# Patent

## Title
Configurable on-die termination

## Abstract

Described are systems that employ configurable on-die termination elements that allow users to select from two or more termination topologies. One topology is programmable to support rail-to-rail or half-supply termination. Another topology selectively includes fixed or variable filter elements, thereby allowing the termination characteristics to be tuned for different levels of speed performance and power consumption. Termination voltages and impedances might also be adjusted.

## Background
FIELD
The subject matter presented herein relates generally to the field of communications, and more particularly to high speed electronic signaling within and between integrated circuit devices.
BACKGROUND
High-speed data communication integrated circuit (IC) dies are known to include both drivers and receivers. The driver of one such IC connects to the receiver of another via one or more signal transmission lines. Both the driver and receiver circuits include termination elements that attempt to match the characteristic impedance of the transmission line to the output impedance of the driver and input impedance of the receiver, as impedance mismatches degrade signal quality and consequently reduce communication speed and reliability.
Some conventional communication systems employ control systems that calibrate the impedance of on-die termination (ODT) elements for improved impedance matching. These systems work well in many applications. Still, high-speed data communication circuits often must achieve ever-greater performance levels, at competitive prices, to satisfy customer demand. Furthermore, different customers typically have different requirements that may not be met by a given ODT configuration. One customer might favor power-efficiency over speed, or may prefer different termination voltages or impedances. There is therefore a need for ODT circuits that offer customers access to a broader range of termination topologies and values.



BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter presented herein is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
FIG. 1 depicts an integrated-circuit die 100 that includes configurable on-die termination in accordance with one embodiment.
FIG. 2 depicts a communication system 200 that employs configurable on-die termination in accordance with another embodiment.
FIG. 3 depicts an IC die 300 in accordance with another embodiment.
FIG. 4 depicts a communication system 400 that employs configurable ODT circuitry in accordance with another embodiment.
FIG. 5 depicts a communication system 500 in accordance with yet another embodiment.
FIG. 6 depicts a configurable RC circuit 600 that can be used in place of the third termination leg of die 510 of FIG. 5, which extends between node 535 and ground.



DETAILED DESCRIPTION
FIG. 1 depicts an integrated-circuit (IC) die 100 in accordance with one embodiment. Die 100 includes a pseudo-differential receiver 105 that compares an input signal RXi, received via a pad 110, with a reference voltage Vref on a like-named voltage terminal or node to produce an output signal RXo. Die 100 also includes programmable on-die termination (ODT) circuitry 115 that can be programmed to provide either of two common termination topologies for high-speed communications: the so-called “rail-to-rail” topology and the so-called “half-supply” topology. The choice of termination topology is then left to the discretion of the user of IC die 100. An external source or internal memory 120 can deliver a signal S/P to temporarily or permanently select one of the two configurations.
ODT circuitry 115 includes two termination legs extending from the communication port between pad 110 and receiver 105. The upper termination leg includes a first termination impedance 125 and a first termination switch 130. Switch 130 includes three switch nodes, two of which are connected to supply voltage Vodt and reference voltage Vref, respectively. The third switch node is coupled to the communication port via termination impedance 125. The lower termination leg includes a second termination impedance 135 and a second termination switch 140 similar to switch 130. Two switch nodes of switch 140 are connected to ground and reference voltage Vref, respectively, while the third is coupled to the communication port via termination impedance 135. Both switches 130 and 140 are two position switches responsive to signal S/P from memory 120 to selectively couple one of the first and second switch nodes to the third switch node.
In rail-to-rail or serial terminations, the communication channel is coupled to each of two opposite supply voltages via a pair of termination impedances. To select a rail-to-rail termination topology, switches 130 and 140 are switched to supply nodes Vodt and ground, respectively. In that case, the input terminal to receiver 105 is coupled to Vodt and ground via respective impedances 125 and 135. Termination voltage Vodt on the like-named supply node is supply voltage Vdd in some embodiments, but may be a different fixed voltage or a variable voltage in other embodiments.
In half-supply or parallel terminations, the communication channel is coupled to a reference voltage between the two supply voltages. To select a half-supply termination topology, switches 130 and 140 are both switched to voltage Vref, in which case the input terminal to receiver 105 is coupled to the reference voltage terminal Vref via parallel impedances 125 and 135. As the name implies, the reference voltage in half-supply terminations is typically half the difference between the voltages on the supply nodes (e.g., Vref=1⁄2(Vdd−Gnd)), but voltage Vref may be a different fixed voltage or a variable voltage in other embodiments.
IC die 100 optionally includes a coupling switch 145 between pad 110 and the input terminal of receiver 105. An external or internal signal, such as from memory 120, can deliver a signal AC/DC to temporarily or permanently open or close switch 145. When switch 145 is closed, receiver 105 is DC coupled to pad 110: when open, receiver 105 is AC coupled to pad 110 via a capacitor 150.
Impedances 125 and 135 may be adjustable and capable of calibration. Suitable calibration methods and circuits are detailed in U.S. Pat. No. 6,924,660 entitled “Calibration Methods and Circuits for Optimized On-Die Termination,” which is incorporated herein. Switches 130, 140, and 150 can be fashioned of transistors, as is well understood by those of skill in the art. Capacitor 150 may also be adjustable using methods and circuits detailed below in connection with FIG. 6.
FIG. 2 depicts a communication system 200 in accordance with another embodiment. System 200 has features in common with IC die 100 of FIG. 1, like-numbered elements being the same or similar. System 200 includes ODT circuitry that can selectively introduce filter elements that may be useful for low power configurations. Further, the selection can be accomplished dynamically in some embodiments, which allows system 200 to select appropriate ODT characteristics for high and low-frequency communication. This flexibility is useful for example in systems that support both a low-frequency, power-saving mode and a high-frequency, high-performance mode.
System 200 includes a transmitter IC die 205 coupled to a receiver IC die 210 via a single-ended communication port made up of pads 215, a channel 220, and related conductors on dies 205 and 210. Die 205 includes a transmitter 225 and a pair of termination legs 230. Legs 230 may be the same or similar to the termination legs detailed in connection with the receiver dies 100 and 210 of FIGS. 1 and 2. Transmitter 225 conveys a signal TX to receiver 105 on die 210 via pad 215 and the other elements of the associated communication port.
IC die 210 includes ODT circuitry 235 that can select either a filtered or unfiltered half-supply termination topology. The termination topology is then left to the discretion of the user of IC die 210. The topology may be fixed, defined at start up, or allowed to change dynamically to support different performance modes. In the depicted embodiment, termination select logic 240 issues a control signal L/H, the state of which identifies either a lower-performance, lower-power mode, or a higher-performance, higher-power mode.
ODT circuitry 235 includes two termination legs extending from the communication port between pad 215 and receiver 105 of die 210. The upper termination leg includes a first termination impedance 245 and a first termination switch.250. Switch 250 includes three switch nodes, two of which are connected to reference voltage Vref, one directly and the other via a filter capacitor 255. The third switch node is coupled to the communication port via termination impedance 245. The lower termination leg is substantially the same. The switches of the upper and lower termination legs are responsive to signal L/H from termination select logic 240.
The switches of both termination legs connect their respective termination resistors directly to voltage node Vref in a high-performance mode, and to voltage node Vref via a respective filter capacitor in a low-frequency mode. Filtering the input signal in the low-frequency mode advantageously dampens high-frequency noise components. The filter capacitors may be adjustable in some embodiments to allow filter tuning. Fixed or adjustable resistors in series and/or in parallel with the filter capacitors can also be included.
FIG. 3 depicts an IC die 300 in accordance with another embodiment. Die 300 includes a receiver 305 that compares an input signal RXi with a reference voltage Vref on a like-named voltage node to produce an output signal RXo. Die 300 also includes programmable ODT circuitry 310 that can be programmed to provide filtered or unfiltered rail-to-rail or a half-supply termination topologies, and thus combines the functionality of the embodiments of FIGS. 1 and 2.
ODT circuitry 310 includes two termination legs. Each leg includes switches 315 and 320, a filter capacitor 325, and a termination impedance 330. Switches 315 and 320 support four modes as follows:

    
    
        1. Unfiltered Rail-to-Rail: Switches 320 are closed and switches 315 of the upper and lower termination legs select nodes Vodt and Ground, respectively.
        2. Filtered Rail-to-Rail: Switches 320 are open and switches 315 of the upper and lower termination legs select nodes Vodt and Ground, respectively.
        3. Unfiltered Half-Supply: Switches 320 are closed and switches 315 both select node Vref.
        4. Filtered Half-Supply: Switches 320 are open and switches 315 both select node Vref.

ODT circuitry 310 can be adapted to support more modes. Additional supply voltages can be supported, for example, and the impedances and capacitances can be adjustable.

    
    


FIG. 4 depicts a communication system 400 that employs configurable ODT circuitry in accordance with another embodiment. The configurable ODT circuitry allows a transmitter die 405 to select between two or more termination voltages when transmitting data to a receiver die 410 over a differential communication channel 415. The resulting output common-mode voltage can thus be tailored to the needs of a receiver on die 410. If, for example, multiple receivers timeshare a common bus but require or benefit from different receive termination voltages, then the associated transmitter or transmitters can use the termination voltage best suited for the receiver with which they are communicating. A communication channel may also support different operational modes that require or benefit from different termination voltages. A transmitter might, for example, support a loop-back communication mode for self test or initialization that employs a first termination voltage, and additionally support one or more operational modes that employ different termination voltages suitable for one or more target receivers.
Die 405 includes a differential transmitter with two identical or nearly identical termination legs. Each leg includes a fixed or adjustable termination impedance 417 and a voltage-select switch 420. The state of switches 420 are controlled using select signal S that may be provided externally or internally, as by a memory 425. Control logic can be included to dynamically alter the states of switches 420, which can alternatively select either of two termination voltages V1 and V2. In other embodiments, a variable voltage source is used in lieu of switches 420 and the two supply nodes.
FIG. 5 depicts a communication system 500 in accordance with yet another embodiment. Communication system 500 includes a transmitting die 505 communicating with a receiving die 510 via a differential channel 515. The transmitting die includes differential output pads 513 coupled via the channel to input pads 517 of the receiving die. In one embodiment, communication system 500 includes a transmitter 520 that employs low-voltage differential signaling (LVDS) for serial data transmission to a corresponding receiver 525, though other types of signaling may also be used. System 500 optionally includes an external differential termination resistor 530 (in phantom).
Die 510 includes programmable ODT circuitry that can select from a number of possible termination topologies. In support of this selectivity, die 510 includes three termination legs that extend from a common node 535, two to the differential input terminals to receiver 525 and one to a reference voltage node, e.g. ground. Each of the first two termination legs includes a termination impedance 540 and a switch 545 connected in series. The third termination leg includes a capacitance 550, a termination impedance 555, and a switch 560. The inclusion of impedances 540 and as associated switches 545 allows for the omission of external resistor 530. The third leg allows for the selective incorporation of a noise-reducing RC filter. The impedances and capacitance of the ODT circuitry of FIG. 5 are variable in some embodiments, which allows filter and termination values to be trimmed for improved performance. Switches 545 and 560 can be controlled by external or internal control signals applied to switch control terminals (not shown). The various capacitive and resistive elements can be similarly controlled.
FIG. 6 depicts a configurable RC circuit 600 that can be used in place of the third termination leg of die 510 of FIG. 5, which extends between node 535 and ground. Circuit 600 includes some memory 605, the outputs of which are coupled to the control terminals of a plurality of transistors 610. The transistors 610 selectively couple one or more differently sized capacitors 615 between node 535 and ground. In addition to controlling the capacitance, the resistance between nodes 535 and ground can be adjusted by selecting various combinations of transistors. The width-to-length ratios of transistors 610 may be varied to provide various impedances so that enabling different combinations of transistors provides different levels of termination impedance.
In the foregoing description and in the accompanying drawings, specific terminology and drawing symbols are set forth to provide a thorough understanding of the present invention. In some instances, the terminology and symbols may imply specific details that are not required to practice the invention. For example, the interconnection between circuit elements or circuit blocks may be shown or described as multi-conductor or single conductor signal lines. Each of the multi-conductor signal lines may alternatively be single-conductor signal lines, and each of the single-conductor signal lines may alternatively be multi-conductor signal lines. Signals and signaling paths shown or described as being single-ended may also be differential, and vice-versa. Similarly, signals described or depicted as having active-high or active-low logic levels may have opposite logic levels in alternative embodiments. As another example, circuits described or depicted as including metal oxide semiconductor (MOS) transistors may alternatively be implemented using bipolar technology or any other technology in which a signal-controlled current flow may be achieved. With respect to terminology, a signal is said to be “asserted” when the signal is driven to a low or high logic state (or charged to a high logic state or discharged to a low logic state) to indicate a particular condition. Conversely, a signal is said to be “de-asserted” to indicate that the signal is driven (or charged or discharged) to a state other than the asserted state (including a high or low logic state, or the floating state that may occur when the signal driving circuit is transitioned to a high impedance condition, such as an open drain or open collector condition). A signal driving circuit is said to “output” a signal to a signal receiving circuit when the signal driving circuit asserts (or de-asserts, if explicitly stated or indicated by context) the signal on a signal line coupled between the signal driving and signal receiving circuits.
An output of a process for designing an integrated circuit, or a portion of an integrated circuit, comprising one or more of the circuits described herein may be a computer-readable medium such as, for example, a magnetic tape or an optical or magnetic disk. The computer-readable medium may be encoded with data structures or other information describing circuitry that may be physically instantiated as an integrated circuit or portion of an integrated circuit. Although various formats may be used for such encoding, these data structures are commonly written in Caltech Intermediate Format (CIF), Calma GDS II Stream Format (GDSII), or Electronic Design Interchange Format (EDIF). Those of skill in the art of integrated circuit design can develop such data structures from schematic diagrams of the type detailed above and the corresponding descriptions and encode the data structures on computer readable medium. Those of skill in the art of integrated circuit fabrication can use such encoded data to fabricate integrated circuits comprising one or more of the circuits described herein.
While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, the embodiments can be adapted for use with various single-ended and differential communication schemes over unidirectional and bidirectional channels. Specific examples include Series Stub Terminated Logic (SSTL) and double-data-rate (DDR) signaling, though this is by no means an exhaustive list. Embodiments may also be used for channels employing various modulation schemes, including those that employ multi-pulse-amplitude-modulation (multi-PAM) and single-PAM signals. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance the method of interconnection, or “coupling,” establishes some desired electrical communication between two or more circuit nodes, or terminals. Such coupling may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. Section 112.

## Claims

1. An integrated-circuit (IC) die comprising:
a. a pad to receive an input signal to the die;
b. a receiver having an input terminal coupled to the pad, a reference terminal, and an output terminal;
c. a first switch having a first switch node coupled to the reference terminal, a second switch node, and a third switch node, wherein the first switch selectively couples the first switch node to the third switch node and selectively couples the second switch node to the third switch node;
d. a first termination impedance coupled between the third switch node and the input terminal;
e. a second switch having a fourth switch node coupled to the reference terminal, a fifth switch node, and a sixth switch node, wherein the second switch selectively couples the fourth switch node to the sixth switch node and selectively couples the fifth switch node to the sixth switch node; and
f. a second termination impedance coupled between the sixth switch node and the input terminal.

2. The die of claim 1, wherein the first switch couples the first and third switch nodes and the second switch couples the fourth and sixth switch nodes in a serial termination configuration, and wherein the first switch couples the second and third switch nodes and the second switch couples the fifth and sixth switch nodes in a parallel termination configuration.

3. The die of claim 2, wherein the first and second switches include respective first and second switch control terminals.

4. The die of claim 3, further comprising a register with an output port coupled to the first and second switch control terminals, the register adapted to store a value that determines whether the first and second switches support a parallel termination configuration or a series termination configuration.

5. The die of claim 1, further comprising a first reference-voltage node coupled to the second switch node and a second reference-voltage node coupled to the fifth switch node.

6. An integrated-circuit (IC) die comprising:
a. a pad to receive an input signal to the die;
b. a receiver having an input terminal coupled to the pad, a reference terminal, and an output terminal;
c. a first switch having a first switch node coupled to the reference terminal, a second switch node, and a third switch node, wherein the first switch selectively couples one of the first and second switch nodes to the third switch node;
d. a first termination impedance coupled between the third switch node and the input terminal;
e. a second switch having a fourth switch node coupled to the reference terminal, a fifth switch node, and a sixth switch node, wherein the second switch selectively couples one of the fourth and fifth switch nodes to the sixth switch node; and
f. a second termination impedance coupled between the sixth switch node and the input terminal; and
g. a capacitor coupled between the first and second switch nodes.

7. The die of claim 6, further comprising a second capacitor coupled between the fourth and fifth switch nodes.

8. The die of claim 6, wherein the capacitor is tunable over a range of capacitances.

9. The die of claim 1, further comprising a third switch and a capacitor coupled in parallel between the pad and the input terminal of the receiver.

10. An on-die termination circuit comprising:
a. a communication port;
b. a first termination leg including a first termination resistance and a first switch, the first switch selectively connecting the communication port to one of a first voltage node and a second voltage node via the first termination resistance;
c. a second termination leg including a second termination resistance and a second switch, the second switch selectively connecting the communication port to one of a third voltage node and a fourth voltage node via the second termination resistance; and
d. a first capacitor coupled between the first and second voltage nodes and a second capacitor coupled between the third and fourth voltage nodes.

11. An on-die termination circuit comprising:
a. a communication port;
b. a first termination leg including a first termination resistance and a first switch, the first switch selectively connecting the communication port to one of a first voltage node and a second voltage node via the first termination resistance;
c. a second termination leg including a second termination resistance and a second switch, the second switch selectively connecting the communication port to one of a third voltage node and a fourth voltage node via the second termination resistance; and
d. an IC-die pad, a third switch selectively connecting the communication port to the pad, and a capacitor connected in parallel with the third switch.

12. A communication system comprising:
a. a first IC die having:
i. an output pad; and
ii. a transmitter coupled to the output pad to transmit data on the output pad;

b. a second IC die having:
i. an input pad; and
ii. a receiver coupled to the input pad to receive the transmit data;

c. a communication channel extending between the output pad of the first IC die and the input pad of the second IC die; and
d. a termination circuit integrated with one of the first and second IC dies and coupled to a respective one of the output pad and the input pad, the termination circuit including:
i. a first switch having a first switch node coupled to a first reference terminal, a second switch node, and a third switch node, wherein the first switch selectively couples the first switch node to the third switch node and selectively couples the second switch node to the third switch node;
ii. a first termination impedance coupled between the third switch node and the input terminal;
iii. a second switch having a fourth switch node coupled to a second reference terminal, a fifth switch node, and a sixth switch node, wherein the second switch selectively couples the fourth switch node to the sixth switch node and selectively couples the fifth switch node to the sixth switch node; and
iv. a second termination impedance coupled between the sixth switch node and the input terminal.

13. The communication system of claim 12, wherein the first switch couples the first and third switch nodes and the second switch couples the fourth and sixth switch nodes in a serial termination configuration, and wherein the first switch couples the second and third switch nodes and the second switch couples the fifth and sixth switch nodes in a parallel termination configuration.

14. The communication system of claim 13, wherein the first and second switches include respective first and second switch control terminals.

15. The communication system of claim 14, further comprising a register with an output port coupled to the first and second switch control terminals, the register to store a value that determines whether the first and second switches support a parallel termination configuration or a series termination configuration.

16. The communication system of claim 12, further comprising a first reference-voltage node coupled to the second switch node and a second reference-voltage node coupled to the fifth switch node.

17. The communication system of claim 12, further comprising a capacitor coupled between the first and second switch nodes.

18. An integrated-circuit (IC) die comprising:
a pad to receive an input signal to the die;
a receiver having an input terminal coupled to the pad, a reference terminal, and an output terminal;
a first switch having a first switch node coupled to the reference terminal, a second switch node, and a third switch node, wherein the first switch selectively couples the first switch node to the third switch node and selectively couples the second switch node to the third switch node;
a first termination impedance coupled between the third switch node and the input terminal;
a second switch having a fourth switch node coupled to the reference terminal, a fifth switch node, and a sixth switch node, wherein the second switch selectively couples the fourth switch node to the sixth switch node and selectively couples the fifth switch node to the sixth switch node;
a second termination impedance coupled between the sixth switch node and the input terminal; and
a register with an output port coupled to the first switch control terminal, the register to store a value that determines whether the first switch supports a parallel termination configuration or a series termination configuration.

19. The IC of claim 18, wherein the register output port is coupled to the second switch control terminal and the value determines whether the second switch supports the parallel termination configuration or the series termination configuration.

20. The IC of claim 18, wherein the first termination impedance includes a tunable capacitance.

21. The IC of claim 20, wherein the output port of the register is coupled to the first termination impedance, and wherein the value selects from among a range of capacitances for the tunable capacitance.

22. The IC of claim 18, wherein the register output port is coupled to the first termination impedance, and wherein the value determines a resistance of the termination impedance.