive voltage like Vdd is applied to the the CEB pin the chip is disabled and entered into a standby mode. When a ground voltage is applied to the CEB pin, the flash memory chip is enabled and a read operation can be performed. When a high negative voltage is applied to the CEB pin, the circuit of the present invention produces an internal high negative voltage to be used for a write operation.

Description:
The instant application claims priority to U.S. Provisional Application Ser. No. 60/280,342 filed Mar. 30, 2001, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor memories and in particular a a circuit for multiple input voltages for use with EEPROM flash memory. 
     2. Description of the Related Art 
     High voltage charge pump circuits and area consuming state machines are built into flash EEPROM memory chips for systems requiring programmability. The use of these chips in systems that do not require programmability are a waste of space and result in increased die cost and noise. For EPROM memories there are no requirements for high voltage charge pump circuits and on chip state machines. Any data changes for EPROM memory relies on electrical program and ultra-violet (UV) light erase operations. The UV erase changes the threshold voltage Vt to a low state and requires a costly package with a quartz window to allow the UV light to reach the surface of the chip die. 
     After a predetermined UV erase time of around 15 minutes, the electrons will be ejected out of the floating gate of all EPROM cells. This results in the Vt of all cells to be lowered to approximately ±1.2V resulting in a logical “1”. The UV erase operation is similar to chip erase in a flash EEPROM. After UV erase, any selected data change from “1” to “0” requires an electric program operation. The program operation is done by either a high current channel hot electron (CHE), or by low current Fowler Nordheim (FN) schemes. For a CHE program, a positive high voltage ranging from +10V to +12V is required. By contrast, FN program operation requires a negative high voltage ranging from −10V to −12V. For today&#39;s low voltage single power supply memory, a positive high voltage is generated from an on chip charge pump. High voltage supply pins, like VPP as required in traditional EPROM memory, are removed from the standard pin configuration of a flash EEPROM. The negative high voltage supply VNN is not needed for the traditional EPROM memory and is created on chip for the flash EEPROM. The VNN can be used either for FN program or erase operations. The penalty for on chip positive and negative charge pumps is an increase in die area and cost. In addition, on chip charge pumps generate substantial noise, which degrades the system performance and causes operation failures at low chip bias voltage Vdd. 
     U.S. Pat. No. 5,848,000 (Lee et al.) is directed toward the use of a plurality of voltage terminals to receive a plurality of voltages to facilitate accurate and flexible read, erase and program operations for a flash memory. U.S. Pat. No. 5,748,538 (Lee et al.) is directed toward providing a flash electrically erasable programmable read only memory (EEPROM), where writing, e.g. erasing and programming, of a selected cell uses an FN tunneling method. 
     For those applications requiring no in system programmability and having a strong demand for low die cost and low noise, the on chip charge pumps and state machine must be removed. Therefore, off chip positive (VPP) and negative (VNN) high voltage power supplies are required. Also, adding VNN to any existing selective pins of an EPROM, a pin compatibility for JEDEC standard has to be maintained. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to remove the on chip high voltage negative and positive charge pumps thereby eliminating the noise and achieving stability of a low Vdd operation and with the negative high voltage VNN being supplied from an external pin for the erase operation; 
     Another object of the present invention is to remove the on chip state machine to reduce the die area and required circuit design for cost reduction; 
     A further objective of the present invention is to add VNN to the preferred pin set of the traditional EPROM, and thereby meeting full compatibility in the read operation for those applications requiring no in system programmability; 
     Still a further objective of the present invention is to add VNN to the preferred pin set of a standard EPROM and maintain the JEDEC standard; 
     Still another further objective of the invention is to provide a novel input circuit solution that can accept three voltages for different operations of flash EEPROM in MTP (multiple time programmable memories) application using the same package pin and without a costly package using a quartz window. 
     The present invention teaches a novel input circuit design that can accept more than three supplied voltages for different operations of EEPROM flash memory using the same package pin. These voltages include a first voltage of positive power supply VDD, or any positive voltage; a second voltage of ground level, or Vss; and a third voltage of a negative power supply VNN, or any negative voltage. The voltages VDD, VSS and VNN are supplied to the same input pad for different flash operations at different times. For example, in one embodiment, the VNN, VDD and VSS are supplied to the CEB pin of an EPROM memory. In this EPROM memory, CEB pin is traditionally used as a chip enable pin. 
     When VDD is connected to the CEB pin, the EPROM chip is disabled and is set into the standby mode. In this condition, the chip will not consume any power and all input and output buffers are disabled. The output buffers are in tri-stated (high impedance) state. When ground is connected to the CEB pin, the chip is enabled. The output buffers are released from tri-state mode and output their stored data. Unlike a UV-EPROM, an electric erase operation is needed for data change for the flash memory. In accordance with the present invention, the electric erase operation requires a negative high voltage which is supplied to the CEB pin. This negative high voltage is approximately around −10V, or less, to allow flash cells to perform an FN erase operation, which is also required in other flash operation such as erase verify. 
     The operating principles of the three voltage input circuit of the present invention is described in reference to FIG.  1  through FIG.  6 . The three different voltages include a first voltage of positive power supply VDD, or any positive voltage; a second voltage of ground level of VSS; and a third voltage of negative power supply VNN, or any negative voltage. According to the present invention, VDD, VSS and VNN are all supplied to the same chip input pad for different flash operations at different times. In order to be consistent with traditional EPROM devices, the selection of the pad for the three voltage input circuit can be used for CEB (chip enable bar), OEB (output enable bar), PGMB (program bar), or other functions depending on the density of the memory. 
     The design targets of the present invention are: 1) To select a traditional EPROM&#39;s pin that can be added to the erase function and make the pin a multiple-function pin. The preferred pin selection is varied for different density of EPROM memory. 2) To allow the selected pin to accept more than three voltages such as VDD, VSS and VNN, or other voltages between VDD and VNN. Where VDD is the positive power supply, VSS is the ground voltage, and VNN is the external negative power supply of around −10V for performing a write operation. The write operation requires the external negative supply of VNN for either program or erase operations according to the present invention. 3) To allow the selected multi-function pin to provide proper external voltages for either read or write operations. For example, when the read function depends on the selected pin, CEB is selected to couple VNN to the chip. Then during the read operation the multi-function pin functions as CEB (Chip-Enable-Bar) to control enable or disable of the chip. During an erase operation the multi-function pin is used to connect a high negative voltage, such as VNN, to the chip. 4) to allow the requirement of device breakdown to be kept below IVNNI+VDD, where IVNNI is defined as the absolute value of the VNN voltage. With VNN=−10V and VDD=+3V, the breakdown voltage of IVNNI+VDD is 13V. 5) to allow the multi-function pin VNN/GEB to consume zero standby current during a read operation. 
     In summary, a total of three voltages, positive, ground and negative, are designed to be coupled to the same input pin, CEB, of a flash memory. The preferred circuit of the present invention connected to the CEB pin is designed to handle the three voltages, to consume zero DC power during a read operation and to be free from breakdown during erase operation. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     This invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 shows the three voltage input circuit of the present invention; 
     FIG. 2 shows the diagram of the non-overlap generator of the present invention; 
     FIG. 3 shows the detailed circuit of the VSS switch of the present invention; 
     FIG. 4 shows a VNN level shifter design of prior art; 
     FIG. 5 shows the novel VNN level shifter of the present invention; and 
     FIG. 6 shows the simulation waveforms for the circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The operating principles of the three input voltage circuit of the present invention will be described with reference to FIG.  1  through FIG.  6 . The three different voltages include a first voltage of positive power supply VDD or any positive voltage, a second voltage of ground level of VSS and a third voltage of a negative power supply VNN or any negative voltage. VDD, VSS and VNN are supplied to the same input pad of a flash memory chip for different flash operations at different time. Thus allowing an EEPROM operation using a pin set of an EPROM. A negative high voltage, VNN, is externally supplied to be used to perform either FN program or FN erase operations depending upon the flash technologies. Program and erase operations are referred to as write operation subsequently in the present invention for simplified description. On-chip charge pumps and the state machine are removed to save area, cost and noise reduction. 
     FIG. 1 shows the overall diagram of the three voltage input circuit. The external input pad is referred as VNN/CEB pad. The circuit connected to the VNN/CEB pad is an input buffer with multiple input functions for different operations. VNN is the negative high voltage supply for write operation and CEB is the chip enable input for read operation. The output of the circuit is VNNI, which produces voltage switching between VSS and VNN during Write operation. During a read operation, VNNI is logically connected to CEB. In either read or write operation, the switching of VNNI between VSS and VNN can be achieved by properly controlling the VNN level shifter  300  and the VSS switch  200 . The non-overlap generator  100  is used to control the switching so that the peak current flowing from the VNN/CEB pad to VSS is substantially reduced with little resulting noise. The resistor divider comprising resistors  40  and  50  connected in series provide an intermediate negative voltage less than VNN for protecting devices from breakdown in the VNN level shifter  300  during a write operation. The VNN_ACTB  30  signal is used to enable the write operation. The ERS_PLS  40  is a signal input that controls switching between VNN and VSS whenever the presence of VNN is detected. 
     In FIG. 1 is shown a preferred embodiment of the present invention in which CEB (chip enable bar) is selected to select the VNN voltage in accordance with the present invention. The VNN/CEB pad  5  is the regular CEB pad of an EPROM memory in a read operation. The selection makes the pin configuration of the flash chip fully compatible with the JEDEC standard of the traditional EPROM memory. The VNN/CEB pad  5  is the only external input pin of this circuit, which couples to the proper voltage supplied from the external pad for multiple operations at different times. When pad  5  is coupled with VNN, a VNN detector  15  will switch the flash memory chip into a write operation. When VNN of −10V is present on the VNN/CEB pad  5 , the detector will generate a same phase signal of 0V called VNN_ACTB  30 . When VNN toggles either from VDD or VSS to VNN, VNN_ACTB  30  will toggle from VDD to VSS accordingly. The circuit output, VNNI  90 , will become the internal VNN supply source. 
     Continuing to refer to FIG. 1, during a read operation, the VNN/CEB pad  5  operates as CEB. When CEB is coupled to ground level, the chip is enabled for a read operation. When CEB is coupled to a VDD level, the chip is powered down and operating current is shut off to below 1 uA. During write operation, the VNN/CEB pad  5  operates as a VNN pad. The VNN/CEB pad  5  becomes the negative high-voltage power supply during a write operation. The NMOS device M 1  with the semiconductor bulk tied to the source node  10  is made in a P-well within an N-well on P-substrate to avoid leakage current due to a forward junction induced by a read operation. Transistor M 1  isolates the internal node of VNN_WELL  10  from the VNN/CEB pad  10  during a read operation. As a result, there is no DC current that will flow from VNN/CEB pad  5  to ground through the resistor divider R 1  ( 40 ) and R 2  ( 50 ). The standby current is less than 1 uA. 
     Continuing to refer to FIG. 1, during a write operation, the VNN/CEB pad operates as VNN. VNN_ACTB detects the presence of the VNN signal of approximately −10V during the write operation. When VNN is −10V, both VNN_WELL ( 10 ) and VNNI ( 90 ) become −10V. When the signal of VNN_ACTB is at a ground level, and the chip will enter into a write operation. ERS_PLS  40  is designed to control the pre-determined write time for a write operation. During a write operation, ERS_PLS stays at a VDD level. The preferred write time depends on the chosen flash technology for the product. The Write time could vary from 1 ms to several hundred ms, depending on memory size for the write operation. The higher the density of the flash memory for a write operation, the longer the write time that is required. The purpose of resistor divider R 1  ( 40 ) and R 2  ( 50 ) is to generate a medium negative voltage VMN  20  to protect the devices in the VNN level shifter  300  and as well as M 3  during a write operation. As an example, VMN is designed to be around −2V when VNN is −10V. The −2V is generated from the resistor divider comprising of 800K ohm for R 1  ( 40 ) and 200K ohms for R 2  ( 50 ) when the VNN_WELL  10  is at −10V during a write operation. The values of R 1  and R 2  are optimized to reduce the write current flowing from the VNN/CEB pad  5  to VSS. The use of the multiple input voltage circuit  500  allows an on chip negative charge pump and a state-machine to be removed from the flash memory chip to save area and cost, and reduce noise. 
     Continuing to refer to FIG. 1, the VNN level shifter  300  and VSS switch  200  are both controlled by the non-overlap generator  100 , which is driven by ERS_PLS  40 . During a write operation, the VNN level shifter  300  is used to connect VNNI  90  to VNN_WELL  10  and thereby coupling −10V to VNNI. During a read operation, VMMI  90  is connected to VSS by the VSS switch  200 . Since M 3  and M 4  are designed to be big devices for better speed, non-overlap switching of M 3  and M 4  is required to reduce the transient current from VNN/WELL to VSS. The non-overlap operation is controlled by the non-overlap generator  100 . The non-overlap generator  100  is optimized to prevent transistors M 3  and M 4  from being turned on simultaneously. This is achieved by controlling both VNNG  70  and VSSG  80  to not be at a high level at the same time. 
     In FIG. 2 a detailed circuit diagram of a non-overlap generator  100  is shown. The delay  11   a  and  11   b  are designed to ensure that no overlapped period exists for the IN and INB signals in order to eliminate the large peak transient current flowing from VNN/CEB to VSS when the circuit toggles from a read to a write operation or vise versa. The IN  50  and INB  60  signals are directly coupled to the inputs of VNN level shifter  300  and VSS switch  200  as shown in FIG.  1 . The input of the non-overlap generator is connected to ERS_PLS  40 , which is the signal used to control the predetermined write period for a write operation. 
     Continuing to refer to FIG. 2, the non-overlap generator  100  comprises of two NAND circuits  12   a  and  12   b , three inverters  14 ,  13   a  and  13   b , two delays  11   a  and  11   b , two outputs IN and INB, and one input ERS_PLS. The delay circuits  11   a  and  11   b  are designed to ensure there is no period of overlap for the signals of IN and INB during the switching from read to write, or vise versa. This eliminates a large peak transient current flowing from VNN/CEB pad  5  to VSS when there is a toggle from read to write operations, or vise versa. The outputs of the non-overlap generator  100 , IN and INB, are directly coupled to the input  50  of the VNN level shifter  300  and input  60  of the VSS Switch  200 . The ERS_PLS signal is used to control the predetermined write period for a write operation. During a read operation, ERS_PLS is kept at ground level, and while in a write operation, ERS_PLS is kept at VDD level to facilitate write time control. 
     In FIG. 3 is shown a detailed circuit of the VSS switch  200 . It comprises a PMOS transistor M 31  with a source connecting to INB in series with a NMOS transistor M 32  with a source connected to VNNI. The output VSSG is connected to the gate of transistor M 4 , shown in FIG.  1 . The common gate of transistors M 31  and M 32  is grounded and common output of the transistors is coupled to VSSG. Transistor M 32  is a NMOS device formed in an N-well on a P-substrate to avoid a forward biased junction during a write operation. The signal of INB switches between VDD and VSS, while VSSG switches between VDD and VNNI. 
     Continuing to refer to FIG. 3, the Vss switch circuit comprises a PMOS transistor M 31  with source connecting to INB. Transistor M 31  is in series with an NMOS transistor M 32  with a source connected to VNNI. The INB input signal is coupled to the output of the non-overlap generator  100 . The output VSSG  80  of the VSS switch circuit  200  is connected to the gate of M 4  as shown in FIG.  1 . The gates of transistors M 31  and M 32  are grounded and the common output is tied to VSSG. Transistor M 32  is a NMOS device formed in an N-well on P-substrate to avoid leakage current caused by a forward junction during a write operation. The signal of INB switches between VDD and VSS, while VSSG switches between VDD and VNNIDuring a read operation, the VSS switch circuit  200  consumes no current. 
     Continuing to refer to FIG. 3, When the input INB is VDD and VNNI is VSS, VSSG is VDD. When INB is switched to VSS, transistor M 31  is turned off. Then when VNNI is made negative, transistor M 32  is turned on and the negative voltage on VNNI is passed to VSSG. This creates less voltage stress for transistor M 31 , which has a voltage of VSS-VNNI. Otherwise if INB is left at VDD, the voltage stress on transistor  31  will be much larger, VDD-VNNI. 
     In FIG. 4 is shown a VNN level shifter circuit  600  of prior art. The single input IN is referred to as “in toggling” between VDD to VSS. The output VNNG is referred to as VNNG switching between VNN and VDD. This circuit is intended to shift the output voltage from VDD to VSS to VNN to VDD. The drawback of this prior art design is that a higher device breakdown voltage is required. For example, If VNN is −10V and VDD is 3V, the requirement for device breakdown voltage is 13V for all devices of M 20   a , M 20   b , M 22   a , M 22   b , M 24   a  and M 24   b.    
     In FIG. 5 is shown a novel VNN level shifter  300  which requires lower device breakdown voltage than prior art. Instead of single input, three inputs are used to achieve the requirement of lower device breakdown voltages. The three inputs include VMN, IN and VNN_ACTB. Instead of VDD being directly connected to the gates of M 34   a  and M 34   b , a medium negative voltage VMN is used to lower the voltage drop across M 34   a  and M 34   b  during write operation. The output VNNG is designed to toggle between VSS and VNN levels, instead of toggling between VDD and VNN levels as with the prior art. 
     Continuing to refer to FIG. 5, the novel VNN level shifter requires a lower device breakdown voltage of 10V. Instead of a single input as with the prior art shown in FIG. 4, three inputs, IN, VMN and VNN_ACTB, are included in the circuit of FIG. 5 to achieve the requirement of lower device breakdown voltages. Unlike the prior art of FIG. 4, where VDD is directly connected to the gates of M 22   a  and M 22   b , a medium negative voltage VMN in FIG. 5 replaces VDD and is connected to the gates of M 34   a  and M 34   b  to lower the voltage drop across transistors M 34   a  and M 34   b  during a write operation. The output VNNG is designed to toggle between VSS and VNN levels, instead of toggling between VDD and VNN levels. 
     During erase or write operation, the voltage inputs of the level shifter, VNN_ACTB goes to VSS and VNN_WELL goes negative (e.g.−10V). The output voltage VNNG of the level shifter is switched between VSS and −10V under the control of input signal IN. An inverter  39  is used to invert the input signal IN and generate a signal INB at the output of the inverter circuit  39 . Signal IN is connected to the gates of transistors M 30   a  and M 32   b , and signal INB is connected to the gates of transistors M 32   a  and M 30   b . Assuming IN is a high voltage VDD, and INB is a low voltage VSS, then during an erase or write operation, when the source voltage VNN_ACTB goes to VSS, the PMOS transistors M 30   a  and M 30   b  cannot be turned on by their gate voltages IN and INB, respectively, and do not control the switching of the level shifter  300 . The level shifter is switched by the NMOS transistor M 32   a  and M 32   b , because IN and INB can turn on transistors M 32   b  and M 32   a , respectively. 
     Continuing to refer to FIG. 5, assuming IN is VDD, it will strongly turn on the NMOS transistor M 32   b , and pull the level of node  31   b  up to VSS since VNN_ACTB is VSS. When VMN is applied with a negative voltage (e.g. −2V), both PMOS transistors M 34   a  and M 34   b  are turned on. Node  33   b  will be charged to VSS by transistor M 34   b . Node  33   b  will turn on NMOS transistor M 36   a  to discharge the node  33   a  to the negative voltage (e.g. −10V) of VNN_WELL. The voltage of node  33   a  then turns off the NMOS transistor M 36   b , so that there is no leakage current flowing from node  33   b  to VNN_WELL through transistor M 36   b . Node  33   a  will pass −10V to node  31   a  through transistor M 34   a . Because the threshold voltage of PMOS transistor M 34   a  drops, node  31   a  will be discharged to VMN+Vtp, approximately −1.3V (when VMN is −2V and Vtp is 0.7V). 
     Continuing to refer to FIG. 5, because voltage INB is VSS, the NMOS transistor M 32   a  will be turned on, which drives node  31   a  to VSS−Vtn=−0.7V when Vtn is 0.7V. Therefore, the voltage of node  31   a  is either −1.3V or −0.7V depending on the PMOS transistor M 34   a  and the NMOS transistor M 32   a . The device sizes of transistors M 34   a  and M 32   a  are selected to limit the leakage current. In this way, all the voltages in nodes  31   a ,  33   a ,  31   b , and  33   b  are between VNN_ACTB (0V) and VNN_WELL (−10V), and source to drain voltage stress is always less than 10V. Compared with the prior art shown in FIG. 4, which has a source to drain voltage of 10V+VDD, the present invention significantly reduces the device breakdown voltage requirement. The purpose of adding the source follower devices PMOS transistors M 34   a  and M 34   b  is to clamp the voltage of nodes  31   a  and  31   b , otherwise node  31   a  will be discharged to −10V and cause a large leakage current to flow through the NMOS transistor M 32   a.    
     Continuing to refer to FIG. 5, during normal operation, ANN_ACTB returns to VDD and VNN_WELL returns to VSS, and the PMOS transistors M 30   a  and M 30   b  can be turned on or turned off by IN and INB. The input VMN is applied with VSS (0V) to turn on the PMOS transistors M 34   a  and M 34   b . The PMOS transistors M 30   a  and M 30   b , long with the cross-coupled NMOS transistors M 36   a  and M 36   b  form a conventional level shifter allowing the output signal VNNG to be switched between VNN_ACTB (VDD) and VNN_WELL (VSS) during the normal operation. 
     FIG. 6 shows simulation waveforms for the circuit of FIG.  1 . Four different voltages levels, −10V, −2V, 0V and VDD, are shown for the various waveforms. VDD varies from 1.8V to 5V. The VNN/CEB waveform shows the use of three of the voltage levels for three different operations, VDD for chip disable, 0V for chip enable and a read operation, and −10V for a write operation. The VNN_ACTB waveform shifts from VDD to 0V when entering a write operation so that the transistors in the level shifter circuit  300  are prevented from breakdown. The ERS_PLS waveform, which is the signal that controls the write period, is shown with a VDD pulse of defined width. The VNN_WELL waveform shows a change from 0V to −10V when a voltage of −10V is applied to the VNN/CEB pad to initiate a write operation. The VMN waveform is shown switching from 0V to −2V when the VNN/CEB signal goes to a −10V, which lowers the voltage drop across transistors M 34   a  and M 34   b  during a write operation. The waveform VNNG (output of the level shifter  300 ) is shown switching from 0V to −10V when −10v is applied to VNN/CEB pad  5 , then switching to 0V when ERS_PLS switches to VDD to initiate a write operation. The waveform VNNG switches back to −10V when the write operation is completed and before the −10V is removed from the VNN/CEB pad  5 , and finally switches back to 0V when −10V is removed from the VNN/CEB pad  5 . The output of the switch  200  is shown in the VSSG waveform, which switches to −10V when a written operation is initiated by signal ERS_PLS. The output VNNI of the VNN/CEB circuit of the present invention is shown switching from 0V to −10V under the timing control of ERS_PLS, which controls the write period for a write operation. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.