(12) United States Patent
`(10) Patent N0.:
`US 6,693,450 B1
`
`Volk et al.
`(:45) Date of Patent:
`Feb. 17, 2004
`
`U8006693450Bl
`
`(54) DYNAMIC SWING VOLTAGE ADJUSTMENT
`
`OTHER PUBLICATIONS
`
`(75)
`'
`
`Inventors; Andrew M_ Volk, Granite Bay, CA
`(US); Warren R_ Morrow, Steflacoom,
`WA (Us)
`
`(73) Assignee:
`
`.
`( * ) Notice:
`
`Intel Corporation, Santa Clara, CA
`(Us)
`.
`,
`.
`.
`Subject to any disclaimer, the term of this
`t
`t
`.
`t
`d d
`d.
`t d
`d
`35
`U.S.C. 154b b 0 d
`.
`pa en is eX en e
`or a Juse un er
`( ) y
`ays
`
`(21) Appl' NO': 09/677’117
`(22)
`Filed:
`Sep. 29, 2000
`51
`I t C] 7
`E52;
`Ci """"""""""""""""
`
`H03K 17 16
`326/33
`’ 326/34; 326/87’
`(58) Field of Search ............................. .. 326/31—34, 82,
`326/83, 87, 91
`
`(56)
`
`References Cited
`
`DeHon et al., IEEE International Solid—State Circuits Con-
`ference,
`1993,
`“Automatic
`Impedance Control,” pp.
`164—165,.
`Gabara et al., IEEE Journal of Solid State Circuits, 1992,
`27(8):]176—1185.
`Knight et al., IEEE Journal of Solid—State Circuits; 1988,
`23(2):457—464.
`
`Kushiyama ct al., IEEE Journal of Solid—State Circuits,
`1993’ 28(4):490_498.
`Pilo et al., IEEE International Solid—State Circuits Confer-
`ence, 1996, pp. 148—149.
`Takahashi et al., IEEE International Solid—State Circuits
`Conference, 1995, “A CMOS Gate Array with 600Mb/s
`Simultaneous Bidirectional I/O Circuits,” pp. 40—41.
`Trotter, et al., IEEE, 1994, “A CMOS Low Voltage High
`Performance Interface , pp. 44—48.
`* cited by examiner
`
`Primary Examiner—Anh Tran
`(74) Attorney, Agent, or Firm—Fish & Richardson PC.
`
`US. PATENT DOCUMENTS
`
`(57)
`
`ABSTRACT
`
`57134311 A *
`575967285 A
`5’677’639 A
`6,060,907 A *
`6,118,310 A *
`691779809 B1 95
`
`7/1992 Bib“ et al~ ~~~~~~~~~~~~~~~ ~~ 307/270
`“1997 Mart/0t .et al'
`10/1997 Maswwwz
`326/87
`5/2000 Vishwanthaiah et al.
`9/2000 Esch’ Jr.
`................... N 327/108
`1/2001 Tom et al.
`................. N 326/83
`
`EP
`JP
`
`FOREIGN PATENT DOCUMENTS
`0 463 316
`1/1992
`11145814
`5/1999
`
`The disclosure presents a device comprising a driver con-
`figured to transmit a signal on a bus line, including a driver
`element configured to pull against termination impedance
`.
`D
`.
`.
`.
`V
`.
`'
`The impedance of the driver element is dynamically adjust-
`able. The disclosure also presents a method of electronically
`adjusting the impedance of the driver element to regulate the
`swing voltage on the bus line.
`
`16 Claims, 5 Drawing Sheets
`
`50\
`
`and
`
` Predriver
`
`
`
`| Tuner ontroller
`
`
`
` Pull-Up
`
`Driver
`55\,
`Element
`32\
`
`
`1
`1
`
`NVIDIA 1004
`NVIDIA 1004
`
`

`

`US. Patent
`
`Feb. 17, 2004
`
`Sheet 1 0f5
`
`US 6,693,450 B1
`
`Receiver
`
`FIG. 1A
`
`\32
`
`Receiver
`
`FIG. 1B
`
`2
`
`

`

`US. Patent
`
`Feb. 17, 2004
`
`Sheet 2 0f5
`
`US 6,693,450 B1
`
`Controller
`
`
`FIG. 2
`
`3
`
`

`

`US. Patent
`
`Feb. 17, 2004
`
`Sheet 3 0f5
`
`US 6,693,450 B1
`
`Element
`
`Driver
`
`FIG. 3
`
`4
`
`

`

`US. Patent
`
`Feb. 17, 2004
`
`Sheet 4 0f5
`
`US 6,693,450 B1
`
`120
`
`
`
`SET DRIVER
`IMPEDANCE
`
`
`
`SENSE VOLTAGE
`0N DATA BUS
`
`
`126
`IS
`
`
`IS
`DATA BUS
`THERE A
`YES
`
`
`VOLTAGE T00
`
`PENDING IMPEDANCE
`L0?W
`ADJUSJMENT
`
`
`
`
`
`ADJUSTMENT
`
` CALCULATE
`IMPEDANCE
`
`
`
`
`
`
`
`IS
`IS
`DATA BUS
`THERE A
`
`
`VOLTAGE T00
`PENDING IMPEDANCE
`
`
`
`HIEH
`ADJUSJMENT
`
`
`
`ADJUSTMENT
`
`
`
`CALCULATE
`IMPEDANCE
`
`ADJUST
`IMPEDANCE
`
`FIG. 5
`
`5
`
`

`

`US. Patent
`
`Feb. 17, 2004
`
`Sheet 5 0f 5
`
`US 6,693,450 B1
`
`
`
`FIG. 6
`
`TRANSCEIVERS SET OWN PULL-UP
`IMPEDANCE TO MATCH BUS
`LINE IMPEDANCE
`
`WHEN TRANSMITTING
`
`FIRST TRANSCEIVER ADJUSTS
`OWN PULL-DOWN IMPEDANCE
`WHEN TRANSMITTING
`
`SECOND TRANSCEIVER ADJUSTS
`OWN PULL-DOWN IMPEDANCE
`
`FIG. 7
`
`TRANSCEIVERS SET OWN PULL-UP
`IMPEDANCE TO MATCH BUS
`LINE IMPEDANCE
`
`IMPEDANCE WHEN RECEIVING
`
`TRANSCEIVERS SET OWN PULL-
`DOWN IMPEDANCE AGAINST
`OWN PULL-UP IMPEDANCE
`
`SECOND TRANSCEIVER ADJUSTS
`OWN PULL-DOWN IMPEDANCE
`WHEN TRANSMITTING
`
`SECOND TRANSCEIVER ADJUSTS
`OWN TERMINATTNG
`
`FIG. 8
`
`6
`
`

`

`US 6,693,450 B1
`
`1
`DYNAMIC SWING VOLTAGE ADJUSTMENT
`
`BACKGROUND
`
`This invention relates to dynamic output
`adjustment.
`A driver is a digital electronic circuit for holding a binary
`value, and communicating it to other circuits to which it is
`connected. The binary value is represented by a voltage
`level. It is common to connect a driver to a data bus for
`
`impedance
`
`10
`
`communicating the binary value to a receiving circuit by
`“driving” the bus to a desired voltage level. In a typical
`parallel interface, one of the voltage levels (HIGH or LOW)
`is a power rail voltage, and the other voltage signal is a
`“swing voltage” away from the power rail voltage. That is,
`the difference between a voltage HIGH signal on bus line
`and a voltage LOW signal is called the “signal swing.”
`The driver has an inherent output impedance. The driver’s
`output impedance when driving the bus to a voltage HIGH r
`level may differ from the driver’s output impedance when
`driving the bus to a voltage LOW level. In addition, the bus
`has an inherent
`transmission line or characteristic
`
`15
`
`impedance, and the receiving end of a parallel-terminated
`system has an input
`impedance, called the termination ,
`impedance. To obtain a high rate of data transfer on the bus,
`the characteristic impedance should closely match the ter-
`mination impedance.
`
`DESCRIPTION OF DRAWINGS
`
`FIGS. 1a and 1b are diagrams of transmitter—receiver
`systems with parallel terminations.
`FIG. 2 is a block diagram of a driver system.
`FIG. 3 is a block diagram of a driver system.
`FIG. 4 is a circuit diagram of a pull-down driver element.
`FIG. 5 is a flowchart illustrating dynamic impedance
`adjustment.
`FIG. 6 is a diagram of a two-transceiver system.
`FIG. 7 is a flowchart illustrating dynamic impedance
`adjustment in a two-transceiver system.
`FIG. 8 is a flowchart illustrating dynamic impedance
`adjustment in a two-transceiver system.
`DETAILED DESCRIPTION
`
`FIG. 1a and 1b are diagrams showing typical parallel
`interfaces. In parallel interface 10 shown in FIG. la, a driver
`12 in a sending circuit 14 transmits data along a bus line 16
`to a receiver 18 in a receiving circuit 20. Driver 12 drives the
`voltage on bus line 16 to a desired value. The dynamic
`voltage on bus line 16 depends upon the ratio of the output
`impedance of driver 12 to transmission line impedance,
`times the pull-up voltage 24 of receiving circuit 20.
`In FIG. 1a, receiver 18 uses a pull-up resistor 22 con-
`nected to the positive voltage supply Vcc 24, thus terminat-
`ing to Vcc. A receiver may also terminate to the opposite
`power rail, such as ground, as shown in FIG. 1b. In FIG. 1b,
`a receiver 26 uses a pull-down resistor 30 connected to
`circuit ground 32. Pull-up and pull-down resistors 22 and 30
`represent pull—up and pull—down termination impedances,
`arid need not be actual resistors.
`
`Driver 12, looking down bus line 16, sees impedance due
`to the characteristic impedance of bus line 16 and due to the
`termination impedance, i.e., the pull-up or pull-down imped-
`ance. The input impedance of the receiver is typically very
`high, and because the pull-up or pull-down impedance is in
`
`40
`
`45
`
`60
`
`65
`
`2
`parallel with the receiver, the receiver’s high impedance is
`not seen by the driver.
`The characteristic impedance and the termination imped-
`ance affect the signal transmitted on bus line 16. Ideally, the
`characteristic impedance and the termination impedance
`should be matched as closely as possible to minimize signal
`reflection. If signal reflections are minimized,
`the swing
`voltage can be safely regulated by dynamically adjusting the
`output impedance of the driver.
`FIG. 2 shows a driver system 50, in Which driver 52
`includes a predriver and logic 54 and driver elements 56.
`Predriver 54 supplies a binary value to driver elements 56.
`Driver elements 56 drive the voltage on a bus line 58 to a
`desired voltage. As Will be described below, the impedance
`of driver elements 56 is programmable and dynamically
`adjustable. The impedance of driver elements 56 is regulated
`by programming signals 66 from a resistance compensation
`controller 64, called the “RCOMP Controller,” and by
`tuning signals 70 from a tuner 72. Tuner 72 senses a voltage
`feedback signal 60 from the bus line 58 to adjust
`the
`impedance and thus regulate the swing voltage.
`FIG. 3 shows driver system 50 configured to transmit data
`on bus line 58 to a receiver with pull-up impedance, such as
`receiver 18 shown in FIG. 1a. Driver system 50 can be
`configured to transmit to a receiver with pull-down imped-
`ance as well, but for simplicity, transmission to a receiver
`with pull-up impedance will be described.
`Driver elements 56 comprise a pull-up driver element 80
`and a pull-down driver element 82. The impedance of
`pull-up driver element 80 and the impedance of pull-down
`driver element 82 are programmable. Because receiver 18
`has pull-up impedance 22, pull-down driver element 82
`pulls against the termination, and therefore dynamic adjust-
`ment to the impedance of pull-down driver element 82 will
`be described.
`
`To facilitate the dynamic adjustment of the output imped-
`ance of driver system 50, the impedance of pull-up driver
`element 80 and the impedance of pull-down driver element
`82 are electronically adjustable. More specifically,
`the
`impedance of pull-up driver element 80 is controlled by a
`pull-up control signal 66 and the impedance of pull-down
`driver element 82 is controlled by a pull-down control signal
`70. The impedance of each driver element 80 and 82 is
`regulated by RCOMP controller 64. In addition, the imped-
`ance of pull-down driver element 82 is dynamically adjusted
`by tuner 72. Pull-up control signal 66 and pull-down control
`signal 70 are digital signals and may be conveyed on a
`plurality of data lines, each line carrying a single control bit.
`Signals 68 from RCOMP controller to tuner 66 likewise are
`digital signals and may be conveyed on a plurality of data
`lines.
`
`For purposes of illustration, it is assumed that a voltage
`HIGH signal on bus line 58 is at or near supply voltage Vcc
`24, and that a voltage LOW signal is ideally a certain swing
`voltage below the supply voltage.
`In FIG. 3,
`the ideal
`voltage LOW level is one-third of supply voltage Vcc 24,
`and is denoted “VSWING.” Driver system 50 is coupled to
`a regulated reference voltage 84 set
`to the magnitude
`VSWING, i.e., one-third of the supply voltage. It is further
`assumed that the relationship between the voltage on bus
`line 58 and the output impedance of driver 52 is known.
`When the termination impedance is closely matched to the
`characteristic impedance,
`the relationship is based upon
`voltage division.
`When bus line 58 is driven LOW by pull-down driver
`element 82, the resulting voltage on bus line 58 should be
`
`7
`
`

`

`US 6,693,450 B1
`
`3
`close to the value of VSWING 84. The actual voltage on bus
`line 58, however, may be above or below VSWING 84.
`RCOMP controller 64 programs the impedance of pull-
`down driving element 82. A resistor 86 connects RCOMP
`controller 64 to circuit ground 32, which in this example is
`the power rail opposite of that used by receiver 18. Avoltage
`divider is formed by resistor 86 and a copy 65 of pull-up
`driving element 80 in RCOMP controller 64 coupled to
`RCOMP line 88. The voltage divider produces an RCOMP
`input voltage 88 that is equal to VSWING voltage 84 when
`the impedance of the pull-up driving element 65 is at a
`desired value. The target impedance of the pull—down driv—
`ing element 82 is set in a similar manner, using a copy (not
`shown) of driving element 82 and another voltage divider
`(not shown) in RCOMP controller 64.
`RCOMP controller 64 initially sets pull-down driving
`element 82 impedance close to a value expected to produce
`a voltage LOW signal equal
`to VSWING 84. RCOMP
`further updates the impedance settings periodically when
`triggered by an update clock input 94. Traffic on bus line 58
`may be suspended during RCOMP updates. In the course of
`actual transmissions, however, the terminating impedance
`may be different from the expected value, or the terminal
`impedance may change due to loading at
`the receiver,
`heating or other factors. As the impedance seen by driver 52
`changes, the swing voltage changes as well, and the voltage
`LOW signal does not remain equal to VSWING 84. To
`compensate for shifts in the swing voltage, the impedance of
`pull-down driving element 82 is dynamically adjusted.
`Dynamic compensation is accomplished by feeding back
`the voltage 60 transmitted on bus line 58 to tuner 72. Tuner
`72 includes a comparator 90, which receives the feedback
`voltage 60 as one input and the regulated VSWING voltage
`84 as another input. Comparator 90 compares the two input
`voltages 60 and 84 and determines which of the two is
`higher, and produces an error signal 92. The polarity of
`comparator 90 shown in FIG. 3 is arbitrary, but for illustra-
`tive purposes VSWING voltage 84 is applied to the nonin-
`verting input. Consequently, when the voltage 60 transmit-
`ted on bus line 58 is the higher of the two voltages,
`comparator 90 generates a voltage LOW error signal, and
`when the regulated VSWING voltage 84 is higher, com—
`parator 90 generates a voltage HIGH error signal 92.
`A tuner controller 100 receives error signal 92. Tuner
`controller 100 compensates for the error by electronically
`increasing or decreasing the impedance of pull-down driver
`element 82. Tuner controller 100 includes an adder 98 to
`
`digitally increase or decrease digital pull-down control sig-
`nal 70, thereby increasing or decreasing the impedance of
`pull-down driver element 82. Tuner controller 100 can make
`adjustments in large or small increments with adder 98. To
`improve driver system 50 stability,
`tuner controller 100
`generally does not let increments exceed a certain amount,
`and does allow some degree of impedance mismatch with
`each adjustment. By repeatedly increasing or decreasing the
`impedance,
`tuner 72 “homes in” on the impedance of
`pull-down driver element 82 that produces a voltage LOW
`signal as close to VSWING 84 as possible. The adjustment
`to the impedance is performed dynamically, i.e., while driver
`52 is performing data transmission.
`Tuner controller 100 includes memory 96 to store data
`about impedance characteristics of bus line 58. Tuner con-
`troller 100 may further be configured to ignore voltage
`HIGH signals transmitted on bus line 58, because such
`signals are not pertinent to impedance adjustment of pull-
`down driver element 82. Tuner controller 100 may also be
`
`10
`
`15
`
`40
`
`45
`
`60
`
`65
`
`4
`programmed with search strategies for finding the best
`impedance of pull-down driver element 82. For example,
`tuner controller 100 may be programmed to make substan—
`tial adjustments at first, followed by smaller adjustments as
`tuner 72 homes in on the best voltage level. Tuner controller
`100 may also be programmed to recognize cases in which
`matching voltage LOW and VSWING 84 is not possible.
`FIG. 4 is a diagram of an exemplary programmable
`pull-down driver element 82. A set of n-channel metal oxide
`semiconductor field-effect transistors (MOSFETs) 106 are
`arrayed in parallel between terminals 102 and 104. Terminal
`102 is connected to output bus line 58, and terminal 104 is
`connected to circuit ground 32. The number and values of
`MOSFETs 106 that are turned on when pull-down driver
`element 100 is enabled determines the impedance between
`terminals 102 and 104. In a preferred embodiment, MOS-
`FETs 106 are sized in a binary progression to allow a wide
`range of impedance programming (e.g., between 25 and 100
`ohms) and with a sufficient number to get a sufficiently small
`granularity (e.g., about 1.5 ohms). MOSFETs 106 may be
`sized in other ways as well, such as logarithmically or
`linearly.
`The gate of each MOSFET 106 is driven by the output of
`one of a set of corresponding AND gates 108. One input of
`each AND gate 108 is coupled to one line of a multi-bit
`control line 112, which corresponds to pull-down control
`signal 70. Each control line 112 enables its corresponding
`MOSFET when HIGH and disables its corresponding MOS-
`FET when LOW. The other input to each AND gate 108 is
`a single bit data line 110, which carries the data to be
`transmitted on bus line 58. The data conveyed on single bit
`data line 110 are supplied by predriver 54. It is assumed that
`a HIGH voltage asserted on single bit data line 110 corre-
`sponds to driving output bus line 58 to its LOW voltage.
`If a MOSFET’s 106 control line 112 is LOW, MOSFET
`106 is turned off. If a particular MOSFET’s control line 112
`is HIGH, the state of that MOSFET depends on single bit
`data line 110. Thus, the values on control lines 112 deter-
`mine which MOSFETs 106 are on, and consequently deter-
`mines the impedance between terminals 102 and 104 when
`single bit data line 110 is HIGH. In the illustrative case of
`MOSFETs 106 sized in a binary progression, a binary
`number is transmitted on control lines 112 with the binary
`number corresponding to an impedance and each control
`line 112 corresponding to a bit of the binary number. Adding
`to or subtracting from the binary number increases or
`decreases the impedance.
`Although the exemplary system described above pulls
`down and functions with a receiver that pulls up,
`the
`structure of a driver system 50 that pulls up against a
`pull-down receiver is similar. In that case, pull-up driver
`element 80 may be controlled by tuner controller 72. The
`structure of programmable pull-up driver element is similar
`to the structure of programmable pull—down 82 except that
`the MOSFETs in the programmable pull-up are p-channel
`devices, the logic gates are OR gates rather than AND gates,
`and the sense of the control lines is opposite that of pro-
`grammable pull-up element 80.
`FIG. 5 presents a flowchart illustrating one mode of
`operation of system 50. The method set out in FIG. 5 is
`applicable to the case in which the data on bus line 58 are
`being transmitted by driver 52 to a receiver 18 with pull-11p
`impedance. A flowchart outlining the method applicable to
`a receiver 26 with pull-down impedance is similar. RCOMP
`controller 64 sets the impedance of driver 56 to match the
`expected value of the termination impedance (120). During
`
`8
`
`

`

`US 6,693,450 B1
`
`5
`
`operation, tuner 72 senses the voltage on bus line 58 (122).
`When the voltage on bus line 58 is too low, tuner 72 adjusts
`the impedance of driver 52. As described above,
`tuner
`controller 100 can adjust impedance up or down and can
`make adjustments in large or small increments. Tuner con-
`troller 100 may be programmed, for example, to adjust the
`impedance by a certain amount. For example, when the bus
`58 voltage is too low for two consecutive cycles, tuner
`controller 100 may be programmed to adjust the impedance
`on the current cycle by the same amount as the previous
`cycle. Tuner controller 100 may be also be programmed to
`anticipate impedance adjustments. When tuner controller
`100 has a pending pre-calculated adjustment (126),
`the
`adjustment can be made (136) without further calculation.
`Otherwise, tuner controller 100 calculates a new adjustment
`(128) and makes the adjustment (136). When the voltage on
`bus line 58 is too high, the process is similar (130, 132, 134).
`Tuner controller 100 can be programmed to forego adjust-
`ment (130) when, for example, feedback signals 60 indicate
`that
`the voltage on bus line 58 is close to but slightly ,
`exceeding VSWING 84. In other words, tuner controller 100
`can be configured to recognize those cases in which a small
`impedance adjustment would make the bus line 58 voltage
`too low.
`
`10
`
`15
`
`40
`
`45
`
`FIG. 6 shows a two-transceiver system 150. Circuits 154 ,
`and 156, which share a bus line 152, include transceivers
`158 and 160, respectively. Both transceivers 158 and 160
`include pull-up and pull-down driver elements and termi-
`nation elements, each driver and termination element having
`an adjustable impedance. While receiving, the pull-up or
`pull-down driver element may be used to form the termi-
`nation element, or alternatively the termination element may
`be an element separate from the driver elements. For
`simplicity, it will be assumed system 150 uses a protocol in
`which a voltage HIGH signal is at or near supply voltage
`Vcc 24 and a voltage LOW signal is a swing voltage below
`Vcc 24. With this assumption, the pull-up driver element
`may be used as the termination impedance while receiving.
`FIG. 7 is a flowchart showing how transceivers 158 and
`160 regulate the swing voltage on bus line 152. Prior to
`transmission,
`the RCOMP controller of each transceiver
`158, 160 adjusts the transceiver’s pull-up impedance to
`match the characteristic impedance of bus line 152 (170).
`During communication between transceivers 158 and 160,
`one transceiver will be transmitting and the other will be
`receiving. One transceiver at a time, transceiver 158 for
`example, controls the voltage on bus line 152. While
`transmitting, transceiver 158 dynamically adjusts its pull—
`down impedance to regulate the swing voltage on bus line
`152 (172). When transceiver 160 transmits and transceiver
`158 receives, transceiver 160 dynamically adjusts its pull-
`down impedance to regulate the swing voltage on bus line
`152 (174). Both transceivers dynamically adjust driver
`impedance to regulate the voltage on bus line 152,
`An alternative technique for regulating the swing voltage
`on bus line 152 is shown in FIG. 8. The RCOMP controller
`of the transceivers sets the transceivers” pull-up impedance
`to match the impedance on bus line 152 (176). The trans-
`ceivers then sets their own pull-down impedance against
`their own pull-up impedance, adjusting the pull-down
`impedance until the proper VSWING voltage value appears
`on bus line 152 (178). While one transceiver is transmitting,
`the tuner of the transmitting transceiver dynamically adjusts
`its pull-down impcdancc until the desired VSWING voltagc
`appears on bus line 152 (180). While receiving, the other
`transceiver then adjusts its termination impedance until the
`proper VSWING voltage value appears on bus line 152
`
`60
`
`65
`
`9
`
`6
`(182). Generally the termination impedance adjustment is
`subject to allowable bus line-to-terminator impedance mis-
`match. In the scenario depicted in FIG. 8, dynamic adjust-
`ments in one component based upon impedances in another
`component are performed by one transceiver.
`A number of embodiments of the invention have been
`described. These and other embodiments are within the
`scope of the following claims.
`What is claimed is:
`1. A device comprising a driver having an output
`impedance, the driver configured to transmit a signal on a
`bus line, the driver including a pull-up driver circuit and a
`pull-down driver circuit arranged in a pull-up pull-down
`configuration,
`the pull-up and pull-down driver circuits
`being independently controllable;
`a tuner to compare a voltage level of the signal to a
`reference voltage and to control at least one of the
`pull-up and pull-down driver circuits as a function of
`comparing the signal voltage level
`to the reference
`voltage to adjust the driver output impedance so that
`the signal voltage level is controlled; and
`a controller configured to electronically set the output
`impedance of the driver to a first value as a function of
`a characteristic impedance of the bus line.
`2. The device of claim 1 wherein the output impedance of
`the driver is adjustable digitally.
`3. The device of claim 1 wherein the tuner comprises
`memory.
`4. The device of claim 1 wherein the tuner comprises a
`comparator configured to receive the signal and the refer—
`ence voltage.
`5. A method comprising:
`providing a driver configured to communicate a signal on
`a bus line having a characteristic impedance;
`setting an output impedance of the driver as a function of
`the bus line characteristic impedance;
`communicating the signal to the bus line;
`while communicating the signal;
`sensing a voltage level of the signal, wherein the voltage
`level is a function of the output impedance of the driver,
`the output impedance including a pull-up driver circuit
`and a pull-down driver circuit;
`generating an error signal as a function of comparing the
`signal voltage level to a reference voltage; and
`dynamically adjusting one of the pull-up driver circuit and
`the pull-down driver circuit so that the output imped-
`ance of the driver is changed to reduce an amplitude of
`the error signal.
`6. The method of claim 5 further comprising calculating
`an adjustment to the output impedance of the driver.
`7. The method of claim 5, wherein adjusting the output
`impedance of the driver comprises adjusting the impedance
`of a driver element.
`
`8. The method of claim 5 further comprising:
`setting a termination impedance of a receiver configured
`to receive the signal on the bus line; and
`adjusting the output impedance of the receiver.
`9. A device comprising:
`a driver configured to transmit a signal on a bus line, the
`driver having a programmable impedance, the driver
`including a pull—up driver circuit and a pull—down
`driver circuit, each of the driver circuits being inde-
`pendently controllable to adjust
`the programmable
`impedance;
`a first controller configured to control each of the driver
`circuits to establish a starting impedance value of the
`driver;
`
`

`

`US 6,693,450 B1
`
`7
`a feedback circuit configured to compare a voltage level
`of the signal with a reference voltage, wherein the
`voltage level is a function of the impedance; and
`a second controller configured to control one of the
`pull-up and pull-down driver circuits independent of
`the other of the pull-up and pull-down driver circuits
`based upon comparing the voltage level to the reference
`voltage to adjust dynamically the programmable
`impedance of the driver.
`10. The device of claim 9, wherein the second controller
`is configured to adjust dynamically the impedance of the
`driver to move the voltage on the bus line closer to the
`reference voltage.
`11. The device of claim 14, wherein driver comprises:
`a signal source;
`a plurality of transistors in parallel;
`a data line coupling each transistor to the signal source;
`and
`
`10
`
`15
`
`a control line coupling to each transistor to the second '
`controller,
`wherein the state of each transistor is a function of the
`signals on the data line and the control line.
`12. The device of claim 11, wherein the output impedance
`of the device is a function of the state of the transistors.
`
`13. A method comprising:
`transceiver
`setting a termination impedance of a first
`configured to receive a signal having a voltage on a bus
`line;
`providing a second transceiver configured to communi-
`cate the signal on the bus line having a characteristic
`impedance, the second transceiver output impedance
`including a pull-up circuit and a pull-down circuit;
`
`8
`setting an output impedance of the second transceiver as
`a function of the bus line characteristic impedance;
`communicating the signal onto the bus line;
`while communicating the signal;
`generating an error signal as a function of comparing the
`signal voltage to a reference voltage;
`sensing the signal voltage on the bus line, wherein the
`signal voltage is a function of the output impedance of
`the second transceiver; and
`adjusting one of the pull-up circuit and the pull-down
`circuit so that the output impedance of the second
`transceiver is changed to control the signal voltage and
`reduce an amplitude of the error signal.
`14. The method of claim 13 further comprising:
`setting a termination impedance of the second transceiver
`configured to receive the signal on the bus line;
`setting the output impedance of the first transceiver con-
`figured to communicate the signal on the bus line; and
`adjusting the output impedance of the first transceiver.
`15. The method of claim 13 wherein adjusting the output
`impedance of the second transceiver comprises moving the
`output impedance to bring the voltage on the bus line closer
`to the reference voltage.
`16. The method of claim 13 further comprising:
`comparing the sensed voltage to the reference voltage;
`and
`
`adjusting a termination impedance of the first transceiver
`based upon the comparison.
`
`10
`1O
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