Multi-rate network transmission circuit

A network transmitter and an associated transmitting method are disclosed. The network transmitter includes a signal converter and a signal driver. The former can convert an input signal into a current signal, and the latter can output a differential transmission signal according to the current signal. The signal driver includes a feedback network which can switch between a first configuration and a second configuration.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to network devices, and more particularly, to a transmitter of the network device and a method thereof.

2. Description of the Prior Art

Network technology is rapidly developed in recent years, and the demand for bandwidth is also growing increasingly. Along with this, the rate of network devices is upgraded from 10/100 Mbps to 1 Gbps. For example, each port of a 1 Gbps Ethernet device has four channels, each of which includes a transmitter and a receiver for transmitting and receiving differential transmission signals respectively, thereby communicating with other network devices.

FIG. 1is a diagram of the transmitter of a conventional Ethernet device. As shown inFIG. 1, the transmitter10lies within an IC chip, and includes a voltage-controlled current source11, a differential amplifier12, and feedback resistors Rf1, Rf2.

To avoid the overshoot of the differential transmission signal, the voltage-controlled current source11performs slew rate control. That is, when the level of the differential transmission signal changes, the rise time (or fall time) is controlled to prevent the signal level from changing too fast. However, when the Ethernet device operates in 1 Gbps transmission rate, the voltage-controlled current source11is required to output a differential current with 17 levels, so as to match with 17 levels of the differential transmission signal defined in the related Ethernet specification. Thus, the circuit within the voltage-controlled current source11would be very difficult to design and would consume much more chip space.

SUMMARY OF INVENTION

It is therefore one objective of this invention to provide a network transmitting unit, thereby simplifying circuit design to save chip space and routing.

Another objective of this invention is to provide a network transmitting unit which can selectively perform slew rate control according to the transmission rate, thereby providing more flexible transmission.

According to one embodiment of this invention, a transmitter of a network device is provided. The transmitter outputs a transmission signal at a transmission rate, which is either a first transmission rate or a second transmission rate. The transmitter comprises: a signal converter configured to convert an input signal into a converted signal; and a signal driver, coupled to the signal converter, configured to output the transmission signal according to the converted signal. The signal driver configures to a first configuration when the transmitter outputs the transmission signal at the first transmission rate, and configures to a second configuration when the transmitter outputs the transmission signal at the second transmission rate.

According to another embodiment of this invention, a transmitter in a network device is provided. The transmitter outputs a transmission signal at a transmission rate, which is either a first transmission rate or a second transmission rate. The transmitter comprises: a signal converter which operates in a first mode or a second mode, and converts an input signal into a converted signal having a plurality of levels; and a signal driver, coupled to the signal converter, for outputting the transmission signal according to the converted signal. The signal converter operates in the first mode if the transmitter outputs the transmission signal in the first transmission rate, and operates in the first mode if the transmitter outputs the transmission signal in the second transmission rate.

According to another embodiment of this invention, a transmitting method used in a transmitter of a network device is provided. The method comprises: receiving an input signal; performing a level conversion on the input signal to output a first signal; determining whether to use a first characteristic or a second characteristic to convert the first signal into a transmission signal according to a transmission rate of the transmitter; and transmitting the transmission signal according to the transmission rate of the transmitter.

DETAILED DESCRIPTION

FIG. 2is a diagram of the first embodiment of the network transmitter according to the present invention. The transmitter20inFIG. 2is for transmitting differential transmission signals. As shown inFIG. 2, the transmitter20comprises a signal converter21, a voltage-mode driver22, a calibration circuit23, and a switch circuit24. The signal converter21converts an input signal S1into a differential current signal, and the voltage-mode driver22outputs a differential transmission signal Tx+, Tx−, via the matching resistors Rm1, Rm2and the pads P, according to the differential current signal. In a preferred embodiment, the values of the matching resistors Rm1, Rm2are adjusted in accordance with outside circuits. In an embodiment, please refer to the U.S. patent application Ser. No. 10/978,628, filed Nov. 1, 2004, titled “Network transmitting unit with correction function”, which has the same assignee with the present invention and is hereby incorporated by reference.

The signal converter21switches between a first mode and a second mode, and outputs a differential current signal having a plurality of levels. When the signal converter21operates in the first mode (i.e. the transmission rate is 1 Gbps), a direct level conversion is performed. That is, the differential current signal is directly changed from a level to a next level, without being changed to any intermediate level during the conversion.

On the other hand, when the signal converter21operates in the second mode (i.e. the transmission rate is 100 Mbps or 10 Mbps), an indirect level conversion is performed. That is, the differential current signal is changed to at least one intermediate level during the conversion from a level to a next level. The number and the change sequence of the intermediate level are determined according to a rise time (or fall time) of the differential current signal, thereby conforming to the related Ethernet specification.

In one embodiment, the signal converter21is a digital-to-analog converter (DAC). In another embodiment, the signal converter21comprises a current cell module including a plurality of current cells, and each level of the outputted differential current signal is corresponding to a possible configuration of the current cells. The current cells can be implemented by switched current cells, each provides a substantially equal current and is connected or disconnected by a switch. In the first mode (i.e. direct level-converting mode), 16 current cells are used since the signal converter21is required to provide a differential current signal having 17 levels.FIGS. 3A and 3Bare diagrams showing how the signal converter21operates in the first mode. InFIG. 3A, the 16 current cells provide 17 possible current cell configurations. A current signal with 17 levels can then be generated by calculating the total current for each of the 17 current cell configurations. In one embodiment, each current cell is coupled to a flip-flop, which generates a corresponding control signal according to the corresponding bit of a received input signal and a clock signal, as shown inFIG. 3B.FIG. 3Cis a partial circuit diagram of the signal converter21. InFIG. 3C, Q andQ(the inverse of Q) are the corresponding control signals outputted by the flip-flops inFIG. 3B. In sum, when the signal converter21operates in the first mode, the level of the outputted differential current signal changes once according to the input signal value during each clock period, without changing to any intermediate level. When the transmission rate is 1 Gbps, the signal converter21does not need to perform the slew rate control since the voltage-mode driver22uses a feedback network with a specific configuration, as described later.

On the other hand, the 100 Mbps Ethernet specification requires a three-level differential transmission signal. Thus, the 16 current cells of the signal converter21are divided into two groups, each of which contains eight current cells, to provide three current cell configurations, as shown inFIG. 4A. InFIG. 4A,1and0represent that the corresponding group of current cells are all connected and disconnected, respectively. Since the signal converter21operates in the second mode (i.e. indirect level-converting mode) at 100 Mbps transmission rate, the current cells within each group become all connected or disconnected in a step-by-step manner.FIG. 4Bshows that the current cells (cell <7:0>) within one group become all connected or disconnected in four steps. InFIG. 4B, four clock signals with various phases, denoted as clock <3> . . . clock <0>, are applied in turn to control the eight current cells (each clock signal for two cells). Another group of current cells (cell <15:8>) operates in the same manner synchronously. Thus, the differential current signal would be changed to three intermediate levels in turn during the conversion from a level to a next level. By adjusting the phase difference among the four clock signals, the time spent for the conversion can be controlled to conform to the related Ethernet specification.

The voltage-mode driver22receives the differential current signal from the signal converter21, and outputs the differential transmission signal. As shown inFIG. 2, the voltage-mode driver22comprises a differential amplifier221and a feedback network222. In the feedback network222, Cf1and Cf2are adjustable capacitors adjusted by the calibration circuit23before the transmitter20starts transmission, as described later. The feedback network222also includes switches SW1and SW2, coupled to the feedback capacitors Cf1and Cf2respectively.

The feedback network222switches between a first configuration and a second configuration, so as to provide two various transfer characteristics. The first configuration is a resistor-capacitor network (RC network). That is, the switches SW1and SW2are kept connected such that Cf1and Cf2are parallel connected with Rf1and Rf2respectively. The second configuration is a resistor network. That is, the switches SW1and SW2are kept disconnected such that Cf1and Cf2are also disconnected from the feedback network222. When the transmitter20operates in 1 Gbps, the signal converter21switches to the first mode (direct level-converting mode) and the feedback network222switches to the first configuration (i.e. RC network). Since the charge/discharge characteristic of the feedback capacitors Cf1and Cf2, the level of the differential transmission signal would change smoothly, not drastically (the degree of smoothness is determined by the RC time constant of the RC network). Thus, by adjusting the capacitor values of Cf1and Cf2, the rise/fall time of the differential transmission signal can be controlled to meet the related specification.

On the other hand, when the transmitter20operates in 100 Mbps, the signal converter21switches to the second mode (indirect level-converting mode) and the feedback network222switches to the second configuration (i.e. resistor network). As mentioned earlier, by adjusting the phase difference among the clock signals used in the indirect level conversion, the time spent for the conversion from a level to a next level can be controlled to meet the related specification.

The calibration circuit23is coupled to the feedback network222, and adjusts the RC time constant of the feedback network222before the transmitter20starts transmission.FIG. 5is a block diagram of one embodiment of the calibration circuit23. The calibration circuit23comprises: a first simulation circuit231for outputting a first simulation signal corresponding to the feedback resistor Rf1; a second simulati on circuit232for outputting a second simulation signal corresponding to the feedback capacitor Cf1; a comparator233for outputting a comparison result according the first and second simulation signals; and a control circuit234for generating a calibration signal according to the comparison result. The calibration signal is provided to the feedback network222to adjust the RC time constant thereof.

In the embodiment ofFIG. 5, the resistor Rsis for simulating the values of the feedback resistors Rf1and Rf2of the feedback network222, and the capacitor Csis for simulating the values of the feedback capacitors Cf1and Cf2of the feedback network222. The first simulation signal is the voltage level at point a, i.e. Va=I1*Rs; the second simulation signal is the voltage level at point b, i.e. Vb=I2/Cs*t, wherein t is the time spent for the charging of the capacitor Cs. In order to generate a specification-conforming RC time constant, the capacitor Csneeds to cause the voltage levels of the points a and b to be equal after being charged for a predetermined time T. That is,
Va=VbEq. (1-1)
I1*RsI=I2/Cs*T
Rs*Cs=I2/I1*TEq. (1-2)

If the comparison result of the comparator233shows that Eq. (1-1) is not met, the RC time constant needs an adjustment. For instance, if the result shows that Vais larger than Vb, it means the charging of Csis not fast enough. Thus, the capacitor value of Csshould be decreased. The control circuit234adjusts the capacitor value of Csaccording to the comparison result (the control circuit234is also coupled to Cf1and Cf2inFIG. 5). When the capacitor value of Csis adjusted to meet Eq. (1-1), Eq. (1-2) represents the RC time constant required for the feedback network222. It is also applicable to adjust Rf1and Rf2only, or adjust Rf1, Rf2, Cf1, and Cf2in the meantime.

InFIG. 2, the switch circuit24is for controlling the feedback network222to switch between the first and second configurations, and controlling the signal converter21to switch between the first and second modes, according to the transmission rate of the transmitter20.

FIG. 6is a diagram of the second embodiment of the transmitter according to the present invention. As shown inFIG. 6, the second embodiment is a combination of the first embodiment inFIG. 2and a current-mode driver61. As to the principle and operation of an embodiment of the current-mode driver61, please refer to the U.S. patent application Ser. No. 10/909,811, filed Aug. 2, 2004, titled “Network device with hybrid-mode transmitter”, which has the same assignee with the present invention and is hereby incorporated by reference.

While the present invention has been shown and described with reference to the preferred embodiments thereof and in terms of the illustrative drawings, it should not be considered as limited thereby. Various possible modifications and alterations could be conceived of by one skilled in the art to the form and the content of any particular embodiment, without departing from the scope and the spirit of the present invention.