# Patent

## Title
On-die termination of address and command signals

## Abstract

A system has a plurality of memory devices arranged in a fly-by topology, each having on-die termination (ODT) circuitry for connecting to an address and control (RQ) bus. The ODT circuitry of each memory device includes a set of one or more control registers for controlling on-die termination of one or more signal lines of the RQ bus. A first memory device includes a first set of one or more control registers storing a first ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the first memory device, and a second memory device includes a second set of one or more control registers storing a second ODT value different from the first ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the second memory device.

## Background
RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 14/088,277, filed Nov. 22, 2013, now U.S. Pat. No. 8,947,962, which is a continuation of U.S. application Ser. No. 12/519,908, filed Jun. 18, 2009, now U.S. Pat. No. 8,599,631, which is a U.S. National stage application filed under 35 U.S.C. §371 of PCT Patent Application Serial No. PCT/US2007/088245, filed Dec. 19, 2007, which claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/876,672, filed Dec. 21, 2006, which are incorporated herein by reference in their entirety.


TECHNICAL FIELD
This disclosure relates generally to the field of semiconductor memories and memory systems.
BACKGROUND
High-speed, readily-expandable memory systems consisting of groups of memory devices or modules are commonly used to store data in computers and other electronic devices. Bits of data are typically stored in semiconductor memory cells on the memory device, with the data being transmitted along data signals lines or data bus connected to the memory modules. To avoid undesirable signal reflections on the data bus, the memory device typically includes some sort of termination circuitry for the data bus lines.
So-called “fly-by” memory modules topologies are being increasingly used for DRAM memory module applications. “Fly-by” memory modules can be terminated either at the hosting motherboard or at the module itself. This scheme often uses extra printed circuit board (PCB) area devoted to termination of the command, address, and control signals (i.e., “RQ” signal or bus lines) on each module in the system, resulting in less area being available for DRAM packages. Also typical memory systems expend significant power for the command, address, and control signals.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the invention to the specific embodiments shown, but are for explanation and understanding only.
FIG. 1 illustrates an example memory system with memory devices that employ ODT circuitry.
FIGS. 2A and 2B are example ODT circuits that may be used in the memory system of FIG. 1.
FIG. 3 illustrates an example method of operation for the memory system of FIG. 1.
FIG. 4 illustrates an example memory module with ODT circuitry that provides multiple termination impedance values.
FIG. 5 is a table that illustrates example termination impedance values corresponding to various input pin logic levels for the memory module shown in FIG. 4.
FIG. 6 illustrates another example memory system with memory modules that employ ODT circuitry.



DETAILED DESCRIPTION
In the following description specific details are set forth, such as device types, system and circuit configurations, component values, signaling methods, etc., in order to provide a thorough understanding of the disclosure herein. However, persons having ordinary skill in the relevant arts will appreciate that these specific details may not be needed to practice the embodiments described.
In the context of the present application, a memory “module” refers to an assembly of memory chips or devices (i.e., semiconductor dies) arranged on a substrate or printed circuit board (PCB) in association with a memory system. A memory module may thus be considered a memory subsystem. The term “data bus” is used throughout to denote data (DQ) signal lines, timing reference signals, and data masking control bits (DM) by which data is communicated to one or more memory modules. Similarly, the term “address/command bus” is used herein to denote the signal lines that carry command, address and control (RQ or CAC) information for communications between a memory controller and one or more memory modules. The term “memory controller”, as used herein, refers to a broad class of generic and application-specific processing devices that are capable of reading data from, and writing data to, one or more memory modules.
In accordance with one embodiment, each memory device or module of a memory system arranged in a fly-by connection topology includes ODT circuitry for selectively terminating one or more of the RQ signal lines. The RQ signals comprise the command, address, control, and clock signals that may be used to write data to the individual memory cells of the modules, and also to read data stored therein. In a typical configuration, the last device of every module connected to the RQ bus has one or more RQ lines terminated by connecting an ODT enable pin to the appropriate voltage supply rail of the device or module.
Other embodiments may also utilize register control, either as an override, or as a replacement, for the enable pin configuration method. For example, in one embodiment register fields may be utilized to override the RQ ODT enable. In a specific implementation, a register bit may allow the clock lines to be discretely terminated on the device or module, while the RQ signals (or a subset thereof) are terminated with ODT circuitry.
FIG. 1 illustrates an example memory module 10 comprising a plurality of DDR3 memory devices 11 arranged in a fly-by (also known as a multi-drop) connection topology, each of the memory devices comprising a single semiconductor chip or die that includes ODT circuitry 12. For example, FIG. 1 shows 16 memory devices 11a-p, each with a respective ODT circuit 12a-p that may be utilized to selectively terminate any or all of the RQ signal lines on RQ bus 14. Each of memory devices 11 also includes an ODT termination pin 15, which is tied to either VDDQ or VSSQ in the module layout, depending on the device's location on RQ bus 14. By way of example, FIG. 1 illustrates ODT circuit 12p of the last memory device (i.e., device 11p) on RQ bus 14 being enabled by connection of ODT termination pin 15p to the positive voltage supply rail, VDDQ. The remaining ODT circuits 12a-o are shown having their respective ODT termination pins 15a-o connected to the negative voltage supply rail, VSSQ, which disables termination on devices 11a-o. 
It is appreciated that the last device (i.e., device 11p) is the one electrically furthest from the memory controller 18. In the fly-by (or multi-drop) connection topology, each control and address (CA) signal is routed from a controller pin to a pin on each DRAM device. The propagation time from the controller to each memory device is different depending on the physical position of the memory device in the module. In the example of FIG. 1, device 11p is the last memory device in the topology, and therefore experiences the longest CA signal propagation time. It is appreciated that chip select (CS) signals may also be routed with the same fly-by connection topology as the CA signals in the example memory module of FIG. 1.
Memory controller 18 is shown driving the signal lines of bus 14, as well as driving a control/address ODT signal line 16 (labeled “CAODT”) connected to ODT circuit 12p. The CAODT control signal pin may be utilized to actively enable and disable a subset (or all) of the RQ signal line terminations. For instance, CAODT pin 16 may be actively driven by memory controller 18 to enable or disable ODT circuit 12p at certain times depending on the transactions been performed in memory system 10. By way of example, when memory controller 18 drives CAODT pin 16 to a logical high level (e.g., VDDQ), ODT circuit 12p is enabled or turned on, such that the impedance for a subset (or all) of the signals at the end of RQ bus 14 is set to a predetermined value. The timing for enabling and disabling the ODT circuitry on the RQ signal lines may be the same is that for the data group signal lines.
Practitioners in the art will appreciate that the impedance value of the termination may be determined by signal integrity studies, and may be different for different “classes” of signals. For example, the chip select signal lines may have a different impedance value as compared to the address signal lines. In the case where memory controller 18 drives CAODT pin 16 to a logical low level (e.g., VSSQ), ODT circuit 12p is disabled or turned off, e.g., a high impedance value for the signal lines terminating at the end of RQ bus 14. It should be further understood that CAODT signal line 16 may be included on RQ bus 14.
In one embodiment, a subset of the RQ signals is terminated at all times by ODT circuit 12p in device 11p. The RQ signals that are terminated at all times include the external clock signals (CK/CKN) and the chip reset signal (RESETN). In the example of FIG. 1, the subset of the RQ signals on bus 14 that are not terminated, unless memory locations in device 11p are being immediately addressed, include the address lines (A[15:0]); bank address (BA[2:0]); chip select (CS[3:0]); clock enable (CKE); row address strobe (RASN); column address strobe (CASN); and write enable (WEN). The termination circuits for these latter signal lines may be dynamically controlled by CAODT pin 16. In other words, the foregoing subset of signal lines may be dynamically controlled such that the impedance value at the end of the RQ bus for these signals depends upon the operations being performed. Termination is normally disabled for non-addressed devices.
Practitioners in the art will appreciate that dynamically controlling (enabling/disabling) the termination at the end of the RQ bus for a subset of signal provides a significant power saving advantage. In one implementation, for example, disabling the termination on most of the RQ pins during extended idle times on the RQ bus, or while in a power-down mode, may save about 300 mW per memory device.
FIG. 2A illustrates an example ODT circuit 20 that may be used in the memory system of FIG. 1. ODT circuit 20 comprises a switch 24 coupled in series with resistor 21 between positive voltage supply rail VDDQ and signal line termination node 23. Similarly, a resistor 32 and switch 25 are coupled in series between node 23 and negative supply voltage rail VSSQ. (It is appreciated that switches 24 & 25 may also be located between node 23 and respective resistors 21 & 32.) The signal line has a characteristic impedance shown by element 28 driven by a driver 29 associated with the memory controller. Both resistors 21 and 32 are shown having the same resistance value, R, such that when both switches 24 & 25 are closed the equivalent resistance terminating the signal line is R/2, with the voltage at node 23 being (VDDQ-VSSQ)/2. With switches 24 & 25 both open, termination is off (e.g., high impedance). Consistent with the example of FIG. 1, switches 24 & 25 may both be simultaneously controlled by CAODT pin 16.
FIG. 2B illustrates another example ODT circuit 30 that may be used in the memory system of FIG. 1 to achieve dynamic termination with multiple impedance values. ODT circuit 30 resembles the ODT circuit of FIG. 2A, except with three parallel arranged circuit legs. Each leg comprises a switch 34 coupled in series with a resistor 31 between VDDQ and signal line termination node 33. Each leg also includes a resistor 42 and switch 35 coupled in series between node 33 and VSSQ. Resistors 31a & 42a have a resistance R; resistors 31b & 42b have a resistance R/2; and resistors 31c & 42c have a resistance R/4.
Switches 34 and 35 of may be selectively opened/closed in order to implement various different termination impedance values applied to different command, control, and address signals of selected memory devices. For instance, in one embodiment, the termination impedance of a signal line may be selected to be “strong” by closing of all of switches 34 & 35, “weak” by closing only switches 34a, 34b, 35a and 35b, or “off” by leaving all of switches 34 & 35 open, depending on signal integrity considerations. In the example of FIG. 2B the characteristic impedance is shown by element 38, with the signal line being driven by a driver 39 associated with the memory controller. Switches 34 & 35 in each leg may both be simultaneously controlled by a different CAODT pin or bit value in a register field.
In a specific embodiment, multi-value RQ termination is supported for memory device by selecting different values of address, control, and command signal line termination impedance to be employed. For instance, multi-value RQ termination may be controlled by three different register fields. A first register field may be used to set an alternate value of termination to be used, while a second register field determines how the alternate value is enabled. A third register field may be used to set a nominal impedance to be used for termination. According to one method of control, the CAODT control pin selects the nominal value of impedance when driven high, and the alternate value of impedance when driven low.
A second method of controlling/selecting multiple termination values for an RQ signal line utilizes the CAODT control pin and an ALTCAODT control pin. FIG. 4 illustrates an example memory device 11 with ODT circuitry 12 that provides for multiple termination impedance values using the CAODT and ALTCAODT control pins. FIG. 5 is a table that illustrates example termination impedance values corresponding to various input pin logic levels for the memory device shown in FIG. 4. As can be seen, when CAODTEN is high (enabled), a logical high value applied to the CAODT control pin enables a nominal impedance value. On the other hand, when ALTCAODT is high, the alternate impedance value is enabled. When CAODTEN and ALTCAODT are both low, termination is off (i.e., a high impedance state).
It is appreciated that in another embodiment the CAODTEN pin may be controlled via a configuration bit. Setting of the configuration bit to “1”, for example, may be equivalent to statically enabling the address on the termination, i.e., hard-tying the CAODTEN pin high.
FIG. 3 illustrates an example method of operation for the memory system of FIG. 1. The process begins at block 44, wherein prior to addressing a selected memory device the termination value of selected RQ lines is enabled for the selected memory device. It is appreciated that, in certain situations, termination values may be enabled for more than one device. For instance, depending where the addressed device is electrically located on the RQ bus, it may be appropriate to disable termination on the addressed device, while simultaneously enabling termination on an adjacent device, in order to achieve optimal signal integrity. In other words, termination of the individual RQ signal lines may be dynamically controlled by the memory controller rather than being statically enabled. Furthermore, in certain cases, termination may specifically be disabled in instances where the terminated device is being addressed and enabled when it is not being addressed. In still other situations, termination may be enabled except during full bus idle conditions or power down states. In other embodiments, each of the memory devices arranged in the fly-by topology may have termination enabled with respect to the RQ signal lines, with the termination impedance value changing during addressing of specific devices.
Once the appropriate termination values have been applied to a subset (or all) of RQ signal lines on the selected memory device(s), the RQ lines may be driven to the appropriate voltage levels for addressing the memory locations within the device (block 45). After data has been written/read from the addressed location, termination is disabled for the subset of RQ lines at the specified device(s) (block 46). As discussed previously, disabling termination has the salutary effect of reducing quiescent power consumption in the memory system.
FIG. 6 illustrates another example memory system 59 with four memory modules 60a-60d, each module comprising a plurality of memory devices (e.g., DRAM chips) 61. Each of the memory devices 61 include ODT circuitry 62, with each of the memory devices of the modules being coupled with a memory controller 68. As in the case of the example of FIG. 1, the respective ODT circuits 62 of memory devices 61 may be utilized to selectively terminate any or all of the RQ signal lines on RQ bus 64. In this example, the RQ bus lines are shown coupled with the memory devices of each module in a stub bus connection topology. In the stub bus connection topology the RQ lines are routed to each module by splitting off of a main bus. In this embodiment, the RQ signals are split off of the main bus and then routed onto each individual module. After being routed past each individual DRAM in a “fly-by” topology, the RQ signals are then optionally terminated by the memory device farthest (electrically) from the controller.
Each of memory devices 61 of each module 60 also includes an ODT termination on/off pin 65, which is tied to either VDD or VSS in the module layout, depending on the location of the device in the module. Tying termination pin 65 to VDD enables termination for that device. Conversely, tying pin 65 low to VSS disables termination at that device. In the example of FIG. 6, only the ODT circuit 62p of the last memory device 61p in each memory module 60 is enabled by connection of ODT termination pin 65p to the positive voltage supply rail, VDD. The remaining ODT circuits 62a-o of the other memory devices 61 on each of modules 60a-60d have their respective ODT termination pins 65 connected to the negative voltage supply rail, VSS, which disables termination on those devices.
A termination impedance value for each device 61p of modules 60a-60d may be selected via CAODT pins 66a-66d, respectively. For example, the ODT circuitry may be configured such that raising CAODT pin 66 to a high voltage potential (logical “1”) result in a nominal termination impedance, whereas lowering pin 66 to a logical “0” voltage produces an alternate (or high) impedance at that device. Multiple CAODT pins (e.g., CAODT & ALTCAODT) for each device may be utilized for setting a variety of different termination impedance values at devices 62p. 
In the same manner as described in conjunction with FIG. 1, memory controller 68 may be utilized to actively drive all, or a subset, of the RQ signal lines of bus 64, as well as driving CAODT signal lines 66a-66d connected to ODT circuits 62p of respective modules 60a-60d. CAODT pin 66 may thus be used to selectively enable or disable ODT circuit 61p of a selected module at certain times depending on the transactions being performed in memory system 59. For example, when a write transaction is initiated to module 60d, the termination on modules 60a-c may be enabled to eliminate reflections from those trace stubs. The termination on module 60d may remain disabled in order to allow a full signal swing on that module. Similarly, where the termination circuitry on devices 61 supports multiple values of termination impedance, a write operation to module 60d may be best achieved having a “strong” termination enabled in modules 60a-c (e.g. low impedance) while module 60d has a “weak” termination (e.g. higher impedance) enabled. In another example, the termination on module 60d may be enabled on a write to module 60d, while the terminations on all other modules are disabled. In still another example, a write to module 60d may be best achieved by having modules 60a-c “weakly” terminated (e.g. high impedance), while enabling a “strong” termination (e.g. lower impedance) on module 60d. 
In another embodiment, termination of the last device of each of a plurality of memory modules may be implemented utilizing a termination component device (or several devices) separate from the memory chips and actively controlled by memory controller 68.
It is also appreciated that the foregoing termination examples are applicable to buffered memory systems configured with a single buffer on each module or multiple buffers paired with one or more memory devices in a fly-by fashion.
In still another embodiment, instead of employing the “stub bus” routing scheme shown in FIG. 6, a “serpentine” routing scheme may be used in which the address lines are routed onto and off of each module in turn. In this latter connection topology, only the ODT circuit of the last memory device of the last memory module is enabled.
It should be understood that elements of the present invention may also be provided as a computer program product which may include a machine-readable medium having stored thereon instructions which may be used to program a computer (e.g., a processor or other electronic device) to perform a sequence of operations. Alternatively, the operations may be performed by a combination of hardware and software. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, propagation media or other type of media/machine-readable medium suitable for storing electronic instructions. For example, elements of the present invention may be downloaded as a computer program product, wherein the program may be transferred from a remote computer or telephonic device to a requesting process by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection).
Additionally, although the present invention has been described in conjunction with specific embodiments, numerous modifications and alterations are well within the scope of the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

## Claims

1. A system comprising:
a plurality of memory devices arranged in a fly-by topology, each of the memory devices having on-die termination (ODT) circuitry for connecting to an address and control (RQ) bus, the ODT circuitry of each memory device including a set of one or more control registers for controlling on-die termination of one or more signal lines of the RQ bus;
wherein a first memory device of the plurality of memory devices includes a first set of one or more control registers to store a first ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the first memory device and a second memory device of the plurality of memory devices includes a second set of one or more control registers to store a second ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the second memory device.

2. The system of claim 1, wherein the RQ bus further includes one or more clock signal lines and the set of one or more control registers in each memory device of the plurality of memory devices includes at least one control value for controlling on-die termination of the one or more clock signal lines by the ODT circuitry of the memory device.

3. The system of claim 1, wherein the ODT circuitry of each memory device of the plurality of memory devices further includes at least one input for receiving an ODT enable signal from a source external to the memory device.

4. The system of claim 3, wherein the ODT circuitry of each memory device of the plurality of memory devices is configured to override the ODT enable signal from the source external to the memory device with an ODT setting specified by the set of one or more control registers in the memory device.

5. The system of claim 4, wherein in a first memory device of the plurality of memory devices, a first subset of the one or more signal lines of the RQ bus is terminated at all times, and a second subset of the one or more signal lines of the RQ bus is selectively terminated in accordance with the ODT enable signal from the source external to the memory device.

6. The system of claim 1, wherein on-die termination of a first signal line of the RQ bus is enabled by the ODT circuitry of a memory device of the plurality of memory devices positioned, on the RQ bus, furthest away from a memory controller and is disabled by the ODT circuitry of all memory devices of the plurality of memory devices other than the memory device furthest from the memory controller/interface.

7. The system of claim 1, wherein the ODT circuitry of a respective memory of the plurality of memory devices is configured, in accordance with the set of one or more control registers in the respective memory device, to enable on-die termination of a first signal line of the RQ bus and to disable on-die termination of a second signal line of the RQ bus.

8. The system of claim 1, wherein the ODT circuitry of a respective memory of the plurality of memory devices is configured, in accordance with the set of one or more control registers in the respective memory device, to enable on-die termination of a first signal line of the RQ bus with an impedance having a value determined by a register field of the set of one or more control registers in the respective memory device.

9. The system of claim 1, wherein
the ODT circuitry of the first memory device of the plurality of memory devices is configured, in accordance with the set of one or more control registers in the respective memory device, to enable on-die termination of a first signal line of the RQ bus with a first impedance having a first value determined by a register field of the set of one or more control registers in the first memory device, and
the ODT circuitry of the second memory device of the plurality of memory devices is configured, in accordance with the set of one or more control registers in the second memory device, to enable on-die termination of a first signal line of the RQ bus with a second impedance having a second value, distinct from the first value, determined by a register field of the one or more control registers in the second memory device.

10. The system of claim 9, wherein the first value corresponds to a weak termination and the second value corresponds to a strong termination, wherein the weak termination corresponds to a higher impedance than the strong termination.

11. A memory device comprising:
address and command (RQ) lines to receive command and address information; and
on-die termination (ODT) circuitry coupled to the RQ lines, the ODT circuitry to enable or disable termination of one or more of the plurality of RQ lines; and
a set of one or more control registers for controlling on-die termination values applied by the ODT circuitry to one or more signal lines of the RQ lines.

12. The memory device of claim 11, wherein the RQ lines includes command and one or more clock signal lines and the set of one or more control registers includes at least one control register for controlling on-die termination of the one or more clock signal lines by the ODT circuitry of the memory device.

13. The memory device of claim 11, wherein the ODT circuitry further includes at least one input for receiving an ODT enable signal from a source external to the memory device.

14. The memory device of claim 13, wherein the ODT circuitry is configured to override the ODT enable signal from the source external to the memory device with an ODT setting specified by the set of one or more control registers in the memory device.

15. The memory device of claim 14, wherein a first subset of the one or more signal lines of the RQ lines is terminated by the ODT circuitry at all times, and a second subset of the one or more signal lines of the RQ lines is selectively terminated by the ODT circuitry in accordance with the ODT enable signal from the source external to the memory device.

16. The memory device of claim 11, wherein the ODT circuitry is configured, in accordance with the set of one or more control registers, to enable on-die termination of a first signal line of the RQ lines and to disable on-die termination of a second signal line of the RQ lines.

17. The memory device of claim 11, wherein the ODT circuitry is configured, in accordance with the set of one or more control registers, to enable on-die termination of a first signal line of the RQ lines with an impedance having a value determined by a register field of the set of one or more control registers.

18. The memory device of claim 17, wherein the value determined by the register field is one of a predefined plurality of values that can be stored in the register field, each of which corresponds to a distinct impedance of the ODT circuitry.

19. The memory device of claim 17, wherein a first value of the predefined plurality of values corresponds to a weak termination and a second value of the predefined plurality of values corresponds to a strong termination, wherein the weak termination corresponds to a higher impedance than the strong termination.

20. A memory module comprising:
a plurality of memory devices arranged on a printed circuit board (PCB) and connected in a fly-by topology to an address and command (RQ) bus having a plurality of address and command (RQ) lines, each of the memory devices having on-die termination (ODT) circuitry for connecting to the address and command (RQ) bus, each memory device including a set of one or more control registers for controlling on-die termination values of one or more signal lines of the RQ bus;
wherein
a first memory device of the plurality of memory devices includes a first set of one or more control registers to store a first ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the first memory device; and
a second memory device of the plurality of memory devices includes a second set of one or more control registers to store a second ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the second memory device.

21. A system comprising:
a plurality of electronic devices arranged in a fly-by topology, each of the electronic devices having on-die termination (ODT) circuitry for connecting to an address and command (RQ) bus, the ODT circuitry of each electronic device including a set of one or more control registers for controlling on-die termination of one or more signal lines of the RQ bus;
wherein the ODT circuitry of each electronic device of the plurality of electronic devices further includes at least one input for receiving an ODT enable signal from a source external to the electronic device, and the ODT circuitry of each electronic device of the plurality of electronic devices is configured to override the ODT enable signal from the source external to the electronic device with an ODT setting specified by the set of one or more control registers in the electronic device.

22. The system of claim 21, wherein a first electronic device of the plurality of electronic devices includes a first set of one or more control registers to store a first ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the first electronic device and a second electronic device of the plurality of electronic devices includes a second set of one or more control registers to store a second ODT value, for controlling termination of one or more signal lines of the RQ bus by the ODT circuitry of the second electronic device.