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VGA Video Graphics Array has become a well-known standard interface in many embedded systems such as video surveillance systems, ATM machines, or video players. It provides a simple method to connect a system with a monitor for showing information/images, or for users to interact with the system. Depending on the needs of these applications, some systems may not require a high display quality. Therefore, VGA monitor controller, which is a logic circuit to control the VGA interface, can be easily realized by FPGA technology with a low cost and high flexibility. However, when moving to a higher display resolution, these FPGAs have to face with timing issues as well as logic resources overhead.
Several FPGA-based designs of VGA monitor controller have been released as commercial Intellectual Property (IP) cores , ,  or open source cores . They provide plenty of features and functionalities to be feasible in different running modes. Most of them can support multiple display resolutions and 24 bits color per pixel as the basic functions of a VGA monitor controller. Some additional functions, for example, the abilities to handle other color modes (e.g. RGBX5551, RGB232, etc.) and to program video timing of the design  might not commonly be used while they usually take more hardware resources. On the other hand, the design in  is more compact than the others, hence, it only works in a fixed display standard; or the lack in design  to support 64 bits data width makes it less efficient in usage. This paper introduces an efficient architecture for VGA monitor controller with all necessary basic functions to work in both graphical mode and text mode.
The efficiency of this design provides many choices for different FPGA devices, where system designers can select a proper display mode or configure the internal pixel buffer to be suitable with the application requirements. The design is intently integrated with Processor Local Bus (PLB) interfaces to be used in Xilinx FPGA-based systems.
II. ARCHITECTURE DESIGN
A. Basic ideas and the needs of an efficient architecture
In order to display an image on screen, VGA monitor controller has to read every pixel data of the image while driving the color signals and synchronization signals of the VGA interface. All pixels are scanned in raster order at a frequency called pixel frequency. To ensure the visual quality, whole image will be re-drawn at a rate determined by refresh rate. The pixel frequency exactly depends on the display resolution and the refresh rate, preferred resolution or higher fresh rate will require higher pixel frequency. Some examples can be seen in Table I.
The size of an image is usually larger than memory resources available on FPGA devices. For example, a 640-480 pixel at 24bpp image has the size of 912.6 Kbytes while the Spartan-3E family has only 81 Kbytes block RAM . The image therefore cannot be stored totally inside the design of VGA monitor controller. It should be held on an off-chip memory (e.g. SDRAM) and transferred into this VGA unit by small data blocks during the display time. For that reason, most of VGA monitor controllers have an internal FIFO memory to temporarily store these data blocks, e.g. ,  or .
To handle the data transfer operations, there are two solutions could be addressed:
•The first solution is to attach a bus master interface to VGA unit to access data on the external memory. Such that the data transfer between memory and VGA unit is consecutive, the bus master interface should support burst transfer mode to speed up this process. In general, this method may give VGA unit more complex due to the appearance of the bus master interface.
•The second solution is to use a Direct Memory Access (DMA) unit to cooperate with VGA unit. The VGA unit therefore does not need to include any bus master interfaces. Every data transfer from the external memory to VGA unit is handled by the DMA. This DMA core can be reused from an existing IP delivered by Xilinx , so we can reduce the development time for VGA unit. A processor, however, has to initiate the operation of the DMA core and handle its interrupt during active time of VGA unit. Consequently, these issues will affect software applications.
In our design, we prefer the first solution to gain the portability and elegance for VGA unit, by using the PLB Master Burst  as the bus master interface. Furthermore, one of the most challenges in the design of VGA unit is to estimate the size of FIFO memory within the core and the buffering strategy. Our goal is to minimize the size of FIFO while it has to ensure a good display quality (without flickering or fragmenting effect). To select a suitable FIFO, there are some factors should be considered, including pixel clock, operating system clock, data bus width, and the bus occupation of the system which contains VGA unit. Obviously, the communication throughput depends on the operating system clock, data bus width, bus latency, and bus occupation. The throughput required by VGA unit is equivalent to the pixel clock. To successfully display, the first throughput must be higher than the second one. If the first throughput is much higher than the second throughput, we can use a lesser amount of FIFO. Otherwise, we need more than that.
B. Overall architecture
To deal with those things above, we propose an efficient architecture for VGA monitor controllers as presented in. It is composed of the following modules:
• PLB Master Burst, which supports burst transfer mode, is used for fast transfer data from an external memory to the FIFO module.
• PLB Slave Single serves the read/write operations from/to VGA monitor controller.
• VGA Registers is a set of registers to hold the control data and other information such as width, height or base address of an image. The accesses to these registers are performed via PLB Slave Single interface.
• VGA BRAMS (FIFO) is a dual-port RAM to temporarily store pixel data during display time. The depth of this FIFO is parameterized to be 64/ 128/ 256/ 512 bytes.
The read and write pointer controllers are implemented with in VGA Controller.
• VGA Data Aligner (VDA) aligns data between the output of the FIFO module and the input of the VGA Interface module. • VGA Interface directly drives the color signals and synchronization signals of the monitor.
• VGA Controller generates video timing according to an expected display standard and controls the operation of other modules, exception of the two bus interface modules.
There are two clock domains in this design: system clock (sys_clk) and pixel clock (pix_clk). The system clock is the source clock for the side of bus interface while the pixel clock is used for the side of VGA interface. The pixel clock frequency is required according to the display standard (as provided by  or ). It can be driven by an on-chip clock generator (using Digital Clock Manager and PLL blocks of FPGA chips) or an off-chip clock generator.
Currently, the architecture can treat with 32 bits or 64 bits data width of the bus master interface without any functions of the Bus Width Adapter (a module inside the PLB MasterBurst).There are different architecture for some modules to be feasible in both 32 bits mode and 64 bits mode, especially for the FIFO and the VGA Data Aligner. For the FIFO module, its data width is always equal to the data width of the bus master interface. Most of the changes take place in the VGA Data Aligner. The next section will give more details about its architecture.
C. VGA Data Aligner
From the overall architecture of VGA unit, the FIFO can output 4 or 8 bytes data per clock cycle (corresponding to the two modes, 32 bits and 64 bits data width of the bus master interface, respectively. However, the VGA Interface module can only push 3 bytes data (or three color components of a pixel) per clock cycle. The VGA Data Aligner therefore will aim to adjust the data flow between FIFO and VGA Interface. On the other hand, it also has to ensure the pixel order (firstin, firstout).
Basically, VGA Data Aligner is formed as the second FIFO of VGA unit. It has two different architectures for the 32bits mode and the 64 bits mode, as depicted in Figure 2 and Figure 3. For the 32 bits mode, the FIFO
has the size of 12bytes and this size will be 24 bytes for the 64 bits mode. The read and write pointer controllers of this FIFO are controlled by the VGA Controller module. The read address and write address specified for each memory cell are shown in every square of these figures, where the read address is on the right and the write address is on the left. The output data width is always 3 bytes, which is equivalent to three color components RGB.
D. VGA Controller
Operations of those unit inside VGA element is controlled by VGA Controller element. For a preferred display standard, VGA Controller will finalize horizontal and vertical timing parameters to generate corresponding video timing and to drive synchronization signals. During the display moment, PLB Master Burst asking the bus to transfer data from external memory to the FIFO module.
The discourses to access the memory and the burst length in every transfer session are
determined by VGA Controller. It also makes a handshake with the bus master interface to control read/write operations of the FIFO and VGA Data Aligner modules. To handle these tasks, VGA Controller includes the following functional modules: Video Timing Generator, Video Address Calculator, FIFO Read/Write Pointer Controller, VDA Read/Write Pointer Controller, as shown in Figure 4. These modules are driven by a finite state machine (FSM).
III. HARDWARE VERIFICATION AND IMPLEMENTATION
A. Verification model
The VGA monitor controller is fulfilled at RTL (VHDL) and simulated by ModelSim simulator. The verification model is created with the usage of the IBM PLB Bus Functional Model (BFM) toolkit . This package offers a set of verification IPs, includes three main components: Master BFM (plbv46_master_bfm), Slave BFM (plbv46_slave_bfm) and Monitor BFM (plbv46_monitor_bfm) to simulate systems which contain the PLB bus (version 4.6). The model is depicted in Figure 5.
In this model, the Master BFM is a bus master core which functions as a processor to initiate and control the operation of VGA unit. The size of an image and its base address will be written to VGA unit by this processor. The Slave BFM is a bus slave core which functions as a memory. A sample image is stored in this memory and will be read by VGA unit during simulation. The monitor BFM checks for bus compliance or violations and notifies warnings or errors to the simulator.
For simulating, we pre-configure the VGA unit to run in a specific mode whose the FIFO depth is 256 bytes and display mode is SVGA 800 - 600. A small image is initialized in the memory. The processor then writes the image effects (width, height, base address) to the VGA unit and makes it active. Bus transactions are described by bus functional language. They will be translated to bus stimulus to run the simulation in ModelSim.
B. Implementation results
The VGA monitor controller has been implemented on different Xilinx FPGA devices with a FIFO depth of 256bytes. To have a fair comparison of performance and resource utilization, we implemented our design and some related works (with minimized functionalities) on Xilinx Spartan3E-1600E. The result comparison is shown in Table II. Our design uses less logic and memory resources in compared with the design  and  while it can achieve a higher performance than the others. Actually, the design  provides some additional functions such as Integrated Test mode and Built-in Power Save mode; or  can support hardware cursor and color lookup-table.
All these stuffs are the causes of logic consumption. The design  has a better result than ours but it only supports VGA standard, its logic circuit is therefore much simpler than the others. Table III shows the comparison of display standard and bus technology supported by these works. The design  uses AMBA Advanced High-performance Bus (AHB), then it can work with higher resolutions than WXGA. The design  uses the open source WISHBONE bus while our design and  support Core Connect PLB bus.
IV. APPLICATION - A CASE STUDY
The VGA monitor controller can be used in
several systems which have video output, such as portable video systems, video games, or digital cameras with video capabilities. In this section, we provide an FPGA-based system which uses VGA monitor controller as a functional module to display visual data in both graphics mode and text mode. This system plays a role as a remote camera system. To build such system, we create a simple system-on-chip with the architecture presented in Figure 7. It is composed of: a PowerPC processor; Ethernet controller unit to communicate with other system via computer network; a memory controller unit, namely MPMC, to interface with an external memory; PS/2 unit to interface with a PS/2 keyboard; VGA unit; and UART unit. The system uses a channel of PLBv4.6 bus to interconnect these components.
The VGA unit is configured to operate in XGA mode, the pixel clock is driven by an internal clock generator. In order to provide good display quality and also meet the timing constraints, we implement the system on Xilinx Virtex-4 ML410 development kit.
A remote desktop PC, which connects to a camera, is used to capture images and send them to the FPGA board. Communication between this PC and the FPGA board is performed on a LAN network. Figure 8 presents the sequence diagram of the whole remote camera system.
To start, the users first make a capturing request on the FPGA board, and then the FPGA board dispatches this request to the PC. Whenever PC receives a proper request, it captures an image from camera, and then sends it back to the FPGA board. After successfully receiving the image, the testing board will decode and display it on the screen. The software application on the FPGA board provides a command-line interface (CLI) to interact with users. Users thereby can invoke a capturing request by executing a specific command on this interface.
Figure 9 presents the result of the experiment, the captured image is shown on both PC application and the FPGA board. The contents (image and text) on the screen of the FPGA board are displayed in 24 bits color - XGA resolution (1024-768). This application is just a case study for demonstrating how this VGA monitor controller can be used in a real system. Its result has already evaluated the functionalities of the VGA unit in both graphics mode and text mode. The design is also used in developing a system-on-chip platform as presented in .
We have presented an efficient implementation of VGA monitor controller using FPGA which has a high thinkable to be used in Xilinx FPGA-based systems. The tinted features such as multiple display resolutions supportive capability, customizable internal FIFO memory, 32/64-bit data bus width, independence to system clock and pixel clock. . . , make the design suitable for several FPGA devices and able to meet different requirements of targeted applications. In addition, a software library to enable text mode is also introduced. These useful features of the design have been validated through real application demonstrations.