Abstract:
A single group of signals interfaces a computer system to either an analog display or a digital display. A video signal in digital format and a video signal in analog format are both supplied to a circuit that multiplexes the digital signal and the analog signal, generating an appropriate output signal for the display, either analog or digital, that is coupled to the computer system. In one example, the signal connector interfaces the computer system to either an analog CRT display or a digital FPD display. The multiplexer multiplexes the analog signal and the digital signal supplied by the computer system and generates an output signal that is suitable for the CRT display or the FPD display, depending on the type of display coupled to the computer system.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to interfaces between computers and video monitors, and more particularly to an interface that multiplexes analog and digital signals for the purpose of using one set of lines between a computer system and either an analog monitor or a digital monitor. 
     2. Description of the Related Art 
     The substantial growth of the personal computer is due, in part, to the technological breakthroughs in computer design. A typical personal computer system available today includes a plethora of peripheral devices undreamed of in years past. Today a computer system available to a typical consumer might include a system processor, associated memory and control logic, and a number of peripheral devices, including a display monitor, a keyboard, a mouse-type input device, floppy and hard disk drives, CD-ROM drives, a laser printer, a color scanner, a modem, network capabilities and even a voice recognition device. 
     An important peripheral available in a personal computer system is the video monitor. From the time of the television set to today&#39;s newer digital-type displays, technology has expanded the capabilities of video monitors. As a result of technological breakthroughs in video monitors, higher resolutions, faster refresh rates, and different types of video monitors are available at reasonable prices. One type of video monitor, the digital video monitor, has been improved upon and is becoming increasingly popular. A typical computer system may provide a connector for both digital and analog video monitors. What is needed is one set of lines that can connect either a digital or an analog video monitor to a computer system. 
     SUMMARY 
     A connector with a single set of signal lines interfaces a computer system to either an analog display or a digital display. A video signal in digital format and a video signal in analog format are both supplied to a circuit that multiplexes the digital signal and the analog signals, generating appropriate output signals for the display, either analog or digital, that are coupled to the computer system. 
     In one example, the set of signals interfaces the computer system to either an analog cathode ray tube (CRT) display or a digital flat panel display (FPD). The multiplexer multiplexes the analog signal and the digital signal supplied by the computer system and generates an output signal that is suitable for the CRT display or the FPD display, depending on the type of display coupled to the computer system. 
     In one embodiment of the present invention, an apparatus includes a video controller, a connector and a circuit coupled to the video controller and coupled to the connector, the circuit selectively coupling one of either an analog video signal and a digital video signal received from the video controller to a video monitor. In another embodiment, an apparatus includes a switch, a first input terminal coupled to the switch for receiving an analog signal, a second input terminal coupled to the switch, for receiving a differential digital signal, and an output terminal coupled to the switch, the switch for multiplexing the analog signal and the differential digital signal, and the output terminal for supplying a multiplexed signal. In a further embodiment, a computer system includes a processor, a video monitor coupled to the processor, a controller, and a circuit coupled to the video controller and coupled to the connector, the circuit selectively coupling one of either an analog video signal and a digital video signal received from the video controller to a video monitor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items. 
     FIG. 1, labeled prior art, illustrates a conventional interface between a computer system and either an analog monitor or a digital monitor. 
     FIG. 2 is a block diagram illustrating a computer system including an interface between either an analog or a digital monitor and the rest of the computer system according to an embodiment of the present invention. 
     FIG. 3 is a block diagram illustrating a computer system including an interface between either an analog or a digital monitor and the rest of the computer system according to a second embodiment of the present invention. 
     FIG. 4 is a schematic circuit diagram illustrating a circuit suitable for use as an interface between a computer system and either an analog or a digital monitor as shown in FIG.  2 . 
     FIG. 5 is a schematic circuit diagram illustrating a circuit suitable for use as an interface between a computer system and either an analog or a digital monitor as shown in FIG.  3 . 
     FIG. 6 is a state diagram that illustrates a first suitable method for controlling the interface shown in FIG. 2 to select the type of monitor coupled to the computer system. 
     FIG. 7 is a state diagram chart that illustrates a second suitable method for controlling the interface shown in FIG. 3 to select the type of monitor coupled to the computer system. 
    
    
     DETAILED DESCRIPTION 
     The following description is intended to be illustrative of the invention and should not be taken to be limiting. 
     In a color display device such as an FPD or a CRT display, video display characteristics are typically provided by a Display Data Channel (DDC) signal upon power-up. A DDC monitor generally includes an Electrically Erasable Programmable Read Only Memory (EEPROM), which stores video display characteristics in a data format called Extended Display Identification Data (EDID) developed by VESA (Video Electronics Standards Association). The current EDID data format is described in Extended Display Identification Data Standard, Version 2, Revision 0, dated Apr. 9, 1996, the disclosure of which is expressly incorporated herein by reference. 
     Referring to FIG. 1, a system block diagram of a computer system  100  is presented. This typical computer system  100  includes a processor and cache  103  electrically coupled to a memory controller  119  via a host bus  108 . The memory controller  119  is further coupled to memory  105  through a memory bus  106 . The memory controller  119  is further coupled to slots  115 , video controller  109  and input/output control (I/O controller)  107  via a PCI (Peripheral Component Interconnect) bus  110 . The I/O controller  107  is electrically coupled to a super input/output controller (Super I/O Controller)  114  and to Basic Input/Output System (BIOS)  113  through an ISA (Industry Standard Architecture) bus  112 . 
     Upon startup of the computer system  100 , the system Power On Self Test (POST) program, typically stored in BIOS Read Only Memory (ROM)  113 , receives EDID information which is sent from an EEPROM located in a DDC monitor that includes either a panel display  111  or a CRT display  112  through the video controller  109  to the processor  103  of a computer system  100 , which reads the information provided in an EDID format. The EDID format information is processed during system setup and the running of the POST program. The processor  103  interfaces with a video controller  109  which is programmed according to monitor characteristics. 
     Monitor characteristics and information in EDID format may include an identification code for the monitor defined by the system manufacturer. Once the computer system  100  obtains monitor identification information, the video controller  109  is programmed as directed by the stored characteristics of the coupled monitor. 
     The video controller  109  generally transmits both digital and analog signals. For a CRT  112 , the video controller  109  transmits analog signals by first converting a digital video signal through a digital-to-analog converter (DAC), not expressly shown. For an FPD  111 , the video controller  109  transmits digital signals without first converting the signal. In a typical desktop computer system  100 , the video controller  109  is generally set up for a CRT display  112 . In a typical notebook or laptop computer system the video controller  109  is generally set up for a digital FPD display  111 . 
     For many years the only type of monitor used with desktop computer systems was the CRT. Currently, however, designers of desktop computer systems supply both CRT and FPD capable systems. One conventional technique, illustrated in FIG. 1, supports both FPD displays and CRT displays by supplying respective analog signals and digital signals to separate interfaces  125  and  126 . The method is disadvantageous because the system  100  will have two sets of signal lines output from the video controller and two connectors. 
     Another method for providing both digital and analog signals to either a CRT or FPD display is to attach an analog-to-digital converter (ADC) at the input terminal of an analog interface on an FPD display. A problem with this method is that the additional conversion degrades the image quality of the video. Another problem with this method is that it adds additional cost to the system. 
     Referring to FIG. 2, a block diagram illustrates a computer system  200  in which a monitor selected from either an analog DDC monitor or a digital DDC monitor is coupled to the rest of the computer system  200 . Like the computer system  100  shown in FIG. 1, the computer system  200  includes a CPU or system processor and cache  203  that is electrically coupled to a memory controller  219  through a host bus  208  and to memory  205  through the memory controller  219  and a memory bus  206 . As in FIG. 1, the memory controller  219  is electrically coupled to a video controller  209  via a PCI bus  210 . A video interface circuit including an analog multiplexer  221  and a digital to differential circuit  220  is electrically coupled to a video controller  209  and to a video monitor, either a digital video monitor  211  such as a FPD or an analog video monitor  212  such as a CRT display. The video controller  209  receives DDC signals from the video monitor. The multiplexer  221  and the digital to differential circuit  220  receive electrical signals from the video controller  209 . The digital to differential circuit  220  may include a Low Voltage Differential Signaling (LVDS) transmitter or another suitable transmitter, such as a transmitter for Transition Minimized Differential Signaling (TMDS) based on PANELLINK technology developed by SILICON IMAGE, located in Palo Alto, Calif., or such as an optical transmitter. One type of LVDS transmitter that is suitable for usage with the computer system  200  is a DS90C363 integrated circuit manufactured by National Semiconductor, Inc. 
     Referring to FIG. 2 in combination with FIG. 4, the illustrative system shows a video monitor, either an analog video monitor  211  or a digital video monitor  212 , electrically coupled to the video controller  209 . The video monitor, either analog video monitor  211  or digital video monitor  212 , is a Display Digital Channel (DDC) monitor with an EEPROM that supplies data in the Extended Display Identification Data (EDID) format. As discussed earlier, the video controller is programmed according to the EDID monitor characteristics. This information is initially supplied through a DDC data line  406  and a DDC clock line  405  from the video monitor to the video controller  209  before being read by the processor  203 . 
     Digital signals generated by the video controller  209  are electrically coupled to the digital to differential circuit  220  that converts 18 bits of Red/Green/Blue (RGB) data and 3 bits of Liquid Crystal Display (LCD) timing and control data, for a total of 21 bits of CMOS/TTL data into three LVDS data streams, and a fourth phase-locked transmit clock line. The three data streams and the clock line are then electrically coupled to an analog multiplexer  221 . Analog signals received from the video controller  209  are also electrically coupled to the analog multiplexer  221 . In the video controller  209 , only the digital signals are received by a digital to analog converter. 
     The analog multiplexer  221  is a high bandwidth, low resistance analog multiplexer. A suitable multiplexer is a P15V330 integrated circuit manufactured by Pericom, Inc. 
     A standard VGA connector is one type of connector suitable to electrically connect the output signal from the analog multiplexer  221 , the DDC clock line  405  the DDC data line  406  to a video monitor, either the analog monitor  211  or the digital monitor  212 . 
     Although FIG. 2 shows the multiplexer  221  and digital to differential circuit  220  as separate from the video controller  209 , other embodiments may include both the multiplexer  221  and the digital to differential circuit  220  as part of the video controller  209 . 
     Referring to FIG. 4, a schematic circuit diagram illustrates an analog multiplexer  421  suitable for usage as an interface between either an analog or a digital monitor and the rest of a computer system. The analog multiplexer  421  is coupled to receive both digital and analog signals from the video controller  209 . A suitable video controller for use with an embodiment described by the claims is the NM2160, manufactured by NeoMagic Inc. Some video controllers could have both an analog and a digital output and support either or both LVDS and TMDS. 
     In one embodiment as shown in FIG. 4 in combination with FIG. 2, a select line  408  and an enable line  407  for the analog multiplexer  421  are electrically coupled between the analog multiplexer  421  and a general purpose input/output (GPIO) signal line from the I/O Control  207 . The video controller  209  is electrically coupled to either an analog video monitor  211  or a digital video monitor  212  through cabling that transmits both a data line DDCDATA  406  that receives EDID data from the video monitor, (either the analog monitor  211  or the digital monitor  212 ) and a clock line, DDCCLOCK  405 . The EDID data is received by the video controller  209 . 
     As discussed earlier, the video controller  209  and the system processor  203  are electrically coupled through the PCI bus  210 , the memory controller  219  and the host bus  208 . The EDID data is electrically transmitted to the processor  203  for interpretation. The interpreted EDID data is used to set up the General Purpose input/output lines output from the input/output controller  207  and electrically coupled to the SELECT  408  to select either the analog lines in the multiplexer  209  or the digital to differential lines output from the LVDS interface  420 . The interpreted EDID data is used to control the General Purpose input/output lines, and is also used to ENABLE  407  the multiplexer after a type of monitor has been selected. The SELECT  408  and ENABLE  407  lines, in alternate embodiments, could be taken from a suitable video controller. 
     The digital to differential circuit  220  shown in FIG. 2 is shown in FIG. 4 as LVDS interface  420 . In other embodiments, however, the LVDS interface could be replaced by a TMDS interface. 
     Referring to FIG. 6 in combination with FIGS. 2 and 4, a state diagram illustrates a first suitable method for controlling the interface to select the type of monitor coupled to the computer system in a method that is controlled by the video controller  209 , the processor  203 , and the I/O controller  207 . In Step  601 , the video controller in cooperation with the processor detects during the POST operation whether a digital or analog video monitor is installed. In the illustrative technique, the display  211  or  212 , sends the EDID format information to the system POST program  213  running on the processor  203  via a DDCDATA line  406  through the video controller  209 . The processor  203  interfaces with the video controller  209  which is programmed according to monitor characteristics. The processor  203  also interfaces with the I/O controller  207  which selects either an analog signal or a digital signal by selectively performing either step  602  SELECT VGA or step  603  SELECT LCD. Upon selection, the enable line to the analog multiplexer  221  will enable communication of either analog signals or digital signals. 
     Referring to FIG. 7 in combination with FIG. 3, a flow chart illustrates a second suitable method for controlling the interface to select the type of monitor coupled to the computer system  300 . In the second illustrative method the monitor is a non-DDC monitor. Thus, unlike the first illustrative method, neither the analog video monitor  311  nor the digital video monitor  312  shown in FIG. 3 is a DDC monitor. Referring to FIG.  5  and FIG. 3, the embodiment now under consideration does not use DDCDATA line  406 , or DDCCLOCK  405 . Instead, as shown in FIG.  5  and in FIG. 7, Pin  10  (labeled  510 ) of the analog multiplexer  521  shown in FIG. 5 is electrically coupled between the analog multiplexer  521  and either the analog video monitor  311  or the digital video monitor  312 . The digital monitor  311  has a cable with Pin  10  (labeled  510 ) open, and the analog monitor  312  has a cable with Pin  10  (labeled  510 ) grounded. If the voltage on Pin  10   510  is HIGH, the select line on the analog multiplexer  521  selects a digital interface. If the voltage on Pin  10   510  is LOW, the select line on the analog multiplexer  521  selects an analog interface. Similar to the above-described embodiment, once the select line chooses an analog or a digital interface, the enable line  507  tied to pins in the analog multiplexer  521  enables the selected analog or digital interface. 
     While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiments described are possible. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.