Running the simulation
1) Get ModelSim. It’s free software.
2) Download the VHDL code from Github.
3) Open ModelSim, and create a new project.
As part of the Homebrew Computer Project, I have been exploring other CPU architectures and their implementations in order to broaden my understanding of CPU design.
An awesome one that I came across was Ben Eater’s 8-bit computer. It is well documented through a series of videos on Youtube and also on his blog. The architecture is simple which allows you to get a firm understanding of all its components.
Since it is a known working design, I saw it as the perfect opportunity to practice implementing a CPU in VHDL.
After months of waiting the new LCD assembly finally arrived!
I noticed the PCB had a different layout than the one I had earlier. Gone was the onboard voltage regulator. Yay cost cutting! Also missing was the ZIF connector. In this one, the LCD flex cable was soldered directly onto the PCB, yay more cost cutting! Given the trouble I had earlier trying to resolder the four chunky traces of the touch screen cable, prospects looked dim for resoldering the many small traces of the LCD screen cable. Drilling mounting holes onto the PCB no longer looked like an option.
Before doing anything, I checked whether the LCD works. I connected it to an Arduino and loaded Adafruit’s graphics test. It sort of worked. But not quite. The screen flickered and what rendered looked glitchy. See the video* below for what I mean. Contrast this with what the test is supposed to look like.
It was time to put the previously disassembled LCD assembly back together. Reconnecting the LCD to the PCB was easy enough (given that it was a ZIF connector). However reconnecting the touch screen would prove to be my undoing.
I had a hard time figuring out how to mount the LCD assembly. It was designed for use as an Arduino shield and thus its PCB had no mounting holes. I wanted a solution that was sturdy but non-permanent so things like glue were not options. I came across these clips from Adafruit, but they didn’t seem rugged enough to support the handling the touch screen (and hopefully handheld computer) was going to receive.
My original plan was to use the FPGA for RAM, either by building the RAM in VHDL using flip flops or by using the FPGA’s dedicated RAM blocks.
Next in line in the awesome tutorial series from Nandland was VGA. I didn’t have a VGA connector for my FPGA, but wondered if I could jury-rig something with what I had on hand.
The wiring diagram below from Grant Seale’s Multicomp gave me a very good idea on how to go about it.
In the Nandland tutorial (image below), three signals are used per channel instead of two so my setup reflects that.
I had five 1Kohm resistors on hand, enough to control one channel. I used two of them in parallel to get the 500 ohms (close enough to 549), two in series to get the 2K ohms, and one on its own for the 1K ohms. Jumper wires were used to connect to the pins (red, green, blue, hsync, vsync). Luckily, all the ground pins were in-line (and the random pin in-between was an unused pin) so I just tied the whole line to ground using one wire (instead of several jumper wires). Here is the final masterpiece:
And the overall setup:
For the homebrew computer, I will need to implement SPI and I2C support to allow the FPGA to communicate with the LCD screen and ADC. To get an idea of how to go about this, I turned to tutorials uploaded on Youtube. But before getting in deep, I started by learning a simpler protocol, asynchronous serial communication.
Here are some additional resources I’ve come across for learning shaders.
Book of Shaders →
Great resource because it starts from the absolute basics and builds up, rather than skipping to all the fancy stuff. Emphasis is placed on understanding the fundamentals and developing an intuitive understanding of how things work.
Good introduction to lighting. The tutorials are based on OpenGL but can be adapted to Three.js (which is what I used to follow along). Here’s a toon shader I made from one of the lessons (click image for animation):
Shader School →
I think it’s a bit useless to work through this one if you do not have prior exposure to GLSL. But once you do, it’s an awesome series. There’s even a section that touches on using the GPU for general purpose computing.
WebGL Workshop →
It is intended to be taken after Shader School. The course teaches you how WebGL (based on OpenGL) works. Basically all the stuff libraries like Three.js do on your behalf. Even if you don’t intend to write your applications using raw WebGL it is useful to understand what is happening behind the scenes.
A neat trick I learned from the course was
discard. You can either render a pixel (default) or specify that it be ‘discarded’ (i.e. skipped). Here is a slicing effect I made using this (click image for animation):
Interactive 3D Graphics →
Though not strictly a shader focused course, it covers so many fundamental concepts with regards to 3D graphics that skipping it would be doing yourself a disservice. The course is taught using Three.js and touches on pretty much everything you need to know about how 3D computer graphics work and how to create them.
I chose to use VHDL to program the FPGA because it is closer to a schematic representation than Verilog. Verilog seems (to me) like learning a new programming language whereas VHDL is more like describing the circuit.
For example, take the construction of a full adder. Here is a video of the structure generated by my code below:
I bought an analog to digital converter for the computer.
The FPGA can only process digital signals (on or off, 3.3V or 0V) but the touch screen outputs analog signals (values ranging from 3.3V to 0V each representing a relative position).
It was my first time soldering (the header pins). As you can see, all the flux was used.
I roughed out the design using Inkscape. (Because unlike Photoshop or Gimp, you can easily move and resize shapes around after creating them).
fillColor. The color scheme is based on this palette with tweaks here and there.
When used with the Arduino SD Library, one can easily read, write, create, and remove files (and directories).
The SD card solution is also very convenient! You can plug the SD card into any regular computer (or phone) and copy onto it any programs you want to run on the Hack Computer (for example these games on Github). (Think game cartridge or USB thumb drive). Similarly, if you write a program and save it onto the SD card when using the Hack Computer, you can then open it up later on any other computer (or phone).
I got it for $20 on eBay and the kit came with a (knockoff) USB Blaster for programming. The idea to use the board came from Charles Cole’s hardware setup of his homebrew Z-Machine. The price is great and I love that it only has the bare minimum components on it.
I worked through this excellent getting started guide by Trey Dahlberg on Youtube and everything seems to be running well.
Update (January 18, 2017):
I recently came across the Multicomp project by Grant Searle. In it he uses this development board and consequentially has a ton of really helpful information on the board and some of the things it’s capable of. The project is described by Hackaday as a ‘pick and mix retrocomputer’ which allows you to use the FPGA as a Z80, 6502, 6800 or whatever CPU mashup you feel inspired to. His project also describes in detail interfacing with various devices including SRAM, SD cards, VGA, composite video, PS/2, USB - basically I found the holy grail! Check out the project page to learn more about the board, and maybe even build a mix and match computer of your own.
Note: if you use this development board, make sure to desolder the four zero ohm resistors as explained in Grant’s project page.
I got it for $10 compared to Adafruit’s price of $40. I came across the screen through this awesome video by Charles Cole where he walks through his FPGA implementation of a Z-Machine. I also bought the Altera mini dev board he recommends ($20 which is a steal) but that’s another post.
My gameplan is
Part 1) Getting the screen to work
Particles are cool! Example,
I had this idea back in 2014 of getting the points from premade 3D models (similar to the video above), and using them to do fancy animations in Threejs. Then I hit a knowledge wall and shelved it. Until today…
Goal - to build a homebrew computer. From scratch.
Inspiration comes from these single board briefcase computers photographed by Rostislav Lisovy.