It’s been a little while since I’ve posted any progress on any of my projects. I’ve been recently employed as a full time design engineer and have finally transitioned back into a schedule that allows me to work on some of my own interests again. I’ve started up working on my NES emulator and had my first success loading a cartridge from an SD Card. I’m stuck again as the emulator locks up when after it’s partially rendered the screen. Debugging has gotten more difficult as time goes on as it now takes 15-20 minutes to compile the design and only having 16 channels on my logic analyzer means recompiling more frequently to figure out what’s gone wrong. I have parts on order to perform an upgrade to 32 channels but until then I expect progress will be a bit slow. I’m hoping to have the parts installed and be able to read the entire address and data buses along with any status bits I need by the end of the week.
A video engineer, or really anybody who cares, would tell you that I’ve created a pretty inaccurate representation of the SMPTE color bar test pattern. As I’m not actually trying to calibrate any monitors I don’t fall into this category. I’ve modeled my test screen off of the Wikipedia description of SMPTE color bars. This project only generates a color simulation, ignoring the I and Q vectors, pluge pulse, and other underlying embedded signals. I’m satisfied with the results of this project and consider the original goal to be accomplished. Modifications to the code will most likely be required when it is integrated into the final NES project, but that is much farther down the road. The updated color generator code as well as a download link to the entire project are provided below.
Continue reading “FPGA NES: VGA (Part 3)”
The heart of the VGA controller described in previous articles is a modified binary counter. The VHDL code provided below is a simple 4 bit counter with clock enable and reset inputs. The 4BitCounter_Test file provides a testbench to stimulate the counter and verify it’s operation.
A counter can also be implemented as an altium designer schematic, writing no VHDL, with little effort. In this case an 8 bit counter is used to simplify connection to the on board LEDs. A clock divider is used to create a 2 Hz clock signal to drive the counter. When uploaded to the FPGA board the LEDs will cycle indicating that the counter is working.
Both the VHDL files and designer schematic are available for download at the end of this post.
Previously a basic VGA controller was designed that had the capability to display a solid color across an entire computer monitor. This post builds on that design in an attempt to verify that the controller is able to correctly display more advanced patterns. In this example the code for the clock sub-circuit remained unchanged. However the color generator code was hacked up to create a moving 100 pixel horizontal and vertical bar. Color selection still works as before with the upper two bits now being used to enable or disable either bar. The animation of the bars works fairly well, however both bars are expected to be a solid color which is not the case as shown in the image below.
I’ve found the bug. It turns out I was not shutting off pixel color when the active pixel left the boundaries of the screen. The code has been modified with the changes from line 73-77. I believe the problem is related to the monitor expecting an absence of data during H-Blank. It looks as though the monitor may be setting the black level by averaging the voltages measured during H-Blank. A fade to black would occur with a persistent input signal if this were the case. This also explains why the vertical bar is unaffected.
Any good video game requires one very fundamental feature: video. The nanoboard includes a VGA port and seeing as I don’t know anything about the VGA protocol it seems like the perfect place to start. The nanoboard also has a built in touch-screen, however I feel like that would be easier to implement and will be covered in a later post. My goal for this section is to display the “SMPTE color bars” on a standard computer monitor.