# Patent

## Title
Output buffer strength trimming

## Abstract

Apparatus and methods for adjusting the buffer strength of an output buffer to match its capacitive load use selectively enabled stages of a multiple stage output buffer. A user can opt for a default capacitive load, or adjust the strength by enabling one or more stages of the multiple stage output buffer.

## Background
RELATED APPLICATION
This is a continuation application of U.S. patent application Ser. No. 10/701,090, filed Nov. 4, 2003 now U.S. Pat. No. 7,064,582 (allowed) titled OUTPUT BUFFER STRENGTH TRIMMING, which application claims priority to Italian Patent Application Serial No. RM2003A000029, filed Jan. 27, 2003, entitled “OUTPUT BUFFER STRENGTH TRIMMING,” which is commonly assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference


TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to buffer circuits and in particular the present invention relates to buffer circuits with adjustable strength.
BACKGROUND OF THE INVENTION
Integrated circuits are common in electronic products. Electronic products often are comprised of integrated circuits interfaced to each other via a data bus or other data paths. Interface specifications for various digital logic families delineate voltage and current levels required for digital signals to be transferred between two or more integrated circuits. Interface specifications are utilized by integrated circuits through the use of output buffer circuits to drive a logical low or logical high signal across a data path. In addition, output buffer circuits are a way of interfacing different digital logic families of integrated circuits.
Typically, output buffer circuits often use an external voltage level, Vccq, as a source of the logic high level. Depending on the design, Vccq can range from 1.6 to 3.3 volts. Output buffer circuits generally use a system ground (GNDQ) as a sink for a logic low output. Output buffer circuits generally use two complementary transistor devices. The first device is a p-channel pull-up metal-oxide semiconductor (MOS) transistor, whose source is connected to Vccq, and whose drain is connected to the output terminal. The second device is an n-channel pull-down MOS transistor, whose drain is connected to the output terminal, and whose source is connected to ground.
The output buffer of an integrated circuit must be sized large enough to provide sufficient sinking and sourcing to a load and to transmit a signal within a short time. This requirement can demand a high rate of change in the current used in the integrated circuit, and can cause significant noise due to power line voltage drops and/or ground voltage line bumps. This noise can upset nominal operation of the circuits that share a power/ground bus with the output buffers. In modern applications, where integrated circuits are assembled in very small printed circuit boards (PCBs) with specialized routing, the load that an integrated circuit must drive can be much smaller than the load used as reference to design the output driver size.
One example of an application of an output buffer is in a memory system. A memory system is commonly used in products such as digital cameras, personal digital assistants, cellular telephones, video game systems and the like. A typical memory system is used to store commands or data that will be used in conjunction with a microprocessor. With the development of faster and faster microprocessors, memory systems must also keep pace. Fast transition times are a factor in the design of increasing circuit speed. This is particularly true with respect to memory systems.
In some applications, a particular integrated circuit will be used in a variety of applications having a very wide range of output loads. For Flash memory circuits, the capacitive load is typically 30 picoFarads (pF). Most applications have loads much smaller than that. With a standard capacitive load, it is easy to size an output buffer during manufacture for a known standard capacitive load. However, since the variety of applications for which typical integrated circuits are used, a standard output buffer size is not sufficient. For example, with a strong output buffer and small load, overshoot and undershoot on output signals occurs. With a weak buffer, the buffer is insufficient to work with an application having a larger load.
Currently, many IC applications are quite sensitive to radio frequency (RF) interference, and the noise generated by a buffer harms the RF performance of the circuit board. This RF interference increases when the switching time of the loads is too fast, which occurs when the load is small.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for adjusting the strength of an output buffer according to the application for which it is to be used.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an integrated circuit on which embodiments of the present invention are practiced;
FIG. 2 is a circuit diagram of an output buffer according to another embodiment of the present invention;
FIG. 3 is a circuit diagram of an equivalent circuit to one configuration of the embodiment of FIG. 2;
FIG. 4 is a block diagram of control logic for an output buffer according to another embodiment of the present invention; and
FIG. 5 is a functional block diagram of a basic flash memory device that is coupled to a processor in accordance with an embodiment of the invention.



DETAILED DESCRIPTION
In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.
The various embodiments of the present invention provide apparatus and methods for adjusting the strength of an output buffer. This is accomplished by programming enable signals for each of a plurality of stages, and connecting the stages in parallel. Each enabled output stage adds to the strength of the output buffer, and the output buffer can be sized to accommodate a specified load.
FIG. 1 is a block diagram 100 of an integrated circuit 102 driving two secondary integrated circuits 104 and 106. The integrated circuit 102 comprises an output driver 108. Output driver 108 is a complementary metal oxide semiconductor (CMOS) structure having a p-channel transistor 110 having a size WP and an n-channel transistor 112 having a size WN. The output driver transistors 110 and 112 are connected to an external data (DQ) pin by a bonding wire 114 having an inductance Ldq. Sources Vccq and Vssq are connected to external pads by bonding wires 116 and 118, respectively, having inductances Lvccq and Lvssq. The total inductance L involved in the DQ rising is therefore L =Ldq+Lvccq. The total inductance L involved in the DQ falling is therefore L=Ldq+Lvssq. Integrated circuit 102 is connected to the inputs 120 and 122 of circuits 104 and 106 respectively.
When circuits 104 and 106 are MOS integrated circuits, their inputs load the output driver 108 with input capacitances Cin1 and Cin2. The routing capacitance on the routing line 126 between the DQ and circuits 104 and 106 is Croute 124. The routing capacitance load Cload seen by the output driver is the sum of the capacitance of the routing line plus the input capacitances of circuits 104 and 106, Cload=Croute+Cin1+Cin2. Suppose ts is the rise and fall time of the routing line 126 that must be satisfied to have the system 100 working with satisfactory performance. The size ratio WP/WN is designed to have a rise/fall time shorter than ts for the specific Cload of the circuit.
Upon a switching, a perturbation dv, which is typically a voltage drop on Vccq or a voltage bump on Vssq, occurs on the power lines. The perturbation follows this equation:

dv≧(4*Cload*Vccq*L)/ts2 

For example, if Cload=100 pF, Vccq=2 Volts (V), ts=2 nanoseconds (ns), and L=2 nanoHenries (nH):

dv≧(4*100 pF*2 V*2 nH)/4 ns2=400 milliVolts (mV)

A noise on the order of 400 mV on the Vccq power line can easily occur. If on a specific application the real load is less than 100 pF (for example 50 pF), and WP/WN remains unchanged, the noise does not decrease because the rise/fall time ts will be less than 2 ns. Reducing Cload can therefore produce an even larger amount of noise. If on a specific application the real load is less than 100 pF (again for example 50 pF), a reduction of WP/WN in order to produce the same rise/fall time ts results in a power line noise reduction of about 50% without affecting switching performance.
The embodiments of the present invention provide methods and apparatus for trimming the sizes WP/WN of an integrated circuit output buffer. In one embodiment, output buffer transistors 110 and 112 are arranged using multiple CMOS stages in parallel as shown in FIG. 2. While only two stages are shown, it should be understood that more stages are easily implementable by one of skill in the art, and are within the scope of the invention.
For example, Flash memories assembled in cellular phone platforms typically have a broad range of interconnect parasitic values. The impedance of the output buffers of Flash devices may not match the requirements of any specific application. While the rise and fall times of output buffers are generally specified for an output load of 30 pF, the capacitive load of real systems can be on the order of 15 pF or less. Test machines at a memory manufacturer's site generally exhibit a significantly larger load capacitance, generally on the order of 50-100 pF. In one embodiment, the output driver strength of a Flash memory can be configured at different levels using programmable/erasable bits or using information stored in volatile latches, to match a wide range of load capacitances. This configuration ensures fast switching speed with small output noise and power consumption at the application level and also in a probe or testing environment.
FIG. 2 is a circuit diagram of an output buffer configuration 200 according to one embodiment of the present invention. Output buffer 200 has two stages 202 and 204. Stage 202 comprises a CMOS structure having p-channel transistor 206 with size WP1 and n-channel transistor 208 with size WN1. Stage 204 comprises a CMOS structure having p-channel transistor 210 with size WP2 and n-channel transistor 212 with size WN2. A signal is presented to the gates of each of the stage 202 transistors. The transistor 206 receives a signal from OR gate 214. OR gate 214 receives as inputs an input data signal and an enable signal dq_trim1 218. Enable signal 218 is inverted with inverter 220 before being input to OR gate 214. The transistor 208 receives a signal from AND gate 216. AND gate 216 receives as inputs the input data signal and the enable signal dq_trim1 218.
Stage 204 is constructed similarly to stage 202, having transistors 210 and 212 fed by OR gate 222 and AND gate 224, respectively. OR gate 222 has as its inputs the data signal and the enable signal dq_trim2 226 which is first inverted with inverter 228. AND gate 224 has as its inputs the data signal and the enable dq_trim2 signal 226.
The stages 202 and 204 provide multiple selectable options for output buffer sizes through selection of the dq_trim signals. The final WP or WN values for output buffer 200, depending upon the enable signals, are the sums of the respective stage transistors that are enabled. When stage 202 is enabled, the WP or WN values are WP1 or WN1. When stage 204 is enabled, the WP or WN values are WP2 or WN2. When both stages are enabled, the WP and WN values are the sum of the values, WP=WP1+WP2, and WN=WN1+WN2. It should be understood that two stages are shown for simplicity and the purposes of illustration only, and that additional stages result in additional choices for determining the size of the output buffers. As such, the invention is not limited to two stages.
FIG. 3 is a circuit diagram representation of an equivalent output buffer structure 300 when both stages 202 and 204 of output buffer 200 are enabled.
Table 1 contains the possible combinations of dq_trim1 and dq_trim 2 signals, and the resulting output buffer possibilities. Each of the allowable configurations as shown in Table 1 are in various embodiments designed to meet specified rise/fall times ts.












TABLE 1







dq_trim1
dq_trim2
WP
WN









1
1
WP1 + WP2
WN1 + WN2



1
0
WP1
WN1



0
1
WP2
WN2



0
0
not allowed
not allowed










Table 2 shows approximate capacitance load values consistent with the requirement that ts must be less than a specified value, e.g., ts≦3 ns, for various choices of WP and WN.











TABLE 2







WP
WN
Cload









WP1 + WP2
WN1 + WN2
100 pF 



WP2
WN2
50 pF



WP1
WN1
30 pF










Trimming of the output buffer size is accomplished in various embodiments in different ways. For example, two ways to trim the output buffer size using the configurations described above are described below. The first uses fuses, a non-volatile technique to generate the dq_trim signals to select different output stages. The second uses latches, a volatile technique to generate the dq_trim signals to select different output stages.
The first technique uses fuses to store the value of the dq_trim1 and dq_trim2 signals. In one embodiment, the fuses are implemented in chips containing Flash electrically erasable programmable read only memory (EEPROM) chips. The choice of fuse structure is well known in the art, and it should be understood that different fuses such as laser fuses and the like may be used without departing from the scope of the invention. Using fuses allows the manufacturer of the integrated circuit employing the output buffer to permanently set at manufacture an output trim configuration according to desired customer needs.
The second technique uses latches that are loaded with a desired configuration every time the customer wishes to use the integrated circuit. The customer decides whether to use this optional configuration to trim the strength of the output buffer according to the application for which the integrated circuit is to be used.
FIG. 4 is a diagram showing one embodiment of control logic for output buffer strength trimming. In circuit 400, a bank of latches 402 is used to store information. In one embodiment, the bank of latches 402 is configured with the same structure and operation as a read configuration register (rcr), the structure and operation of which are well known in the art.
The circuit 400 produces dq_trim signals for enabling the various stages 404 and 406 of a multiple stage output buffer according to parameters of operation determined at manufacture, by application, or both. In this embodiment, each of the stages 404 and 406 comprises two CMOS transfer gates, stage 404 having transfer gates 408 and 410, and stage 406 having transfer gates 412 and 414. Depending upon the configuration desired by a user, either the fuse_dq_trim signals will be used by the stages, or the rcr signals generated by latch bank 402 will be used. Fuses fuse_dq_trim1 and fuse_dq_trim2 are set during manufacturing to provide a standard capacitive load if enabled. A fuse such as those described above sets the operation mode of the circuit 400. The signal from that fuse is f_dis_rcrtrim, which is an input to OR gate 416.
If the fuse f_dis_rcrtrim has value 1 (which is its default value in one embodiment), the signal en is 0 and enb is 1. The signals dq_trim1 and dq_trim2 are in this configuration connected to fuses fuse_dq_trim1 and fuse_dq_trim2, and provide a permanent output drive strength setting associated with a specific capacitive load. The output buffer strength is permanently set by the fuses. In this configuration, the customer does not have the ability to use the latches to trim the output buffer strength.
In another configuration, the fuse f_dis_rcrtrim is programmed during the manufacturing process to a value of 0. In this configuration, the user has the option of trimming the output buffer strength as desired using the three latches rcr_en (enable latch 418) and rcr1 and rcr2 (trim latches 420 and 422). If the user does not want to apply functionality of using the latches to selectively enable various output buffer stages to adjust the output driver strength, then the rcr_en latch 418 is not programmed. The value of rcr_en is then at its default of 1, and dq_trim1 and dq_trim1 are connected to fuse_dq_trim1 and fuse_dq_trim2 respectively as described above.
If the user desires to selectively adjust the output buffer strength of the multiple stage buffer, using the latches to trim the output driver strength, the rcr_en latch 418 is programmed to a value of 0. Latches 420 and 422 generating outputs rcr 1 and rcr 2 respectively are programmed with values DIN(j) and DIN(k). The respective outputs from latches 420 and 422 are connected to CMOS components 408 and 412 respectively. The OR gate 416 output is 0, so enb is 0 and en is 1. The signal rcr_en is then 0, and dq_trim1 and dq_trim2 are connected to latches rcr1 and rcr2 respectively. The latch values are used to adjust the strength of the output buffer as desired. Default power up values for rcr1 and rcr2 are 1. In this configuration, the CMOS components 408 and 412 of stages 404 and 406 are enabled, and depending upon the rcr1 and rcr2 values, either or both of the stages are enabled, resulting in a selectable output buffer size.
Output buffer strength trimming is static in the sense that the buffer strength is set for a given Cload. No feedback from actual rise/fall time is used to trim the output strength. Trimming on a permanent basis as discussed above is configured permanently if fuse f_dis_rcrtrim is programmed to a 1 value. This is set after the rise/fall time performances and Cload are known. Trimming is optionally set up using latches 420 and 422 programmed by the user for a specific rise/fall time performances and PCB load.
It should be understood that enabling the signals f_dis_rcrtrim, rcr_en, and the like can be accomplished in a number of ways, and that such enablement is within the scope of one of skill in the art. Further, the embodiments of the present invention, while described with respect to two output stages, are scalable to greater than two output buffer stages. As the number of stages increases, the area on an integrated circuit also increases, but the number of stages is not limited.
The embodiments of the present invention use no feedback from rise/fall time of the circuit. There is no internal monitoring of the rise/fall time. For example, to drive C of 10 pF with a desired rise time of 5 ns, the WP/WN values are assigned to make the buffer strength to yield a 5 ns rise time when driving 10 pF capacitive load. No adaptive monitoring and adjustment of the rise time is required or necessary.
FIG. 5 is a functional block diagram of a basic flash memory device 601 that is coupled to a processor 603. The memory device 601 and the processor 603 may form part of an electronic system 600. The memory device 601 has been simplified to focus on features of the memory that are helpful in understanding the present invention. The memory device 601 includes an array of memory cells 605.
The memory cells may be non-volatile floating-gate memory cells arranged in rows and columns, with the rows often arranged in blocks. A memory block is some discrete portion of the memory array 605. The memory cells generally can be erased in blocks. Data, however, may be stored in the memory array 605 separate from the block structure.
A row decoder 609 and a column decoder 611 are provided to decode address signals provided on address lines AO-Ax 613. An address buffer circuit 615 is provided to latch the address signals. Address signals are received and decoded to access the memory array 605. Column select circuitry 619 is provided to select one or more columns of the memory array 605 in response to control signals from the column decoder 611. Sensing circuitry 621 is used to sense and amplify data stored in the memory cells. Data input 623 and output 625 buffers are included for bi-directional data communication over a plurality of data (DQ) lines 627 with the processor 603. The DQ lines 627 provide access to data values of memory cells of the memory array 605. A data latch 628 is typically provided between data input buffer 623 and the memory array 605 for storing data values (to be written to a memory cell) received from the DQ lines 627. Data amplified by the sensing circuitry 621 is provided to the data output buffer 625 for output on the DQ lines 627. The data output buffer 625 includes at least one output buffer circuit in accordance with an embodiment of the invention.
Command control circuit 631 decodes signals provided on control lines 635 from the processor 603. These signals are used to control the operations on the memory array 605, including data read, data write, and erase operations. Input/output control circuit 633 is used to control the data input buffer circuit 623 and the data output buffer circuit 625 in response to some of the control signals. As stated above, the flash memory device 601 has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of flash memories is known to those skilled in the art. As is well known, such memory devices 601 may be fabricated as integrated circuits on a semiconductor substrate.
CONCLUSION
The various embodiments of the present invention provide apparatus and methods for adjusting the strength of an output buffer. Each of multiple stages of an output buffer may be enabled, and each enabled stage adds to the strength of the output buffer. The output buffer contains a bank of latches for programming the enable values to enable at a user's specification appropriate stages for providing an output buffer strength suitable to the capacitive load of the application.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.

## Claims

1. An output buffer, comprising:
a first stage and a second stage, the first and second stages having outputs connected parallel to one another, the first stage to provide buffer strength upon receipt of a first stage enable signal, and the second stage to provide buffer strength upon receipt of a second stage enable signal, wherein each output stage comprises:
a complementary metal oxide semiconductor (CMOS) structure having a p-channel MOS device and an n-channel MOS device;
an AND gate having an output connected to a gate of the n-channel transistor, and having two inputs, an input connected to a data signal and another input connected to one of a plurality of enable signals provided by one of a plurality of enable signal generators; and
an OR gate having an output connected to a gate of the p-channel transistor, and having two inputs, an input connected to the data signal and another input connected to a complement of the one of the plurality of enable signals.

2. The output buffer of claim 1, wherein the first and the second stages are additive when both are enabled.

3. An output buffer circuit, comprising:
a first output buffer stage to provide an output buffer strength in response to a first stage enable signal; and
at least one second output buffer stage, wherein each second output buffer stage is adapted to selectively provide additional buffer strength in response to a respective second stage enable signal;
wherein each output buffer stage comprises:
a complementary metal oxide semiconductor (CMOS) structure having a p-channel MOS device and an n-channel MOS device;
an AND gate having an output connected to a gate of the n-channel transistor, and having two inputs, an input connected to a data signal and another input connected to one of a plurality of enable signal generators each to provide one of a plurality of enable signals; and
an OR gate having an output connected to a gate of the p-channel transistor, and having two inputs, an input connected to the data signal and another input connected to a complement of the one of the plurality of enable signals.

4. A memory device, comprising:
an array of memory cells;
a plurality of data lines for access to the memory cells; and
an output buffer coupled between the array of memory cells and the plurality of data lines, wherein the output buffer comprises:
a first stage and a second stage, the first and second stages having outputs connected parallel to one another, the first stage to provide buffer strength upon receipt of a first stage enable signal, and the second stage to provide buffer strength upon receipt of a second stage enable signal, wherein each output stage comprises:
a complementary metal oxide semiconductor (CMOS) structure having a p-channel MOS device and an n-channel MOS device;
an AND gate having an output connected to a gate of the n-channel transistor, and having two inputs, an input connected to a data signal and another input connected to one of a plurality of enable signals provided by one of a plurality of enable signal generators; and
an OR gate having an output connected to a gate of the p-channel transistor, and having two inputs, an input connected to the data signal and another input connected to a complement of the one of the plurality of enable signals.

5. A system, comprising:
a processor;
a memory device coupled to the processor, the memory device comprising:
an array of memory cells;
a plurality of data lines for access to the memory cells; and
an output buffer coupled between the array of memory cells and the plurality of data lines, wherein the output buffer comprises:
a first stage and a second stage, the first and second stages having outputs connected parallel to one another, the first stage to provide buffer strength upon receipt of a first stage enable signal, and the second stage to provide buffer strength upon receipt of a second stage enable signal; wherein each output stage comprises:
a complementary metal oxide semiconductor (CMOS) structure having a p-channel MOS device and an n-channel MOS device;
an AND gate having an output connected to a gate of the n-channel transistor, and having two inputs, an input connected to a data signal and another input connected to one of a plurality of enable signals, each enable signal provided by one of a plurality of enable signal generators; and
an OR gate having an output connected to a gate of the p-channel transistor, and having two inputs, an input connected to the data signal and another input connected to a complement of the one of the plurality of enable signals.

6. A method of adjusting an output buffer strength, comprising:
selectively enabling at least one output buffer stage in response to only a respective one of a plurality of enable signals for the respective stages, one of the plurality of enable signals presented to each stage, the plurality of enable signals selectable from a plurality of enable signal generators, wherein selectively enabling at least one output buffer stage comprises:
providing a first enable signal to a first enable input of each of the output stages; providing a second enable signal to a second enable input of each of the output stages; and
selecting between the first and the second enable signal and a selectable signal, the selectable signal provided by a selectable input fuse to generate an active selectable input enable signal and to provide a non-volatile enable signal set.

7. A method of adjusting an output buffer strength, comprising:
selectively enabling at least one output buffer stage in response to a single one of a plurality of enable signals for the respective stages, wherein selectively enabling comprises:
providing an enable signal to each of the at least one buffer stage, wherein each enable signal is stored in a latch; and
enabling each output stage for which its enable signal is active.

8. A method for adjusting a strength of an output buffer, comprising:
generating enable signals for a plurality of parallel output buffer stages, wherein generating enable signals further comprising selecting one of a plurality of enable signal generators to generate the enable signals;
enabling each stage having an enable signal; and
combining the strength of each enabled stage into a total buffer strength;
wherein enabling each output stage comprises:
a complementary metal oxide semiconductor (CMOS) structure having a p-channel MOS device and an n-channel MOS device;
supplying two input signals to an AND gate having an output connected to a gate of an n-channel metal oxide semiconductor (MOS) transistor in a complementary MOS, the input signals comprising a data signal and one of a plurality of enable signals; and
supplying two input signals to an OR gate having an output connected to a gate of a p-channel MOS transistor in the CMOS, the input signals comprising a data signal and a complement of the one of the plurality of enable signals.

9. A method of trimming an output buffer, comprising:
enabling at least one of a plurality of parallel connected output stages, each enabling accomplished by providing a single one of a plurality of enable signals to each of the output stages, the one of the plurality of enable signals provided by selecting between a plurality of enable signal generators each providing a separate plurality of enable signals; and
combining a strength of each of the enabled output stages into a total output buffer strength, wherein enabling at least one of a plurality of parallel connected output stages further comprises:
providing a first enable signal to a first enable input of each of the plurality of parallel connected output stages:
providing a second enable signal to a second enable input of each of the plurality of parallel connected output stages; and
selecting between the first and the second enable signal and a selectable signal, the selectable signal provided by a selectable input fuse to generate an active selectable input enable signal and to provide a non-volatile enable signal set.