a2 United States Patent
`US 6,356,105 Bl
`(0) Patent No.:
`Mar.12, 2002
`(45) Date of Patent:
`Volk
`
`
`US006356105B1
`
`IMPEDANCE CONTROL SYSTEM FORA
`CENTER TAPPED TERMINATION BUS
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`(54)
`
`(75)
`
`Inventor: Andrew M. Volk, Granite Bay, CA
`(US)
`
`(73)
`
`Assignee:
`
`Intel Corporation, Santa Clara, CA
`(US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21)
`
`(22)
`
`61)
`
`(52)
`
`(58)
`
`Appl. No.: 09/606,846
`;
`Filed:
`
`Jun. 28, 2000
`
`Int. C17 oo. eeeeeeeeeeeeees H03K 17/16; HO3K 3/01
`
`ULSI: sesscvovssernscevernroseves 326/30; 326/21; 326/86;
`327/108
`
`Field of Search .........00....cee 326/30, 21, 86,
`326/90; 327/108
`
`4,992,677 A
`5,528,172 A
`6,115,773 A
`6,118,310 A
`6,188,237 Bl
`
`*
`2/1991 Ishibashi ct al. ........... 307/443
`*
`6/1996 Sundstrom ................. 326/80
`
`*
`vee 710/129
`9/2000 Capps,Jr. etal.
`
`*
`..........
`9/2000 Esch, Jr.
`.. 327/108
`* 2/001 Suzuli et al. oo... 326/30
`
`* cited by examiner
`
`Primary Examiner—Michael Tokar
`Assistant Examiner—Vibol Tan
`(74) Attorney, Agent, or Firm—Peter Lam
`ABSTRACT
`
`(57)
`
`Amethod and apparatus for an impedance control system tor
`a center tapped termination bus. One method of the present
`invention comprises comparing an output potential of a
`buffer with a pair of reference potentials. The output imped-
`ance of the buffer is adjusted to cause the buffer output
`voltage swing to match the reference potentials.
`
`
`
`
`
`
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`
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`
`27 Claims, 7 Drawing Sheets
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`

`

`U.S. Patent
`
`Mar.12, 2002
`
`Sheet 1 of 7
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`U.S. Patent
`
`Mar. 12,2002
`
`Sheet 2 of 7
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`U.S. Patent
`
`Mar. 12,2002
`
`Sheet 3 of 7
`
`US 6,356,105 B1
`
`PROCESSOR BUS
`
`MEMORY
`CONTROLLER
`HUB
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`U.S. Patent
`
`Mar.12, 2002
`
`Sheet 4 of 7
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`Mar.12, 2002
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`

`U.S. Patent
`
`Mar. 12, 2002
`
`Sheet 6 of 7
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`US 6,356,105 B1
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`U.S. Patent
`
`Mar. 12,2002
`
`Sheet 7 of 7
`
`US6,356,105 B1
`
`SYSTEM STARTUP/RESET
`702
`
`SAMPLE BUFFER OUTPUT
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`OUTPUT WITH REF
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`

`

`US 6,356,105 B1
`
`1
`IMPEDANCE CONTROL SYSTEM FOR A
`CENTER TAPPED TERMINATION BUS
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to the field of
`computers and computer systems. More particularly,
`the
`present invention relates to an impedance control system for
`a center tapped termination bus.
`
`BACKGROUND OF THE INVENTION
`
`10
`
`Data buffers are used on a variety of integrated circuit
`devices to receive and drive data values between one device
`and another circuit coupled with to it. These data butters are
`commonly connected to data buses that connect the output
`of a buffer to a receiving buffer of another device. Since a
`numberof output buffers are often coupled to an individual
`bus, interface protocols have been developed to regulate
`signaling. Different aspects of a bus standard specify the
`impedanceof a buffer when the bufferis driving a signal, the
`terminating impedance of a buffer when the buffer is not
`driving a signal, and signal voltage swing.
`Double data rate (DDR) dynamic random access memory
`(DRAM)systemsuse a center-tapped termination (CTT)to
`control bus reflections. The buffers that drive the DDR bus
`
`15
`
`20
`
`are usually strong buffers with a series resistor to compen-
`sate the buffer to a target impedance that gives a target swing
`on the bus. An example of such system is shownin FIG. 1.
`FIG. 1 illustrates an example of a prior art system 100
`having a DDR bus 106. In this example, there are three
`agents (memory controller 102, DDR DRAM 114, DDR
`DRAM 118) coupled to the DDR bus 106. These agents 102,
`114, 116 can be a driver or a receiver on the bus 106 at
`different
`times. The buffer of memory controller 102 is
`coupled to the bus 106 through a series resistance R, 104.
`This resistance can be internal to the buffer itself and is
`
`2
`of this example receives a number of input signals: PENB
`[0::x] 202, DATA 204, OE 206, CLOCK 208, and NEN[0::x]
`210. PENB[0::x] 202 is a number of trim signals used to
`adjust the strength of adjustable P type transistor device P1
`230. Similarly, NEN[0::x] 210 is a number of trim signals
`for adjusting the strength of adjustable N type transistor
`device N1 232.
`
`The pull-up P1 230 and pull-down N1 232 transistors are
`made from multiple legs that can be cnabled or disabled by
`the strength control bits PENB[0::x] 202 and NEN[0::x]
`210, respectively. The OR gates 222 and AND gates 224 are
`only illustrative in purpose. The necessary logic varies
`depending on the particular implementation.
`DATA input 204 is coupled to data latches 218, 220
`through logic 212, 216 controlled with output enable signal
`OE 206. The outputs of data latches 218, 220 are coupled to
`logic 222, 224. Data latches 218, 220 latch and output DATA
`204 to logic 222, 224 when OE 206 and CLOCK 208 are
`activated. ‘The source terminal of P1 230 is connected to a
`VCCsupplyvoltage and the gate terminals are connected to
`the outputs of logic 222. The source terminal of N1 232 is
`connected to a ground potential and the gate terminals are
`connected to the outputs of logic 224. The drain terminals of
`P1 230 and N1 232 are connected together with a first
`terminal of resistor R, 234. R, 234 also serves to linearize
`the voltage-current output characteristics of the buffer 200 as
`well as help set the proper buffer impedance. The second
`terminal of Rs 234 is connected to pad 236.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention is illustrated by way of example
`and is not
`limited by the figures of the accompanying
`drawings, in whichlike references indicate similar elements,
`and in which:
`
`TIG. 1 illustrates an example of a prior art system having
`a DDR bus;
`integrity on the bus 106.
`present to ensure good signal
`FIG. 2 is a prior art schematic diagram of a programmable
`Similarly, the buffers of DDR DRAMs 114, 118 are also
`strength buffer;
`coupled to the bus 106 through R, 112 and 116, respectively.
`FIG. 3 is a block diagram of one embodimentof a system
`The bus 106 of this example includes a pair of impedances
`employing an impedance control system for a center tapped
`R,, 108 and R;, 110, which are termination resistances to
`termination bus;
`help control signaling quality on the bus 106. Note that the
`FIG. 4 is a schematic diagram ofafirst embodiment of an
`termination resistances R;, 108 and R;, 110 can be replaced
`impedance compensation mechanism for a center tapped
`with a single resistor with a value equal to R;, 108 and R,.
`termination bus;
`110 in parallel and connected to a different
`termination
`voltage cqual to Vrrp,(R71/(R7,+R;-)), the Thevenin volt-
`FIG. 5 is a schematic diagram of an alternative embodi-
`age.
`ment of an impedance compensation mechanism;
`FIG. 6 is a schematic diagram of one embodimentof a
`voltage swing reference generator; and
`FIG. 7 is a flow diagram illustrating how buffer imped-
`ance is compensated for one embodiment of the present
`invention.
`
`The series resistors R, 104, 112, 116 plus the buffer
`impedanceare set to give good signal integrity on the bus .
`106. The series resistor value may be different for the
`memory controller 102 and the DDR DRAM 114, 118. The
`CTTresistors set the reference level for the bus 106 around
`
`whichthe signal swingis set. The signal swing is determined
`by the ratio of buffer impedance plus R, to the Thevenin ,
`resistance of the CTT (R;, 108 inparallel to Rz, 110).
`Some drivers have been developed that
`integrate the
`resistor R, into the buffer driver. But some semiconductor
`processes do not have resistances with sufficiently good
`accuracy and temperature coefficient
`that can produce a
`buffer that will meet the DDR specifications. However, an
`impedance compensation capability is available that can be
`adapted to facilitate this need. A standard prior art
`synchronous, impedance compensated buffer is shown in
`FIG. 2.
`
`60
`
`65
`
`FIG. 2 is a prior art schematic diagram of a programmable
`strength buffer 200. The programmable strength buffer 200
`
`9
`
`DETAILED DESCRIPTION
`
`A method and apparatus for an impedance control system
`for a center
`tapped termination bus is disclosed. The
`described impedance compensation mechanism may be used
`to control impedance for butters in integrated circuits, but
`have other applications as well. The embodiments described
`herein are described in the context of a memory controller
`and a DDR businterface, but are not so limited. Although
`the following embodiments are described with reference ta
`memory controllers, other embodiments are applicable to
`other integrated circuits or logic devices. The same tech-
`niques and teachings of the present invention can easily be
`applied to other types of circuits or semiconductor devices
`
`

`

`US 6,356,105 B1
`
`10
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`15
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`20
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`40
`
`3
`that use a center tapped termination bus where the signal
`swingseither side of a reference voltage (i.e. the Thevenin
`voltage of FIG. 1) or require impedance compensation.
`In the following description, for purposes of explanation,
`numerous specific details are set forth in order to provide a
`thorough understanding of the present invention. One of
`ordinary skill in the art, however, will appreciate that these
`specific details are not necessary in order to practice the
`present invention. In other instances, well-known clectrical
`structures and circuits have not been set forth in particular
`detail in order to not necessarily obscure the present inven-
`tion.
`Embodimentsof the invention eliminate externalresistors
`while not requiring a special process with precision resis-
`tors. Unlike prior impedance or resistance compensation
`(RCOMP) methods, methods of the present invention use
`the termination available on any pin to perform the com-
`pensation. The signal swing is directly set by the compen-
`sation method. ‘he present disclosure also includes a pro-
`vision for varying the signal swing to improve signal quality
`at the receiver and adaptable to possible changes in the DDR
`specification. Sometestability capabilities such as Veer
`margining are also described.
`Embodiments of the present invention can be used to
`control the buffer strength of the DRAM when in DDR ~
`mode. This mechanism allows the DDRinterface to perform
`with better signal integrity at a lower system cost. One
`circuit is self-adjusting to accommodate a programmable
`signal swing level. A circuit designer developing a chipset
`supporting DDR may wish to use an embodiment of the
`present invention to compensate the chipset outputs, espe-
`cially as the data rate of busesrise.
`Referring now to FIG. 3, a computer system 300 is shown.
`System 300 inclides a component, such as a memory 3
`controller hub 316, employing an impedance control mecha-
`nism in accordance with the present invention, such as in the
`embodimentdescribed herein. System 300 is representative
`of processing systems based on the PENTIUM® Pro, PEN-
`TIUM® II, PENTIUM® III, Itanium® microprocessors
`available from Intel Corporation of Santa Clara, Calif.,
`although other systems (including PCs having other
`microprocessors, engineering workstations, sel-lop boxes
`and the like) may also be used. In one embodiment, sample
`system 300 maybe executing a version of the WINDOWS™
`operating system available from Microsoft Corporation of
`Redmond, Wash., although other operating systems and
`graphical user interfaces, for example, may also be used.
`Thus, the present invention is not limited to any specific
`combination of hardware circuitry and software.
`FIG. 3 is a block diagram of one embodimentof a system
`300. System 300 is an example of a hub architecture. The
`computer system 300 includes a processor 302 that pro-
`cesses data signals. The processor 302 may be a complex
`instruction set computer (CISC) microprocessor, a reduced
`instruction set computing (RISC) microprocessor, a very
`long instruction word (VLIW) microprocessor, a processor
`implementing a combination of instruction sets, or other
`processor device, such as a digital signal processor, for
`example. FIG. 3 shows an example of an embodimentof the
`present invention implemented in a single processor system
`300. However, it is understood that other embodiments may
`alternatively be implemented as systems having multiple
`processors. Processor 302 is coupled to a processor bus 310
`that transmits data signals between processor 302 and other
`components in the system 300. The elements of system 300
`perform their conventional functions well knownin theart.
`
`
`
`60
`
`65
`
`10
`10
`
`4
`System 300 includes a memory 320. Memory 320 may be
`a dynamic random access memory (DRAM)device, a static
`random access memory (SRAM) device,
`flash memory
`device, or other memory device. Memory 320 may store
`instructions and/or data represented by data signals that may
`be executed by processor 302. A cache memory 304 can
`reside inside processor 302 that stores data signals stored in
`memory 320. Alternatively,
`in another embodiment,
`the
`cache memory may reside external to the processor.
`An impedance control mechanism 306for a center tapped
`termination bus also resides in memorycontroller hub 316.
`Alternate embodiments of an impedance control mechanism
`306 can also be used in microcontrollers, embedded
`processors, graphics devices, DSPs, and other types of logic
`circuits.
`
`A system logic chip 316 is coupled to the processor bus
`310 and memory 320. The system logic chip 316 in the
`illustrated embodiment is a memory controller hub (MCH).
`‘The processor 302 communicates to the MCH 316 via a
`processor bus 310. The MCH 316 provides a high bandwidth
`memory path 318 to memory 320 for instruction and data
`storage and for storage of graphics commands, data and
`textures. The MCH 316 directs data signals between pro-
`cessor 302, memory 320, and other components in the
`system 300 and bridges the data signals between processor
`bus 310, memory 320, and system I/O 322.
`In some
`embodiments, the system logic chip 316 provides a graphics
`port for coupling to a graphics controller 312. The MCH 316
`is coupled to memory 320 through a memoryinterface 318.
`The graphics card 312 is coupled to the MCH 316 through
`an Accelerated Graphics Port (AGP) interconnect 314.
`System 300 uses a proprietary hub interface bus 322 to
`couple the MCH 316 to the I/O controller hub (ICH) 330.
`The ICH 330 provides direct connections to some I/O
`devices. Some examples are the audio controller, firmware
`hub (BIOS) 328, data storage 324, Icgacy I/O controller
`containing user input and keyboard interfaces, a serial
`expansion port such as Universal Serial Bus (USB), and a
`network controller 334. The data storage device 324 can
`comprise a hard disk drive, a floppy disk drive, a CD-ROM
`device, a flash memorydevice, or other mass storage device.
`The present enhancement
`is not
`limited to computer
`systems. Alternative embodiments of the present invention
`can be used in other devices such as, for example, handheld
`devices and embedded applications. Some examples of
`handheld devices include cellular phones, Internet Protocol
`devices, digital cameras, personal digital assistants (PDAs),
`and handheld PCs. Embedded applications can include a
`microcontroller, a digital signal processor (DSP), system on
`a chip, network computers (NetPC), set-top boxes, network
`hubs, wide area network (WAN) switches, or any other
`system which usesa latch type mechanism for other embodi-
`ments.
`
`FIG. 4 is a schematic diagram of a first embodiment of an
`impedance compensation mechanism for a center tapped
`termination bus. A standard buffer pull-up and pull-down
`driver set is used to drive a copy of the CTT resistors. The
`pull-up and pull-downare enabled in turn. While the pull-up
`is enabled, the PENB counter 450 is enabled to count up or
`down depending on whether the voltage at the RCOMPpad
`472 is higher or lower than the reference voltage VUP 426.
`VUP 426 is the desired upper output swing level for the
`buffer of FIG. 2. The input differential
`input buffer
`(comparator) COMP! 458 detects the RCOMPpad voltage
`and compares it to VUP 426 and allowsthe control logic
`state machine to determine whether to count up or down to
`
`

`

`US 6,356,105 B1
`
`5
`find the optimum pull-up strength. This process is repeated
`for the pull-down transistor using the NEN bits 424,differ-
`ential input buffer COMP2 460, and the VDN reference
`voltage 428. VDN 428is the desired lower output swing
`level for the buffer of FIG. 2. When the strength bits are set
`for the correct swing levels, then the rest of the memory
`controller interface is updated with the strength bits.
`This embodiment comprises of a control
`logic state
`machine 402,
`logic to adjust pull-up and pull-down
`transistors, an input pad, and comparators to compare the
`input voltage against references. The upper and lower halves
`of the mechanismin FIG. 4 are symmetric. CLOCKsignal
`412 is coupled to the control
`logic state machine 402,
`pull-up counter 450, pull-down counter 456, LATCH1 452,
`and LATCH2 454. CLOCK 412 allows the mechanism to
`
`10
`
`15
`
`operate synchronously and ensure proper operation.
`The control logic state machine 402 provides a numberof
`signals: PUCNTEN 404 to enable the pull-up up/down
`counter 450, PUCNTU/D 406 to direct the pull-up counter
`450 to count up or down, PUENRB408to enable the pull-up.
`selection logic 462, PDCNTEN 418 to cnable the pull-down
`up/down counter 456, PCNTU/D 416 to direct the pull-
`down counter 426 to count up or down, and PDEN 420 to
`enable the pull-down selection logic 464.
`The pull-up counter 450 provides a series of selection ,
`signals PENB[0::x] 422 through logic devices 462 to select
`and enable various legs of the adjustable P type transistor
`device Pl 466. Pl 466 has a plurality of legs that can be
`selectively enabled to adjust the strength and operation of
`the device. The numberof control bits used to control the ,
`buffer strength depends on the range and granularity needed
`to meet the DDRspecification. The strengths of P1 466 and
`N1 468 are adjusted by enabling and disabling different legs
`of the transistor devices. PENB[0::x] 422 and NEN[0::x]
`424 are coupled to a plurality of logic gates 462 and 464,
`respectively, that control the adjustments of P1 466 and N1
`468. For example, as more legs of P1 466 are enabled,
`current can be conducted morestronglyand the resistance of
`reduced. The source terminal of P1 466 is connected to a
`
`40
`
`45
`
`VCC supply voltage and the drain terminal of P1 466 is
`connected to a first terminal of resistor R, 470 and the drain
`terminal of N1 468. Resistor R, 470 linearizes the output of
`the buffer formed by P1 466, N1 468 and pad 472. The
`second terminal of R, 470 is connected to the RCOMPpad
`472. The resistor R, 470 is shownin the output path as a way
`to make the driver have a more linear output impedance
`which will give better signal integrity. The larger the value
`of R, 470, the more linear the buffer output. However, a
`larger R, 470 also requires a stronger (larger) buffer to get
`to the target impedance. One benefit of a larger buffer isa .
`better granularity.
`The pull-down portion of the circuit operates in a similar
`fashion. The pull-down counter 456 provides a series of
`selection signals NEN[0::x] 424 through logic devices 464
`to select and enable various legs of the adjustable N type ;
`transistor device N1 468. N1 468 has a plurality of legs that
`can be selectively enabled to adjust the strength and opera-
`tion of the device. The source terminal of N1 468 is
`
`connected to a ground potential and the drain terminal of N1
`468 is connectedto a first terminal of resistor R, 470 and the
`drain terminal of P1 466.
`
`60
`
`6
`from a center tapped termination bus. The upper and lower
`halves of this mechanism are operated separately. When the
`pull-up P1 466 is adjusted, the pull-down portion of the
`circuit is disabled. Similarly, when the pull-down N1 468 is
`adjusted,
`the pull-up portion of the circuit
`is disabled.
`Hence, PUCNTEN 404 and PDCNTEN 418 are mutually
`exclusive for activation.
`
`For this embodiment, the buffer upper swing impedance
`is adjusted first with the pull-up portion of the mechanism.
`Then the buffer lower swing impedance is adjusted with the
`pull-down portion of the circuit. For alternative
`embodiments, the lower swing canbe adjusted first and then
`the upper swing. Similarly, the upper and lower swings do
`not have to be fully adjusted before the other swing is
`adjusted. For instance, the impedance compensation adjust-
`ments can alternate between the upper swing and then the
`lower swing, causing both swing levels to be almost simul-
`taneously adjusted.
`Comparator COMP1 458 has the potential of RCOMP
`472 coupledto a first input and a reference voltage VUP 426.
`VUP 426 is a reference voltage that indicates the desired
`upper swing of the buffer. COMP1 458 compares RCOMP
`472 with VUP 426 andoutputs the result to latch LATCH1
`452. The latch mechanism helpsto stabilize the result for the
`control
`logic state machine 402 as the trim process is
`happening. This stabilization can be helpful especially as
`RCOMP472 approaches the appropriate reference voltage.
`LATCH1 452 latches the result and drivesthat to the control
`logic state machine 402. Control logic state machine 402
`enablesthe pull-up up/down counter 450 accordingly and P1
`adjustment logic 462. Based on the comparator result, the
`control logic state machine 402 either toggles the counter
`450 to count up or down to activate or deactivate corre-
`sponding legs of P1 466. This portion of the circuil operates
`as an analog to digital converter. The adjustments to P1 466
`alter the buffer impedance, and thus the voltage potential at
`RCOMP472. COMP1 458 again samples RCOMP472 and
`compares the newly adjusted voltage to VUP 426. The
`control logic state machine 402 evaluates the comparison
`result determines whether RCOMP 472 is at the desired
`
`level or if the pull-up 466 needs to be adjusted again.
`Depending on the particular implementation, this operation
`repeats until RCOMP472 hasreacheda tolerable level oris
`equal to VUP 426.
`The pull-down portion of the impedance compensation
`mechanism is configured in a similar manner and operates in
`a similar fashion. Comparator COMP2 460 hasthe potential
`of RCOMP472 coupled to a first input and a reference
`voltage VDN 428. VDN 428 is a reference voltage that
`indicates the desired lower swing of the buffer. COMP2 460
`compares RCOMP472 with VDN 428and outputs the result
`to latch LATCH2 454. LATCH 2 454 latches the result and
`
`drives that to the control logic state machine 402. Control
`logic state machine 402 enables the pull-down up/down
`counter 456 accordingly and N1 adjustment
`logic 464.
`Based on the comparator result,
`the control
`logic state
`machine 402 either toggles the counter 456 to count up or
`downto activate or deactivate correspondinglegs of N1 468.
`The adjustments to N1 468 alter the buffer impedance, and
`thus the voltage potential at RCOMP 472. COMP2 460
`again samples RCOMP 472 and compares the newly
`adjusted voltage to VDN 428. The control
`logic state
`machine 402 evaluates the comparison result determines
`whether RCOMP 472 is at
`the desired level or
`if the
`
`The logic devices for adjusting the legs of P1 466 and N1
`468 are shown with OR gates 462 and AND gates 464 in
`FIG. 4 for illustrative purposes. The logic of can vary
`depending on the parlicular implementation.
`is in
`When this impedance compensation circuit
`operation, the RCOMPpad 472 is supplied with a voltage
`
`65
`
`pull-down 468 necds to be adjusted again. Depending on the
`particular implementation,
`this operation repeats until
`RCOMP472 has reached a tolerable level or is equal to
`VDN 428.
`
`11
`11
`
`

`

`US 6,356,105 B1
`
`7
`There are several variations on this scheme. The one
`described above assumes a separate compensation buffer
`that updates the interface at regular intervals. The same thing
`can be done with any of the buffers of a standard interface.
`Only the two extra differential
`input buffers have to be
`attached to the pin used for RCOMP.The inputs of the input
`buffers should also be filtered to remove noise, but is not
`shown in these figures. The interface can be quieted for a
`brief time and the buffer would first be set high and the
`pull-up strength bits set, then set low and the pull-downbits
`set. This operation isfirst done at reset time, as in previous
`interfaces, and then again during DRAM refresh or other
`slack times. If the previous valucs are uscd to start the
`search,
`the time to perform subsequent searches can be
`minimized. Pull-up and pull-down devices can also be
`adjusted in alternate slack times to minimize any given
`search time. Adjustments can be made more frequently, but
`limited to a single bit of adjustment per update. This method
`
`more than covers the worst case impedance drift. If the
`
`10
`
`15
`
`updates are done during quiet times, then there is no need to :
`store and forward buffering between the RCOMPbuffer and
`the rest of the bus interface.
`
`8
`ence point. The embodiment of FIG. 6 shows four taps each
`for an upper reference and a lowerreference, although more
`could be provided. The “center” of the reference resistor
`string is shown coupled to the reference switching voltage
`for the interface. The resistors in the divider need to have
`values muchlarger than of the divider used to set the voltage
`externally so as not to disturb that external reference voltage.
`By having this connection, the internal swing values can be
`referenced to the external value and better adapted to the
`system settings. The connection to the reference voltage
`could also be omitted and the reference swing voltages can
`be simply a division of the local power supply. Once the
`specification is frozen, the default divider valuc can beset.
`The selection mechanism can also be used as a design for
`test (DFT) mechanism to margin the voltage swing andtest
`the output buffer characteristics. This or a similar voltage
`divider stack can also be used to provide the DDRREF
`voltage to the interface input buffers through a similar kind
`of mux. This reference generatorcircuit also allows the input
`voltage margin to be automaticallytested.
`The reference generatorcircuit includes a resistor chain to
`generate an upper swing reference voltage and a lower
`swing reference voltage. AP type transistor P1 620 heads up
`a first end of the resistor chain. The source terminal of P1
`
`Care should be taken not to produce glitches on clocks
`and strobes. If the DDR specification has a large enough
`range of acceptable impedances, the compensation cycle at
`rest may be sufficient. Extra granularity can be added to the
`buffers to allow for drift. Since computer system tends to
`heat up after reset, the direction of drift is known and the
`initial value can be set (biased) to account for such drift.
`FIG. 5 is a schematic diagram of an alternative embodi-
`mentof an impedance compensation mechanism to linearize
`a buffer. The circuit of FIG. 5 is similar to the embodiment
`of FIG. 4. The internal impedanceof the buffer of FIG. 4 is
`due to RS 470, P1 566, and N1 568. For this present
`embodiment, the impedanceis separated out for the pull-up
`and pull-down portions of the mechanism. One such situa-
`tion may be where the upper and lower portions of the
`mechanismsare located at different areas of the chip. In
`another implementation, the impedanceof P type and N type
`devices can be different and the additional resistance of
`
`°
`
`40
`
`620 is coupled to a VCC supply potential and the gate
`terminal 603 is coupled to a reference circuit enable signal
`VREFEN602 through an inverter 634. The drain terminal of
`P1 620 is coupled to a first terminal of R1 630. The first
`portion of the resistor chain is formed with R1 630, R2 632,
`R3 634, R4 636, and R5 638. The second terminal of R5 638
`is coupled to a pad 624. For this embodiment, the potential
`coupled to the pad 624 is a DDRREFsignal. DDRREFis a
`reference voltage for a center tapped termination bus. Alter-
`native embodiments can have another type of reference
`signal.
`The midpoint of the resistor chain should be centered
`about the reference voltage as on the pad 624. For a receiver
`on the center tapped termination bus, an external reference
`voltage is coupled to pad 624.
`Also coupled to DDRREEFpad 624 is the second portion
`different values is needed to compensate.
`of the resistor chain. The second portion of the resistor chain
`is formed with R6 640, R7 642, R8 644, R9 646, and R10
`The area of interest is the buffer portion on the right hand
`648. The second terminal of R10 648 is coupled to the drain
`side of FIG. 5. The source terminal of pull-up P1 566 is
`45
`coupled to a VCC supply potential and the drain terminal of
`terminal of N type transistor N1 622. The gate terminal of
`
`P1 566 is coupled toafirst terminal of R1 569. The second N1 622 is coupled to VREFEN 602 and the source terminal
`
`terminal of R1 569 is coupled toafirst input of comparator of N1 622 is coupled to a ground potential.
`COMP1 558and also to the RCOMP pad 572. The source
`The first portion of the resistor chain provides a plurality
`terminal of pull-down N1 568 is coupled to a ground
`of voltage potentials at taps VIII0 631, VIII2 633, VIII2
`potential and the drain terminal of N1 568 is coupled to a
`635, VHI3 637 ranging from below the VCC supply voltage
`first terminal of R2 571. The second terminal of R2 571 is
`to above DDRREF 624. However, selecting DDRREF 624
`coupled to a first terminal of comparator COMP2 560 and
`is the sameas selecting zero swing up from DDRREF, which
`also to RCOMP572.
`is not desirable. Similarly, selecting VCC is impractical
`since even if P1 devices 466, 566, were able to zero ohms,
`RCOMP472, 572, will never reach VCC for any non-zero
`value of R1 470 or 569. Taps VHIO 631, VHI1 633, VHI2
`635, VHI3 637 are coupled to the inputs of a multiplexer
`(mux) 616. Selection signals VUPSELO 604 and VUPSEL1
`608 are used to control the selection of which input value is
`driven to the mux output VUP 606. VUP 606 is the upper
`swing reference voltage used to adjust the buffer upper
`swing impedance as in the mechanism of FIGS. 4 and 5.
`The second portion of the resistor chain provides a
`plurality of voltage potentials at taps VLO3 641, VLO2 643,
`VLO1 645, VLOO 647 ranging from below DDRREF624to
`above the ground potential. However, selecting DDRREF
`624 is the same as selecting zero swing down from
`
`The operation of the mechanism in FIG. 5 is similarto that
`of FIG, 4. The control logic state machine 502 samples the ;
`potential at RCOMP 532 and adjusts the buffer upper
`(ower) swing with pull-up Pl 566 (pull-down N1 568) in
`order to match RCOMP 532 to VUP 526 (VDN 528).
`FIG. 6 is a schematic diagram of one embodiment of a
`voltage swing reference generator. This circuit operates as a
`digital to analog converter. Since the DDRspecification is
`not finalized, circuit designers mayfi