FPGA 6502 Emulator: First Life

There’s an amazing project known as Visual 6502.  They have created a transistor level net-list of the 6502 processor which can in turn be converted into synthesizable VHDL.  As nice as this is, I’ve decided that would be far too easy and am going to go ahead and emulate the 6502 core before I fool around with a proper simulation.  I’ve already poured through the available documents and laid out a framework for my emulation.  The capture below is the first result from that process.  That is the emulator executing the opcode 0x01 (Inderect X indexed OR with the accumulator)  The A register isn’t actually visable but the processor is successfully loading the data from my simulated ROM and jumping to the correct memory locations based on the code I’ve provided.  There is still a long way to go before the process is complete, but it is well on its way.



FPGA NES: VGA (Part 3)

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.

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FPGA: 4 Bit Counter

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.


Altium 8 Bit Converter Schematic
Altium 8 Bit Converter Schematic


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FPGA NES: VGA (Part 2)

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.

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