Digital transmission circuit and method providing selectable power consumption via single-ended or differential operation

The present application is related to U.S. patent application Ser. No. 10/289,777, Publication No. 20040203483, entitled “INTERFACE TRANSCEIVER POWER MANAGEMENT METHOD AND APPARATUS”, filed on Nov. 7, 2002, having at least one common inventor and assigned to the same assignee. The specification of the above-referenced patent application is herein incorporated by reference.

1. Technical Field

The present invention relates generally to communication link circuits, and more particularly, to digital signal transmitters having selectable drive capability and power consumption.

2. Description of the Related Art

Interfaces between present-day system devices and also between circuits have increased in operating frequency and complexity. In particular, high-speed serial interfaces include transmitters and receivers that typically consume relatively large amounts of the power budget of an integrated circuit. However, depending on channel conditions and parameters, the maximum output signal level of a transmission circuit may not be required for proper signal reception at the remote end of the interface. For example, the channel physical length may be shorter in some applications than in others, reducing signal degradation and thereby reducing transmit power requirements for the same receiver complexity.

Due to limited design resources and the need to satisfy the requirements of multiple interface applications, customers and channel conditions, transmitters and receivers within above-described interfaces are typically designed for the worst-case bit error rates and environmental conditions, leading to relatively complex receivers and high power transmitters. As a result, it is not always possible to provide a transmitter having lower power consumption when a high channel quality is available.

The above-incorporated Patent Application discloses an interface in which the transmitters and receivers have adjustable and/or adaptive parameters for finely tuning an interface to manage power consumption. One of the controllable parameters is the transmitter power level of the interface driver circuit(s). However, typical transmit power adjustment performed by changing the driver voltage and/or current levels is not always a preferable mechanism to adjust transmit power. Communications links within and between computer subsystems have reached bandwidths of between 5 gHz and 10 gHz and interface frequencies can be expected to increase in the future. Driver circuits operating at such high frequencies do not typically scale operation well over voltage or bias current adjustments, as internal impedances change with such adjustments causing mismatch and loss of power. Delay also typically increases with reduced transmitter power, compromising the integrity of the data window.

Transmitter drivers as described above are not generally simple digital buffers or inverters that switch power rail levels using an effectively near-zero impedance onto the interface line, but are typically linear driver circuits providing multiple stages of amplification or switches having progressively increasing and controlled signal current levels. The cascaded driver circuits progressively raise the power level of the signals internal to an integrated circuit or subsystem to the level required for transmission across the interface channel. Such complex circuits are susceptible to impedance changes due to power level adjustment and therefore such adjustment may not result in optimum performance for a given power consumption level. For example, if the mismatch between stages and at the input of the driver circuit increases when the transmit power level is dropped, the transmit power decrease will not be linear. Such non-linearity indicates an inefficiency of the driver at the lower power level. Similarly, if the transmitter is designed so that impedance matching is ideal at the lower power level, then at higher power consumption levels, the output power will not provide a linear increase with the consumption level. Delay necessarily increases with decreased signal levels through the stages due to device capacitance changes and charge effects relative to the reduced signal strength.

It is therefore desirable to provide an interface transmitter circuit having selectable power consumption that is efficient in both high and low power modes.

The objective of providing an interface transceiver having selectable reduced power consumption while retaining power efficiency is achieved in a method and apparatus.

The apparatus is a transmitter circuit for transmitting a digital signal and having multiple cascaded driver stages. The final driver stage has an output connected to one or more interface signal lines and has a pair of differential inputs. The power level of the transmitter circuit is controlled by selectively enabling one or more pre-driver circuits providing the input to one or both of the differential inputs, providing a reduction in power consumption. If the final portions of both pre-driver circuit paths are disabled, then a single-ended to differential signal converter is used to provide the inputs to the final driver stage, selected via a multiplexer. In fully differential mode, both pre-driver paths are fully are enabled and the single-ended to differential converter is disabled.

The selection and power control process may be programmable by a logic connection, register bit or via a signal from an interface quality measurement circuit. A remote transmitter may also be power-managed at the other end of the interface by transmitting a control signal to the remote transmitter.

The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like components, and:

FIG. 1 is a block diagram of transceivers connected by an interface in accordance with an embodiment of the invention.

FIG. 2 is a schematic diagram of a transmission circuit in accordance with an embodiment of the invention.

FIG. 3 is a signal diagram depicting signals within the circuit of FIG. 2 .

FIG. 4 is a flowchart depicting a method in accordance with an embodiment of the invention.

With reference now to the figures, and in particular with reference to FIG. 1 , there is depicted a block diagram of transceivers 12 A and 12 B connected by an interface or channel 10 in accordance with an embodiment of the invention. Transceivers 12 A, 12 B may be located within a device such as a computer peripheral, a computer system, or within integrated circuits interconnected within a system. Interface 10 may be a single two wire bi-directional interface as depicted, or may be a full-duplex single wire interface or a bus having multiple transceivers in a half-duplex or full-duplex configuration. Transceivers 12 A and 12 B are connected to interface 10 each using a receiver 14 A and 14 B and a transmitter 16 A and 16 B, but the present invention is applicable to transmitters in general, and it should be understood that a transmitter in accordance with an embodiment of the invention may be incorporated in devices for connection to any of the above-specified types of interface 10 , as well as other forms of electrical signal interconnection.

Transmitters 16 A, 16 B of the present invention incorporate power control mode selection inputs SELA and SELB that reduce the power consumption of the transmitter when channel conditions permit. Blocks that are unneeded within transmitters 16 A, 16 B are disabled by isolating one or more power supply rails from the block or by designing the block with an enable signal input that prevents or substantially reduces current flow through the block. Lower-power alternative signal path blocks within 16 A, 16 B that support only a fraction of the worst-case interface specification for the interface may also be substituted in place of higher-power consumption blocks that support transmission at the highest rates under the worst-case conditions for the interface.

Thus, the above-described transmitter circuits provide a selectable power consumption that can be used to provide lower power usage and dissipation within transceivers 12 A and 12 B, when channel conditions are good, while maintaining low bit error rates (BERs) using a higher power consumption state when channel conditions are poor.

The selection of power consumption states via select input SELA may be hard-wired or externally programmed using an external signal terminal 17 or may be programmed using a bit register 19 within transceiver 12 A. Receiver 14 A, transmitter 16 A or both may be controlled by one or more selection signals, for example, multiple bits may be provided for each of transmitter 16 A and receiver 14 A so that power consumption may be very finely traded off for receiver processing power or transmitter signal strength, etc. Alternatively, a single bit or external terminal may be used to set a single binary power consumption selection for both transmitter 16 A and receiver 14 A. Details of receivers having selectable power consumption are detailed in the above-incorporated Patent Application and those receivers may be used in concert with the transmission circuits of the present invention.

Transceiver 12 A is an example of a transceiver having external power mode selection via register programming or external connection. Wired or register selection of power modes is very useful in integrated circuits and systems, including computer systems, communication systems or peripherals. In such systems, external terminal 17 can be hard-wired depending on the application (e.g., known short shielded cable length attached to a peripheral dictates a high channel quality or connection of two transceivers on a high-quality circuit board also dictates high channel quality).

Transceiver 12 B is an example of a transceiver having automatic channel-quality-based power level selection in response to a measurement performed by interface quality measurement block 18 , which may be an eye-diagram circuit, an error detection circuit or other mechanism for detecting that the channel quality is less than a desired threshold. Select signal SEL B is provided by an output of interface quality measurement block 18 and automatically selects higher or lower transmitter power in conformity with the measured channel quality.

Another type of transceiver power consumption control is provided by an interface link wherein a register such as programmable register 19 may be set via reception of a command code sent over interface 10 and received by a receiver such as receiver 14 A. Remote interface link control is useful for informing a transceiver about channel conditions when the transceiver being programmed has no ability to determine the channel quality or does not have information regarding channel conditions (such as cable length). Also, return of channel quality from the remote side of an interface provides absolute information about the transmitted signals, where measurement at the local side provides only an estimate based on reception of signals from the remote side. Therefore, in practice, it would be desirable to provide interface quality measurements at both ends of the interface and return channel quality measurement information to the originating side for controlling transmitter power in accordance with the present invention.

Referring now to FIG. 2 , details of a transmitter 20 in accordance with an embodiment of the invention are depicted. A digital serialization block 21 receives a plurality of Data In signals for serialization over interface 10 . At the output of digital serialization block 21 four signals are provided in the depicted example: two differential pairs representing two distinct data signals. The circuit depicted in FIG. 2 is only illustrative of an example and should not be construed as limiting the invention. The present invention is equally applicable to signals that are not serialized/multiplexed at all.

Pre-buffers 22 A and 22 B receive a respective polarity signal from each of the differential pairs and provide a boosted signal to the inputs of a corresponding multiplexer 23 A and 23 B. Multiplexers 23 A and 23 B have select inputs connected to transmit clock signals that select one of the distinct data signals, providing the last stage of serialization. Thus, the outputs of multiplexers 23 A and 23 B represent a differential serialized data stream. However, in low power mode, power mode control 29 disables pre-buffer 22 B, multiplexer 23 B and another prebuffer 24 B that normally receives half of the differential pair provided by multiplexers 23 A and 23 B. The above listed blocks are generally high power consumers and therefore disabling them results in a substantial reduction of power consumption by transmitter 20 . In the depicted embodiment, pre-buffer 24 A is also disabled and the function of providing a differential pair to output driver 28 is provided by a single-ended to differential converter block 26 designed for low power consumption. The output of output driver 28 is depicted as a differential pair, which generally would be the type of signaling used on such an interface in order to achieve the advantages of channel quality sufficient that the low power mode can be used under appropriate channel conditions. However, the internal architecture and power control of the present invention can be applied to a single-ended interface driver that has differential inputs.

Single-ended to differential converter block can be provided by a capacitively-coupled amplifier or comparator 27 as shown, or another digital or analog circuit capable of providing a differential output from a single-ended signal. Single-ended to differential converter block 26 is disabled by power mode control 29 when the high-power operating mode is selected. Multiplexers 25 A and 25 B have a select input coupled to power mode control 29 for selecting between outputs of single-ended to differential converter block 26 and the outputs of pre-buffers 24 A and 24 B.

Alternatively, if output driver 28 is designed such that it will operate with a single-ended input, the + input of output driver 28 may be appropriately biased and pre-buffer 24 A can be left enabled, in which case multiplexer 25 A is not needed. Multiplexer 25 B may be needed depending on the output level of pre-buffer 24 B when disabled.

Disabling the various blocks listed above is generally accomplished in one of two manners: one or more power supply rails are disconnected from the blocks via power control transistors, or the blocks are designed with internal devices that block some or all of the current conduction through the block, as in the case with digital circuits having internal circuits that can suppress switching action in response to a disable input.

The result of the power control of transmitter 20 is that the delay and signal strength through transmitter 20 is changed depending on power mode. Increased delay due to lower-power operation is tolerable when channel conditions permit, as is the lower signal strength provided when the low-power mode is selected. Such conditions can either be detected via the feedback mechanisms detailed with respect to FIG. 1 above, or may be selected based on expected channel conditions and architectures such as interface connection type and environment.

Referring now to FIG. 3 , a signal diagram depicting the timing and relative amplitude of signals within transmitter 20 is shown. Data signals D 0 − and D 1 − are shown for illustration (signals D 0 + and D 1 + would be complementary to them and are not shown). The outputs of multiplexers 23 A and 23 B are relatively low amplitude signals representing the multiplexed data signals as a differential pair PreBuff Out− and PreBuff Out+. (PreBuff Out+ is suppressed in low-power mode, as multiplexer 23 B and pre-buffer 22 B are disabled.) Next, the outputs of prebuffers 24 A and 24 B, Diff Out + and Diff Out − are shown as they would occur in high-power mode and the outputs of single-ended to differential converter 26 , SEC Out+ and SEC Out− are shown as they would occur in low-power mode. Tdelay depicts the increased internal delay due to the lower-power operation of single-ended to differential converter 29 versus pre-buffers 24 A and 24 B, and the relatively lower amplitude of the SEC Out signals is also due to the lower-power operation. (It should be noted that any resulting skew from the single-ended to differential conversion is tolerable in low-power mode, as the low-power mode condition corresponds to a much wider jitter margin due to favorable link and channel conditions.) Finally, the TxOut signal is depicted as a dashed line (high power mode) and a dotted line (low power mode) to illustrate the increased delay and lower amplitude of the signal provided to interface 10 . Tmargin represents the maximum potential delay under which the above power control scheme can used in low power mode. If the required delay margin at the remote interface is met when the low power mode delay of transmitter 20 is as depicted, then single-ended to differential converter 26 (and/or other blocks within transmitter 20 that are operational when in low power mode) can be scaled to operate at the power level producing that delay when transmitter 20 is in low power mode.

Referring now to FIG. 4 , a control method in accordance with an embodiment of the present invention is shown in a flowchart. The depicted method illustrates the operation of the above-described circuits using feedback from interface quality measurement block 18 . First, interface channel quality is measured (step 40 ) and channel quality information is sent to the transmitter side of the interface (step 41 ). If the interface channel quality is sufficient to support a lower power consumption state within transmitter 20 (decision 42 ), single-ended internal operation of transmitter 20 is selected (step 43 ). If the interface channel quality is not sufficient (decision 42 ) then the differential internal operation topology is selected (step 44 ). While the above-illustrated method provides a sophisticated control of signaling on interface 10 , it should be understood that such control is not necessary to practice the present invention, which encompasses selection of the internal operating topology of transmitter 20 via programmatic or hardwired control as well. In such cases, decision 42 is made a priori, predicted during operation or determined in some other manner such as during an interface check or upon detection of error conditions. Step 43 or 44 is then performed in response to the hardwired or programmatic signal.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.