Abstract:
A very low voltage swing is used to achieve very high data rates (up to 4 Gbps double data rate) at very low power consumption. A differential signaling approach is used for noise rejection, and a constant current approach also is used to minimize switching noise.

Description:
GOVERNMENT LICENSE RIGHTS 
     The United States Government may have certain rights in some aspects of the invention claimed herein, as the invention was made with United States Government support under award/contract number NBCHC010038 for the Department of Interior/National Business Center. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to communications between and transfer of signals between integrated circuit chips, interconnections between such chips and methods of connecting integrated circuit chips for high speed communications therebetween. 
     2. Description of the Related Art 
     The coupling of signals between integrated circuit chips, either on the same circuit board or between boards, is of critical interest. Metal interconnections (i.e., metal backplanes) appear to have reached their speed limits, which are estimated to be in the 100s of Giga-bits-per second (Gbps), for each backplane. Discrete channel fiber-optical connections are being incorporated on top of the metal interconnections, but they too are limited to approximately 10 Gbps per channel. 
     The LVDS standard (Low Voltage Differential Signaling) is currently a popular standard and is based on differential data transmission. This popularity is mainly driven by the ability of LVDS to deliver high speed transmission without large power consumption. LVDS is a differential scheme, which uses two signal lines (traces or conductors) to convey information, with increased noise tolerance in the form of common-mode rejection being achieved. Because of the improvement in signal-to-noise rejection, the signal swing could be dropped to only a few hundred millivolts. Specifications for the LVDS standard may be found in ANSI/TIA/EIA-644-1995 Electrical characteristics standard titled: “Electrical Characteristics of Low Voltage Differential Signaling (LVDS) Interface Circuits.” 
     In addition, LVDS offers other benefits that include low voltage power supply compatibility, low noise generation, high noise rejection, robust transmission signals, and integration. For these reasons, it has been deployed across market segments wherever the need for speed exists. Even with all these benefits, there are some limitations in certain applications, including support of multipoint bus configurations, operation from still lower power rail voltage, and extended receiver common-mode range. 
     Of particular interest and need is the ability to increase transmission power efficiency in terms of data transmission speed per unit of power. 
     SUMMARY OF THE INVENTION 
     The present invention solves this need by providing a new design for low power, high speed chip-to-chip communication (either on the same circuit board or between boards on a common backplane). 
     According to the present invention, a very low voltage swing is used to achieve very high data rates (up to 4 Gbps double data rate) at very low power consumption. A differential signaling approach is used for noise rejection, and a constant current approach also is used to minimize switching noise. 
     The invention provides according to one preferred embodiment a complete link for high speed, low power chip-to-chip communication, including driver and receiver configurations. 
     According to an embodiment of the present invention, the differential swing and common mode are decreased so as to effectively increase the data speed and lower the power usage of integrated circuit chip I/O cells. To accomplish this, according to one exemplary embodiment of the invention novel designs of amplifier, inverter, ESD (electrostatic discharge) protection, current mirror, and current driver components are provided to achieve the desired data transfer speed and power consumption goals. 
     In particular, according to one aspect of the invention, an input/output interface circuit for transferring data between integrated circuit chips is provided, which includes a driver circuit including an inverter stage and a constant current driving stage, that receives an input data signal and develops a differential voltage signal corresponding thereto, a receiver circuit that receives the differential voltage signal from the driver circuit, including a differential in/differential out amplifier stage that receives the differential voltage signal and outputs an amplified differential signal, a differential in/single-ended out amplifier stage that receives said amplified differential signal and outputs a voltage swing signal, and an inverter stage for increasing drive strength of said voltage swing signal for transfer into a chip core. 
     According to a second aspect of the invention, a method of providing low power, high speed data transfer between integrated circuit chips is provided, including the steps of converting data output from an integrated circuit chip to a differential voltage signal having a common mode range of 1 V, and a voltage swing of 100 mV, converting the differential voltage signal to a constant current driving signal and applying said constant current driving signal to a differential receiver circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a chart showing a modified differential voltage swing and common mode range according to one preferred embodiment of the invention; 
         FIG. 2  is a circuit diagram of a receiver interface according to one preferred embodiment of the present invention; 
         FIG. 3  is a circuit diagram of a driver interface according to one preferred embodiment of the present invention; and 
         FIG. 4  is a chart showing input waveform vs. output waveform for the circuit designs of  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates the difference between a conventional LVDS signaling standard and the modified design according to one preferred embodiment of the invention. As shown, according to a conventional LVDS standard, a common mode voltage range of 2.4 V, and a differential signal swing of 400 mV at 1.25 V are used. According to one embodiment of the present invention, a modified common mode range of 1 V is used, along with a differential signal swing of 100 mV at 0.55 V. In order to realize the use of such modified levels, new designs of circuit components for both a receiver and driver circuit are provided. 
     Referring to  FIG. 2 , one preferred embodiment of a receiver circuit  20  according to the present invention is disclosed. A first stage of the receiver circuit is an ESD protection stage  21 , including a pair of modified transmission gates, a first of which receives a high differential voltage input Vih, and a second of which receives a low differential voltage input Vil, across a resistor R 1 . A second stage of the receiver is a differential in/differential amplifier stage  22 . Transistor matching was used to increase the circuit&#39;s sensitivity to small differential signals. A first input MXP8 of stage  22  receives the high differential voltage Vih from ESD protection stage  21 , and a second input MXP9 of stage  22  receives the low differential voltage Vil from ESD protection stage  21 . Differential voltage signal are outputted at nodes NR 2  and NR 3 . 
     A third stage of the receiver circuit is a differential in/single-ended out amplifier stage  23 . This stage receives differential voltage inputs at gates MXP 11  and MXP 12 , and outputs a single voltage signal at output node NR 6 . The output voltage signal from amplifier stage  23  is a weak, large swing (i.e. nearly rail-to-rail) signal. 
     The final stage of the receiver circuit  20  is an inverter series stage  24 , which includes a number of inverters arranged in series to increase the drive strength of the receiver into the core of a chip. While only two inverters are shown in the example of  FIG. 2 , it will be recognized by those of skill in the art that a greater or lesser number of inverters also may be used in accordance with specific application/chip design considerations. The inverter stage receives the single voltage output signal from amplifier stage  23  at input X 17 , and outputs a final output voltage signal Dig_Out at output node NR 8 . 
     Referring now to  FIG. 3 , an exemplary embodiment of a driver circuit  30  according to the present invention is described. The driver circuit is composed of three main stages. The first stage is an inverter stage comprised of two groups  31 ,  32  of three inverters each. One of the inverters of group  32  is used as a signal delay to compensate for the propagation time required for the data signal to transition to the next state. The inverter stage  31 ,  32  receives an input signal Dig_In, processes the signal and provides the signal at each of nodes ND 2  and ND 7 . 
     The next stage is a current driving stage  33 . The current driving stage  33  provides driver differential voltage signals Vin and !Vin at nodes ND 4  and ND 9 . The final stage is an ESD protection stage  34  formed of modified transmission gates. The ESD protection stage receives the differential voltage signals and passes them through to a receiver. There are two areas of ESD protection in the driver circuit  30 . In addition to the ESD stage  34 , the transistors on either side of nodes ND 4  and ND 9  provide increased ESD protection. The main purpose of these transistors is to modify the DC offset of the output differential signal from the driver. 
       FIG. 4  is a chart showing input v. output waveforms for the I/O transfer circuit. The input signal to the driver, which typically is from the core of a chip, is indicated by the wave having an arrow pointing to the driver input at the bottom of the figure. The output waveform of the receiver is indicated by the wave having an arrow pointing to the receiver output at the bottom of the figure. The 100 mV differential signal, with a DC offset of approximately 600 mV, is indicated by the arrows pointing to the input of the receiver at the bottom of the figure. 
     Table 1 below shows the relationship between input signal frequency and average power consumed. As shown, the power requirements of the circuit increase as the frequency (i.e., speed) of the input signal increases. As seen, the circuit exhibits a non-linear increase in efficiency as frequency increases. When the clock rate of the input signal is more than doubled from 0.6 GHz to 2 GHz, the average power increases only slightly, at less than 50%. The Data Rate Per Unit Power column shows the number of bits per second transmitted for each watt of power used. The final column shows the improvement factor of the present invention over existing LVDS designs. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Simulated I/O power at various speeds. 
               
             
          
           
               
                   
                   
                   
                   
                 Data Rate 
                   
               
               
                   
                   
                   
                 Data 
                 Per Unit 
                   
               
               
                   
                 Average 
                 Clock 
                 Rate 
                 Power 
                 Improvement 
               
               
                   
                 Power 
                 Rate 
                 @DDR 
                 @DDR 
                 Over 
               
               
                   
                 (mW) 
                 (GHz) 
                 (Gbps) 
                 (Gbps/W) 
                 Existing Cell 
               
               
                   
               
             
          
           
               
                 Existing LVDS 
                 46.5 
                 0.6 
                 1.2 
                 26 
                 1 
               
               
                 (0.18) 
               
               
                 MSP cell (0.18) 
                 11.5 
                 0.6 
                 1.2 
                 104 
                 4 
               
               
                 MSP cell (0.18) 
                 12.3 
                 1 
                 2 
                 162 
                 6 
               
               
                 MSP cell (0.18) 
                 13.3 
                 1.5 
                 3 
                 216 
                 9 
               
               
                 MSP cell (0.18) 
                 14.2 
                 2 
                 4 
                 282 
                 11 
               
               
                   
               
             
          
         
       
     
     Table 2 shows simulated I/O power at 2 GHz over three simulated cases of voltage and temperature ranges. In the worst-case conditions, the temperature is high (125 C) and the voltage is low (−10% of normal level), and the transistors do not respond well to stimuli. For the nominal case, all of the parameters remain at normal operating conditions. Finally, in the best case conditions the temperature is low (−55 C), the voltage is high (+105 of normal level), and the transistors respond better than average to stimulus. The simulated average power is lowest for worst-case conditions and increases as conditions improve. The first row results were obtained before the simulated circuit layout was implemented. The second row was collected after insertion of the driver. The extracted driver included metal capacitance and diodes. The last row shows the power required for a design implementing LVDS in 0.25 um technology. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE2 
               
             
             
               
                   
               
               
                 Simulated I/O power at 2 GHz over voltage and temperature ranges 
               
             
          
           
               
                   
                 Worst Case 
                 Nominal Case 
                 Best 
               
               
                   
                 T = 125° C. 
                 T = 25° C. 
                 T = −55° C. 
               
             
          
           
               
                   
                 V −10% = 1.62 V 
                 V −5% = 1.7 V 
                 V −5% = 1.7 V 
                 V = 1.8 V 
                 V +5% = 1.89 V 
                 V +10% = 1.98 V 
               
               
                   
                 lo = −4.1 mA 
                 lo = −4.8 mA 
                 lo = −6.5 mA 
                 lo = −7.6 mA 
                 lo = −12.1 mA 
                 lo = −13.6 mA 
               
               
                   
                 Diffo = 100 mV 
                 Diffo = 120 mV 
                 Diffo = 180 mV 
                 Diffo = 200 mV 
                 Diffo = 350 mV 
                 Diffo = 375 mV 
               
             
          
           
               
                   
                 Ones 
                 Clk 
                 Ones 
                 Clk 
                 Ones 
                 Clk 
                 Ones 
                 Clk 
                 Ones 
                 Clk 
                 Ones 
                 Clk 
               
               
                   
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
                 (mW) 
               
               
                   
                   
               
             
          
           
               
                 Latest Design @ 2 GHz 
                 5.69 
                 7.17 
                 6.81 
                 8.52 
                 9.12 
                 11.02 
                 11.17 
                 13.34 
                 18.34 
                 21.43 
                 21.78 
                 25.29 
               
               
                 Driver Extract @ 2 GHz 
                 5.28 
                 7.03 
                 6.47 
                 8.27 
                 8.51 
                 10.49 
                 10.43 
                 12.67 
                 17.39 
                 19.73 
                 20.40 
                 23.19 
               
             
          
           
               
                 Atmel Driver @ 600 MHz 
                 25.00 
                 45.00 
                 70.00 
               
               
                   
               
             
          
         
       
     
     The invention having been fully described above with reference to the drawing figures, it will be apparent to those of skill in the art that certain modifications, variations, and alternative constructions are possible, while remaining within the spirit and scope of the invention.