1. Field of the Invention
The present invention relates generally to the transmission of digital data. More particularly, the invention relates to a high-speed data transmission between integral circuits (ICs) or chips.
2. Description of Related Art
Over last years, operating speed of computer processors has increased dramatically, resulting in additional demands on speeds of transmission of digital data. This has lead to very high frequencies in data transmission lines. At high frequencies some effects occur not observed at low frequencies and connected with the behavior of receiving loads (wires, circuit boards and integrated-circuit packages that make up a digital product).
Small lengths of signal conductors such as shown in FIG. 3a, can act like analog transmission lines, producing reflections which may distort received signals as shown in FIGS. 4a-4c and produce effects such as ringing, bouncing, and overshoot. Such distortion, combined with other sources of noise, such as cross-talk between conductors as shown in FIG. 5b, may produce faulty operations in circuits which otherwise appear to have satisfactory design specifications. That is, at high frequencies load elements directly affect electrical performance. Accordingly, such transmission line effects, especially with respect to circuits having critical timing requirements for signal reception, are becoming the matter of increasing importance.
One high frequency effect of particular concern in the present invention is illustrated in FIG. 5a. When a period of alteration of a signal, i.e. data rate, approaches the settling time of a signal on the data line, inter-symbol interference (ISI) occurs depending on the history of previously transmitted bits. Inter-symbol interference refers to the timing and signal quality impact of the previous state of the data line on the current state.
Charge storage both in the transmitting devices and the state of the transmission line causes a delay in the signal and a change in the slew rate. For example, if a transmitter is driving a very high frequency signal that is 1, 0, 1, 0 then each cycle the driver and transmission line must change statexe2x80x94with very high frequency systems each of these signals will approximate a triangular wave. When the same system transmits 1, 1, 1, 0 then the level of the signal which is already a 1, will be higher than in the case where the state is inverting on every cycle.
Thus, as seen in FIG. 5a, a signal will reach different values depending on how long the present state of the signal is maintained, whereby the cross-point of the signal and the reference shifts, thus causing a skew. To eliminate the skew, a history of the signal shall be analysed and a clock adjusted accordingly.
Another high frequency effect of particular concern in the present invention as shown in FIGS. 4a-4c, is that of reflections which also cause ISI in the line. ISI is a function of the mismatch between the driver impedance, the transmission line impedance, and the transmission line length.
Moreover, a transmission line is generally not uniform. It can include unequal-impedance parts (as shown schematically by 32 and 33) such as conductive traces on different boards or in different layers of the same board, connectors, and/or cables. Interfaces between these parts will also produce reflections which will contribute in affecting the signal.
FIG. 3a illustrates a conventional communication link configuration with a driver 31 and a receiver 34 coupled to a transmission line comprising portions 32 and 33 of different impedance Z1 and Z2. Data that applied to an input terminal of the driver 31 appears at an output terminal of the driver 31, allowing a data signal, such as shown in FIG. 4a, to be transferred via the transmission line 32. If there is a significant impedance mismatch between the transmission lines 32 and 33, a reflection from the line discontinuity (on the border of portions 32 and 33) can occur, so that the signal actually transmitted will be as FIG. 4b, not as shown in FIG. 4a (where t1 is a propagation time in the line portion having impedance Z1). The presence of reflections in a transmission line can cause an increase in the skew and a corresponding decrease in the overall bus transfer rate. Thus, it would be beneficial to have a device adapted to handle these kind of reflections, to enhance thereby bus transfer rate.
This effect can occur also when the impedance of a transmission line does not match the impedance of a terminating load 35 on the driving or receiving end of the line. Ideally, a terminating load R2 will sink a transmitted signal immediately upon the signal arriving at the load. However, the terminating load often does not match the transmission line impedance because of variations in output resistance which can result from the production variations of elements and/or variations in power supply voltage and/or temperature. In this case the load will sink only a portion of the signal upon the signal""s initial arrival. The remaining portion of the signal will be reflected back onto the transmission line after a time t1+t2, so that the signal at the load 35 would be as shown in FIG. 4c (left part). If the receiving circuit provides a terminating load R2 that matches the impedance Z2 of the transmission line, the reflected signal portion will sink on reaching the receiving end 34. Otherwise, the reflected signal portion will be partially reflected again and thus return to the receiving end after a time 3t1+t2 as shown in FIG. 4c (right part). Thus, substantial portions of the reflected signal can go back and forth until they damp in the transmission line. Each time the reflected signal portion comes to the receiving end it affects the main signal by producing an additional skew. This will reduce the precision with which the signal can be measured and therefore will not allow increasing the speed of data transmission.
Efforts were made to eliminate, or at least greatly reduce, signal reflections by matching, insofar as possible, the impedance of a terminating load 35 and that of a transmission line portions 32 and 33.
Known are various means for impedance matching disclosed in U.S. Pat. Nos. 4,719,369; 5,134,311; 5,162,672; 5,811,197; 5,602,494; 6,191,663; 6,175,250; 6,157,215; 6,130,563; 6,127,862; 6,118,310; 6,087,853; 6,060,907; 5,955,894.
Generally, these patents disclose IC output drivers comprising a circuit which compensates for the variations in output resistance.
With the driver according to U.S. Pat. No. 6,118,310, a portion of the signal reflected on the receiving end back onto the transmission line will sink on its driving end and thus not affect the received signal. However, this driver will not help in sinking the above-mentioned reflections produced at the interfaces between the unequal-impedance parts of the transmission line. These reflections can go back and forth between the receiver and respective interface, not reaching the driver end where they could be terminated.
Thus, even though impedance-matching means are used in the prior art, reflections and inter-symbol interference still exist and affect the signal by producing additional skew. Furthermore, topological restrictions and transmission line discontinuities can make impedance balancing impractical or very difficult (and therefore costly) to implement. Even more significantly, signal distortions associated with transmission line effects are due to both reflections and re-reflections of signals, and taking steps to suppress or reduce both type of reflections may be even more difficult, costly and less useful.
Known is a skew reducer described in U.S. Pat. No. 5,953,521, wherein, to compensate for the skew caused by an inter-symbol interference in a signal pattern transmitted, predetermined delays are applied to a data to be transmitted via a transmission line. However, this means can use only two predetermined delay values, while in practice, a multitude of delays may be required. Moreover, the delay values cannot be adjusted to actual transmission channel characteristics because they are fixed at a production stage. And the last, but not the least important issue is that the known means allows to compensate for inter-symbol interference only, not cross-talk effects. At the same time, the cross-talk from adjacent lines is still another actual problem of high-frequency data transmission. Say, if both signals move from 0 to 1 at the same time, the rising edges contain high frequency components that will be radiated from each line to the other and cause both signals to rise faster than if they were to switch in isolation. Conversely if one line moves from 0 to 1 and the other moves from 1 to 0, the signal slew rate will be lower than that where the lines change in isolation.
This effect will be discussed with reference to FIG. 3b and FIG. 5b. 
In FIG. 3b, a two wires"" configuration carrying a digital signal is shown. A signal, such as shown in FIG. 5b, is transmitted along a transmission line 1 driven by a driver 41, the transmission line 2 being quiet. Nevertheless, small signals will appear in the transmission line 2 due to cross-talk effect, as illustrated in FIG. 5c. Therefore, if a signal shown in FIG. 5d is to be transmitted along a transmission line 2, the actual shape of the signal would be as shown in FIG. 5e. It is evident that this situation would result in additional skew as shown in FIG. 5e. 
This cross coupling is strongly pattern dependent: if an 8-bit bus for example has all signals moving from 0 to 1, the skew between signals at the receiver will be very different from where a hexadecimal AA pattern is transmitted. Moreover, the historical pattern dependency still remains and requires a line by line compensation based on the historical state of that line, for example, the last three bits transmitted.
Integrated circuit technology has continually advanced to produce increased device performance largely by shrinking the dimensions of the physical structures fabricated on semiconductor chips. For example, the trend today is to produce devices in signal lines having physical dimensions in the sub-micron range. As structures have been scaled down in size, factors such as noise and signal interference have become major problems. Basically, the close physical distance between adjacent signal lines leads to unintentional coupling and interference.
A variety of techniques have been used to reduce crosstalk between adjacent signal lines. In the field of random access memory arrays, shielded bit line architectures have been employed in which two pairs of opposed bit lines associated with a common sense amplifier have an adjacent unselected line pair that is clamped to AC ground to shield the selected line pair from dynamic coupling effects. This approach is described in U.S. Pat. No. 5,010,524. A similar approach is disclosed in U.S. Pat. No. 5,646,556, which teaches an apparatus for pre-charging pairs of bus conductors to alternating rails in order to minimize crosstalk and speed degradation problems. Another example of modifying the spacing and physical arrangement of the signal lines to prevent crosstalk is described in U.S. Pat. No. 5,475,643.
Other practitioners have approached the problem from a different perspective. For example, U.S. Pat. No. 5,596,506 teaches a method for predicting respective magnitudes of crosstalk voltages before the actual fabrication of an integrated circuit (I/C) chip. Following circuit simulation, the signal line layout of the chip is modified according to an algorithm, which includes changing driver circuits, moving signal lines, and inserting buffer circuits into the chip. U.S. Pat. No. 5,311,074 attempts a more radical approach to reducing crosstalk by reducing signal line voltages through a large scale cell array region of the chip. The signal voltages are restored back to their original logic levels upon exiting the cell array.
Thus, in a high performance digital bus, to obtain a good signal quality, it is important both to eliminate cross-talk effects in the transmission line, and also to reduce inter-symbol interference (ISI). Most often, as has been discussed above, it is impractical to match the driver impedance to the line impedance. Thus, an unfulfilled need exists for an apparatus and method capable of compensating for the intersymbol and crosstalk effects in coupled busses.
The object of the invention is to reduce the skew of signals at a receiver end (at the end of a transmission line) so as to compensate for the effects of cross-talk and various signal reflections, settling time influence, or other kind of inter-symbol interference (like frequency dependent line resistance due to skin effect).
According to the invention, information on signal distortions caused by inter-symbol interference and by cross-talk in the whole transmission path is stored in a storage device and used for compensation for signal skew in these lines. By changing parameters of the driver as a function of the historical data pattern in the line and information on skew caused by inter-symbol interference, on one hand, and, of the current data pattern in all lines and information on skew caused by cross-talk in neighbouring lines, on the other hand, it is possible to compensate for the skew caused by both these reasons.
As a result, a system is proposed which can use the obtained summary information for dealing with various problems caused by different transmission line effects including cross-talk of neighboring lines, pattern effects, etc as mentioned above, and even those, the exact origin of which is unknown. The proposed system may comprise a look-up table containing data for compensating both cross-talk and inter-symbol interference effects. Alternatively, sub-systems for compensating for different effects may be independent. For example, the pattern dependency managing cross-talk may be independent of the system dealing with the historical pattern dependency.
The present invention provides a means to vary parameters of a signal as a function of the data pattern being transmitted by changing the driver strength or by adding or subtracting delay elements to the signal.
According to one aspect of the present invention, a data transmission means for high-speed transmission of digital data is proposed, the data transmission means comprising:
at least one driver for driving a transmission line and
a timing deskewing means connected thereto, wherein
the timing deskewing means comprises
a storage means for recording and storing information on skew caused by inter-symbol interference and cross-talk influence in the transmission line, for at least one data pattern transmitted through the transmission line; and
an adjustment means for generating and applying a correction to the timing position of a signal transition between two logical levels, the correction being generated on the basis of the information stored in the storage means, so as to compensate the above skew.
The combination of the above features enables to eliminate both the crosstalk influence and inter-symbol interference influence which result in a skew at the receiver and to enable transmission and reception of electronic communications orders of magnitude faster than is possible using prior art technique.
In another aspect of the invention, a method of eliminating skew caused by inter-symbol interference and cross-talk effects in transmission lines for high-speed transmission of digital data is proposed, the method comprising:
transmitting of digital data through the transmission line provided by at least one driver;
measuring a skew for at least one data pattern transmitted through the transmission line; and
recording and storing information on skew caused by inter-symbol interference and cross-talk influence in the transmission line, for at least one data pattern transmitted through the transmission line; and
generating and applying a correction to the timing position of a signal transition between two logical levels, the correction being generated on the basis of the information stored in the storage means, so as to compensate for the above skew.
In still another aspect of the invention, a transmission system for high-speed transmission of digital data is proposed, the transmission system including at least one transmission line electrically connecting a transmission means and a receiving means; wherein the transmission means is the transmission means in accordance with the present invention.
The driving circuit may comprise at least one driver, or a plurality of drivers.
Preferably, the storage means stores parameters on previous data patterns.
As an example, a driver with variable drive strength may be used, e.g. a plurality of drive transistors connected in parallel with the gates controlled to determine the number of transistors that are turned on.
In another example embodiment, a driver with a variable delay is used. The delay may be incorporated into the data signal line of the driver as shown in FIG. 1b. Alternatively, the delay may be incorporated into the clock line as shown in FIG. 1c.