Patent Application
20120280723
1. Field of the Invention
This invention is related to the field of integrated circuits and, more particularly, to driver circuits.
2. Description of the Related Art
Integrated circuits (ICs) generally include a core circuit that implements the various operations that the IC is designed to perform, a driver circuit that drives output signals from the core circuit to an external circuit, and a receiver circuit that receives input signals from the external circuit and provides the received signals to the core circuit. The driver and receiver circuitry buffer and isolate the core circuit from the external circuit, thus handling the larger loads, higher current flows, higher voltages, noise, etc. that are typically involved in external communications.
In some embodiments, the driver circuits can be required to provide a specified output impedance. For example, driver circuits for signals that are part of an industry standard interface may be required to provide an output impedance that is specified by the standard. The output impedance can be required to be a specified value, within a specified tolerance, across a range of process conditions, temperature conditions, and supply voltage conditions.
An integrated circuit (IC) may be configured to communicate signals to an external device (e.g., a memory) via drivers. Each driver may include multiple driver circuits arranged in parallel with respect to each other. Each driver circuit may in turn include multiple driver sub-circuits. Based, for example, on the load presented by the external device and/or the design of the external device, the interconnect between the IC and the external device, the presence or absence of termination, etc., a control circuit may provide signals that enable individual ones of driver circuits to result in a selected driver strength (or, equivalently, a selected driver impedance). The control circuit may also provide impedance control signals that enable or disable individual sub-circuits within one or more driver circuit, to thereby control the output impedance of each such driver circuit to account for variations in process, temperature, and supply voltage.
In some embodiments, a driver circuit may include a first sub-circuit having a first pull-up transistor, a first pull-down transistor coupled to the first pull-up transistor at a first node, and a first resistor coupled between the first node and an output pad or pin. The driver circuit may also include a second sub-circuit having a second pull-up transistor, a second pull-down transistor coupled to the second pull-up transistor at a second node, and a second resistor coupled between the second node and the same output pad or pin. Additionally or alternatively, the driver circuit may include several other similarly configured sub-circuits. By selectively enabling sub-circuits in the driver circuit, a desired output impedance for the driver circuit may be realized (or at least approximated within the granularity available via parallel combinations of the sub-circuits.
In certain embodiments, the resistance of a first resistor may be different from the resistance of a second resistor. For example, across various sub-circuits of the same driver circuit, the resistors may be designed with “binary weighting,” such that the second resistor is approximately twice the size of the first resistor, a third resistor is approximately twice the size of the second resistor (and four times the size of the first resistor), and so on. Moreover, each sub-circuit may be configured such that each resistor provides approximately 80% of the output impedance of that sub-circuit, and each of the pull-up and pull-down transistors provides the remaining 20%.
In some embodiments, variations in the load of the external device (coupled to the output pad or pin) and/or in the operating conditions of the IC (e.g., temperature, process speed, voltage, etc.) may prompt the control circuit to issue impedance control signals which enable or disable individual ones of the driver sub-circuits so as to maintain a desired output impedance.
In other embodiments, a method may include determining a programmable drive strength of an interface circuit having multiple driver circuits and enabling a subset of the driver circuits based, at least in part, on the programmable drive strength. The method may also include setting, based at least in part on an operating condition of the interface circuit, an impedance of one or more driver sub-circuits within the subset of enabled driver circuits. In some cases, adjusting the impedance may include enabling different ones of the pull-up or pull-down transistors of the one or more driver sub-circuits within the driver circuit. Furthermore, enabling the different ones of the pull-up or pull-down transistors may include independently enabling a different number of pull-up and pull-down transistors in each driver circuit for the same signal.
The following detailed description makes reference to the accompanying drawings, which are now briefly described.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to.
Various units, circuits, or other components may be described as “configured to” perform a task or tasks. In such contexts, “configured to” is a broad recitation of structure generally meaning “having circuitry that” performs the task or tasks during operation. As such, the unit/circuit/component can be configured to perform the task even when the unit/circuit/component is not currently on. In general, the circuitry that forms the structure corresponding to “configured to” may include hardware circuits. Similarly, various units/circuits/components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase “configured to.” Reciting a unit/circuit/component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, ¶6 interpretation for that unit/circuit/component.
Turning now to
Driver circuit 140 may be coupled to receive the one or more drive signals from the control circuit 180, and may be configured to drive its output at pin 190 “high” (“pull-up”) or “low” (“pull-down”) to allow integrated circuit 100 to communicate with external device 120. Generally, a drive signal may be considered to be asserted in either the high state (“pull-up”) or the low state (“pull-down”), and deasserted in another state (i.e., “tristated”). In some embodiments, driver circuit 140 may not actively drive its output when placed in a “tristated” configuration.
As illustrated, driver circuit 140 receives supply voltage VIO that is used to communicate with external device 120 at output pin 190, and core circuitry 160 is provided supply voltage Vcore. External device 120 is also provided VIO. Integrated circuit 100 is further supplied VSS (e.g., ground) to which the VIO and Vcore voltages are referenced. In some cases, VIO may be higher than Vcore voltage during operation. For instance, in low-power applications, VIO may be approximately 1.2 volts. In other cases, VIO may be approximately 3.3 volts, and Vcore may be approximately 1.8 volts (or less, such as 1.1 volts or even less than 1 volt). While
Core circuit 160 operates with Vcore supply voltage, and thus the drive signals generated by core circuit 160 generally swing between VSS and Vcore. Driver circuit 140 may receive these drive signals and drive output 190 to VIO (in response to an assertion of a pull-up signal) or to VSS (in response to an assertion of a pull-down signal). In some embodiments, control circuit 180 may include one or more level shifters to shift signals that are supplied to the driver circuit 140. For example, a pull-up signal may be shifted so that it swings between VIO and a voltage that is higher than VSS. Level shifting may be user to control transistors in the driver circuit 140 without a voltage drop across its transistor's terminals that would exceed specifications. In such embodiments, control circuit 180 may also be coupled to receive the VIO voltage, as illustrated in
Control circuit 180 is configured to control driver circuit 140. For example, control circuit 180 may be programmable (e.g., in a register) to pull up, pull down, or tristate the output of driver circuit 140. Software may write the register to drive these desired values. Additionally or alternatively, control circuit 180 may control the output of driver circuit 140 automatically according to an interface specification for external device 120. Moreover, control circuit 180 may generate a reference voltage for driver circuit 140 (not shown). For example, such reference voltage may include Vcore, or may be generated from Vcore (e.g., using a band gap generator, etc.). In some embodiments, output pin 190 may be a general purpose I/O (GPIO) pin connected to any external device and controlled by software.
While output pin 190 is illustrated explicitly in
In some cases, driver circuitry 160 may be used within integrated circuit 100, for example, if integrated circuit 100 supports multiple voltage domains within core circuitry 160. And, in some embodiments, external device 120 may be integrated into integrated circuit 100; but may be external to core circuitry 160 and its Vcore voltage domain.
Core circuitry 160 may generally include circuitry that implements various logical operations that integrated circuit 100 is designed to perform. For example, if the design includes one or more processors, core circuitry 16 may include circuitry that implements the processor operation (e.g., instruction fetch, decode, execution, and result write, etc.). Such processors may include general purpose processors and/or graphics processors in various embodiments. If the design includes a bridge to a peripheral interface, for example, core circuitry 160 may include the circuitry that implements the bridge operation. If the design includes other communication features such as packet interfaces, network interfaces, etc., core circuitry 160 may include circuitry implementing the corresponding features. Integrated circuit 100 may generally be designed to provide any set of operations. Generally, core circuitry 160 may comprise any combination of one or more of the following: memory arrays, combinatorial logic, state machines, flops, registers, other clocked storage devices, custom logic circuits, etc.
In some embodiments, the system shown in
Turning now to
In an illustrative, non-limiting embodiment, main driver 260 includes six (6) driver circuits 300-320, each driver circuit having approximately the same impedance (e.g., 240Ω) and provides three selectable drive strengths (e.g., 60, 48, and 40Ω). In operation, when four (4) of the six driver circuits are enabled, the drive strength of main driver 260 is 60Ω. Adding a fifth driver circuit in parallel with the other four drive circuits results in a selected drive strength of 48Ω. And adding yet a sixth driver circuit yields a selected drive strength of 40Ω. In this manner, the drive strength of main driver 260 may be selected by control circuit 180 using drive strength select signals 220.
In addition to providing programmable drive strength, driver circuits within main driver 260 may also provide impedance adjustment/control, as described in more detail below. That is, while the drive strength control may be programmable to selected a desired drive strength, the actual drive strength achieved in response to the programmed strength may vary due to variations in semiconductor fabrication process, operating temperature, and operating voltage. The main driver 260 and more particularly each driver circuit 300-210 may support impedance control to adjust the actual impedance over the various operating conditions.
Turning now to
As illustrated, pull-up transistors 410a-z are p-type metal-oxide-semiconductor (PMOS) transistors, pull-down transistors 420a-z are n-type MOS (NMOS) transistors, and resistors 430a-z are polysilicon resistors (“poly resistors”). In some embodiments, poly resistors 430a-z may be sized to be approximately 10× the size of the minimum semiconductor feature or dimension so as to avoid processing variations as well as to support current density restrictions. Thus, the poly resistors 430a-z may be resistant to variation due to variations in the semiconductor fabrication process used to manufacture the integrated circuit 100. In other embodiments, other types of semiconductor devices may be used that achieve similar functionality as described herein. For example, instead of polysilicon resistors, one or more of sub-circuits 400a-z may include diffused resistors, ion-implanted resistors, or thin-film resistors. Furthermore, transistors 410a-z and 420a-z may include any suitable type of transistor, including, for example, field-effect transistors, metal-oxide semiconductor transistors, etc.
Still referring to
Supplementary pull up and pull down signals may be input into other sub-circuits 400b-z of driver circuit 300 in response to impedance control signals received by main driver 260. For example, a resulting series resistance of pull-up transistor 410a with resistor 430a of sub-circuit 400a may be configured to result in a specified output impedance under a particular operating condition (e.g., process speed, resistance, voltage, temperature, or combinations thereof). Also, a series resistance of pull-up transistor 410b and resistor 430b may be configured such that, when sub-circuit 400b is enabled in parallel with sub-circuit 400a, driver circuit 300 produces a different output impedance at output 190.
In some embodiments, each of resistors 430a-z has a resistance approximately 4 times larger than the resistance of its respective pull-up or pull down transistor 410a-z and 420a-z. In other words, resistor 430a may be designed such that it is responsible for approximately 80% of the series resistance of the combination of resistor 430a with pull-up transistor 410a and/or the combination of resistor 430a with pull-down transistor 420a. Furthermore, in some embodiments resistors 430a-z may be designed using “binary weighting” (e.g., 1×, 2×, 4×, 8×, 16×, etc.). For example, resistor 430b may have a resistance that is approximately twice the resistance of resistor 430a, resistor 430c (not shown) may have a resistance that is approximately four times the resistance of resistor 430a (and twice that of resistor 430b), resistor 430d (not shown) may have a resistance that is approximately eight times the resistance of resistor 430a (twice that of resistor 430c and four times that of resistor 430b), and so on. Additionally or alternatively, sub-circuits 400a-z may be designed such that the series resistance of each sub-circuit (in a pull-up and/or pull-down configuration) is binary weighted with respect to other (e.g., preceding) sub-circuits.
By various combinations of the sub-circuits 400a-z, the impedance presented by the driver 300 may be tuned to adjust for variations in the impedance supplied by the driver 300. At any given point in time, the configuration of sub-circuits 400a-z may account for the current operating temperature and voltage, and thus the impedance supplied by the driver 300 (and ultimately by the main driver 260) may be adjusted to track the desired output impedance of the driver.
Referring to
In some embodiments, calibration circuitry (not shown) within control circuit 180 may determine which drive strength to select in a particular situation. For example, a drive strength of 60Ω may be suitable for a certain set of operating conditions or a specific load presented by external device 120. As these operating conditions or loads change over time, the calibration circuit may determine that a more appropriate drive strength is now 48Ω, for example. As such, control circuit 180 may issue another or a different drive strength selection signal 220 that enables a fifth driver circuit in parallel with the first four (4) circuits, thus resulting in the desired 48Ω drive strength. In some cases, control circuit 180 may itself be controlled software that allows it to respond to these variations in load or operating conditions by changing the drive strength of main driver 260 in a programmable manner.
At 520, the method may enable specific ones of driver circuits 300-320, for example, based on an initial operating condition. Typically, however, programmable drive strength may assume that each driver circuit 300-320 is capable of providing a fixed output impedance. Therefore, to ensure that main driver circuit 260 maintains a relatively constant output impedance that is approximately the desired fixed output impedance, control circuit 180 may also provide impedance control signals 230 that determine which sub-circuits (400a-z) within each drive circuit 300-320 are enabled at a given time (in addition to providing drive strength signals as discussed above). At 530, these impedance control signals, when processed by pre-driver logic 240, provide supplemental impedance control pull-up and pull-down signal pairs 250 to sub-circuits 400a-z of main driver 260.
At 540, the method may determine whether there is a change in operation condition that suggests or requires that impedance should be adjusted or modified. For example, operating conditions at output 190 may have changed (e.g. temperature increased or decreased, supply voltage increased or decreased, etc.). If so, control circuit 180 may provide modified impedance control signals 230 that modify which sub-circuits 400a-z of which driver circuits 300-320 should be enabled. Specifically, software may be executed periodically (e.g., on the order of once a second or once every few seconds) to detect temperature changes and to modify the impedance controls to adjust for temperature changes. Because the series resistance of two or more sub-circuits is different from each other (e.g., binary weighted), control circuit 180 has substantial flexibility in ensuring that each driver circuit 300-320 maintains an approximately constant output impedance, within the granularity possible via various parallel combinations of the sub-circuits 400a-z, and therefore enables main driver 260 to provide the selected drive strength as operating conditions vary.
In certain embodiments, control circuit 180 may be configured to provide impedance control signals 230 that independently enable different numbers of pull-up and pull-down transistors for each sub-circuit 400a-z for a same drive signal 210. For example, control circuit 180 may determine that three sub-circuits (e.g., 400a-c) of a driver circuit (e.g., 300) be enabled for a pull-up portion of drive signal 210, but only two sub-circuits (e.g., 400a and 400b) of the same driver circuit for a pull-down portion of the drive signal. The selection of a different number of pull-up and pull-down transistors may be used, for example, to compensate for variations in the fabrication of the various transistors 410a-z and 420a-z in each sub-circuit 400a-z.
As shown in
In some embodiments, a system may incorporate embodiments of the above described integrated circuit. Turning next to
Peripherals 630 may include any desired circuitry, depending on the type of system 600. For example, in an embodiment, system 600 may be a mobile device (e.g., personal digital assistant (PDA), smart phone, etc.) and peripherals 630 may include devices for various types of wireless communication, such as Wi-Fi™, Bluetooth®, cellular, global positioning system, etc. Peripherals 630 may also include additional storage, including RAM storage, solid state storage, or disk storage. Peripherals 630 may include user interface devices such as a display screen, including touch display screens or multi-touch display screens, keyboard or other input devices, microphones, speakers, etc. In other embodiments, system 600 may be any type of computing system (e.g., desktop and laptop computers, tablets, network appliances, mobile phones, personal digital assistants, e-book readers, televisions, and game consoles).
External memory 620 may include any type of memory. For example, external memory 620 may include SRAM, nonvolatile RAM (NVRAM, such as “flash” memory), and/or dynamic RAM (DRAM) such as synchronous DRAM (SDRAM), double data rate (DDR, DDR2, DDR3, etc.) SDRAM, Rambus® DRAM, etc. External memory 620 may include one or more memory modules to which the memory devices are mounted, such as single inline memory modules (SIMMs), dual inline memory modules (DIMM5), etc.
Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.
