Abstract:
Methods and apparatus for trimming a reference circuit. A representative technique includes transmitting a constant signal (e.g., a constant current or voltage). The constant signal is received (e.g., at a device pin or other contact). The constant signal is compared to a reference signal. Variables are obtained for program/erase pulses from a user. The reference circuit signal is adjusted to match the constant signal by sending program/erase pulses to the reference circuit. The program/erase pulses are set based on the variables for program/erase pulses and a result of comparing the constant signal with the reference signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to the field of nonvolatile memories (NVMs). More particularly, the invention relates to embedded NVMs. Specifically, a preferred implementation of the invention relates to trimming reference circuits for use in any NVM product, including stand alone and embedded NVMs. 
     2. Discussion of the Related Art 
     A reference circuit is typically used as a reference against which the performance of other circuits may be measured. In NVMs, the reference circuit typically needs to be calibrated to appropriate threshold voltages before it may be used. 
     Conventional embedded threshold voltage reference bits are typically trimmed to their desired value by a manual method. This manual method requires a tester for the chip first to program the bit, typically using an externally-timed program mode. The tester then reads the bit&#39;s threshold voltage using a threshold voltage read mode. If the threshold voltage is too high, the tester applies an erase pulse, typically using an externally-timed erase mode. The tester then continues the process of reading the threshold voltage and erasing until the threshold voltage reaches the desired value. 
     To test a flash EEPROM, a tester provides a constant current through a device pin, and logic external to the NVM module will trim the reference cells to the correct target voltage range, as determined by the manufacturer. For NVM products, a reference circuit must be trimmed to the correct threshold voltage such that all operations (read, program, erase, etc) work correctly. Current methodology requires the tester to measure current through a high precision Parametric Measurement Unit (PMU), make decisions on whether to apply program or erase pulses, and apply the program or erase pulses until the PMU measures a current within the target range. 
     Currently, automated flash EEPROM array reference circuit trimming utilizes complicated external voltage, current measurements, and timing control mechanisms that are supported by using production test platforms. The process requires synchronization of DUTs (Devices Under Test) that are being tested, which in itself utilizes complex control software that is run on intelligent test platforms. These intelligent test platforms may automate the process, but the platforms require complex hardware and software to use. They typically use high precision power supplies and PMUs. Due to the time needed to calibrate each reference circuit, this entire process results in long test times, long resolution times to synchronize high precision supplies and PMUs, and may only allow a limited number of DUTs to be tested in parallel due to tester hardware limitations in parts such as the power supplies and PMUs. 
     Accordingly, the need for a simple and inexpensive test and trim tool for NVM reference circuits has not been fully met. The current testing tools on the market require complex and expensive software and testing equipment and take a long time to accomplish their task, resulting in lost time. There is a need for a solution that will quickly and simply test and debug NVM reference circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
     The accompanying FIGURE is a block diagram representative of a hardware embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     In general, the context of the invention can include an on-chip self trim circuit. The context of the invention can also include a flash EEPROM tester. 
     An embodiment of the present invention may be used in microcontrollers with embedded nonvolatile memories (NVMs) and stand alone nonvolatile memories. It may also be used with any type of trimmable reference device, including but not limited to a non Floating Gate type NVM cell (SONOS; silicon-oxide-nitride-oxide-semiconductor) which may require a fixed reference cell to improve a read disturb issue (a natural characteristic of the SONOS technology). It can be a cost effective solution for testing flash memory. A reference cell which is identical to the core cells is preferred as it will exhibit the same behavior as the core cells in regards to voltage, temperature, and frequency. 
     Trimming methods in use today require an intelligent test platform equipped with high precision power supplies and PMUs to perform the reference circuit trimming operation. An intelligent test platform is one that can control the reference test current/voltage and send commands to the chip being tested as changes are perceived to be necessary by the test platform. 
     Another method currently in use self trims the NVM reference cell by comparing a current to a current across a fixed reference resistor and trying to match the voltage across the reference resistor. Hardware variabilities and process variations make this method less accurate than the techniques described in the present disclosure. 
     In an embodiment of the invention, a tester provides a constant signal (e.g., a constant current or voltage) through a contact such as but not limited to a device pin (representing a much cheaper tester hardware requirement), and internal logic within the NVM module self-trims reference cells to the correct target range. This hardware design allows for flexibility in the design implementation to make this invention economically feasible. This embodiment allows for testing in low cost ranged test systems as reference circuit trimming are self contained within the DUT. 
     One embodiment of the invention is shown in the accompanying FIGURE. The FIGURE is a block diagram of a hardware implementation. It shows that test registers  100  are coupled to a state machine  110 , which may be coupled to a grouping of logic and hardware elements called the ref_all block  120 . This ref_all block  120  compares a test current with the current in the chip and trims the reference cells accordingly. This ref_all block  120  contains a “Reference Array High Voltage Write Control” block  130  that is coupled to a reference array  140 . This reference array  140  is coupled to both a reference current mirror  150  and a comparator block  160 . The comparator block  160  also receives input from an external current source. The reference current mirror  150  outputs to a receiving block that is outside of the ref_all block  120 . 
     In this embodiment, an external current source  170  is applied to the ref_all block  120  through the comparator  160 . The comparator  160  compares the external current with the current of the DUT reference circuit/array  140 . The results of the comparison is transmitted to state machine  110 . In a different embodiment, external current source  170  may actually be a voltage source (not illustrated as such). The external current source  170  may also be replaced by a band gap circuit (not illustrated) that is internal to the NVM. 
     The self trimming state machine  110  is unique in that it is programmable to tweak and optimize signal and transistor conditions as the silicon process is improved. (i.e. flow, bias conditions, pulse width, etc.) The state machine  110  receives inputs from the test registers  100  whose contents are input by the user. The contents of the test registers  100  may include variables for program/erase pulses/signals sent to the reference circuit/array  140  such as pulse width, bias conditions, and an algorithm which controls the program/erase signals. In a different embodiment, state machine  110  may be software or hardware, or it may be realized with a CPU, as is known in the art. 
     Based on the results from the comparator  160  and the user-input variables stored in the test registers  100 , the state machine  110  instructs the “Reference Array High Voltage Write Control” module  130  to send program/erase pulses to the reference array  140 . These pulses change the transistor characteristics of the DUT by changing the bias on the transistor, which in turn, changes the reference circuit voltage. 
     The current of the reference circuit/array  140  after the program/erase pulses have been applied is output to both the comparator  160  and a reference mirror  150  which outputs the current of the reference circuit/array  140 . This current output by the reference mirror  150  may be used for transistor characterization purposes, i.e. IV curves. 
     One embodiment of the present invention, as applied to an EEPROM, is described as follows. A long erase pulse is applied to the EEPROM to erase a reference bit (refbit) below a target vt. Then, a program voltage is set in a test register, and a pulse width is set in a pulse width control register. The external reference current is then set, and the refbit is programmed by the external reference to match the external reference current. 
     A special self trimming reference circuit, an embodiment of the invention, may be embedded into a Flash EEPROM module, along with the reference circuit (cells). Where the reference cells are set and how they may be efficiently trimmed requires the use of characterization data on the actual reference module. This happens well after the final design of the reference module. 
     Embodiments of the invention (e.g., those utilizing a constant current through a device pin) advantageously allow one to meet an inexpensive tester-hardware requirement. Further, such embodiments provide for more robust trimming resolution and accuracy (e.g., for a NVM reference cell). 
     All timing sequences for timing the reference cells may be internally controlled by the circuit through synchronization and voltage switching by the program/erase pulses. Test time throughput is maximized by eliminating the overhead of tester/DUT handshaking for synchronization, current measurements and voltage. 
     The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. 
     All the disclosed embodiments of the invention disclosed herein can be made and used without undue experimentation in light of the disclosure. It will be manifest that various substitutions, modifications, additions and/or rearrangements of the features of the invention may be made without deviating from the spirit and/or scope of the underlying inventive concept. It is deemed that the spirit and/or scope of the underlying inventive concept as defined by the appended claims and their equivalents cover all such substitutions, modifications, additions and/or rearrangements.