Wednesday, April 22, 2015

DIY FPGA-based HDMI ambient lighting

Ambient lighting is a technique that creates light effects around the television that correspond to the video content. It has been pioneered by Philips under the brand Ambilight. In this project we will create a basic FPGA-based ambient lighting system that reads the video signal over HDMI. This means we are not limited to computer output. We can use it together with DVD players, video game consoles, etc.

The complete source code for the entire project is available on GitHub.

Tuesday, October 14, 2014

Zero-confirmation bitcoin transactions

Bitcoin transactions and confirmations

Bitcoin transactions are incorporated into the bitcoin blockchain, that acts as a shared ledger. Once the transaction has been part of the blockchain for a long enough period of time, it is accepted as truth by all the nodes in the network and becomes an unalterable part of the bitcoin permanent record. This makes it irreversible and resistant to both network events and deliberate tampering in the form of double-spending attacks.

A block that is added to the blockchain that contains the transaction is called a confirmation. So a newly broadcast transaction is said to have zero confirmations. When it's included in a block, it has one confirmation. Another block is mined on top of the block containing the transaction, we have two confirmations. Generally, a transaction is not considered definitive unless it has six confirmations. On average this takes about an hour.

In this post we will develop a crude system for handling zero-confirmation transactions without incurring into any appreciable risk.

Wednesday, July 30, 2014

Retro Pinball: Reverse Engineering the Bally AS-2518-51 Sound Module Assembly

One of my friends has a beautiful 1980 Nitro Ground Shaker pinball machine. It's in great condition, but there is absolutely no sound output. As I was trying to figure out a way to troubleshoot the sound module, I found myself falling deeper and deeper into the 8-bit pinball rabbit hole. In this post I will try and explain the inner workings of the amazing Bally AS-2518-51 Sound Module Assembly.


The 2518-51 Sound Module is based on the famous Motorola 6800 series microprocessor (U3 in the picture and in the schematic). Sound synthesis is performed by a General Instrument AY-3-8910 (U1). There's also a 6820 Peripheral Interface Adapter (PIA) (U2) to interface the AY38910 to the MPU, a ROM (U4) that contains game-specific code and data and a RAM chip (U10). The microprocessor is clocked by a 3.58 MHz crystal. Due to the built-in clock divider the 6800 operates at 895 kHz.

The Bally AS-2518-51 Sound Module Assembly as found in Nitro Ground Shaker
The pinball system interfaces with the sound module using a 15 pin connector (J1) that provides both power and input. The only output of the sound module happens through the speaker connector (J2).

Wednesday, August 8, 2012

Complex CPU Addressing modes, for free!

One of the most powerful features of a central processing units is the ability to access memory using complex indexing modes. These modes are essential to implement compound data structures such as arrays, structures and classes in an efficient manner. Our FPGA soft-processor core will need to do all these things too.
The instruction set architecture (ISA) that we have developed in the previous post support only basic register indirection, like this:

MOV [R1], R2
MOV R1,[R2]

where the [R1] denotes the memory address pointed to by the register R1.

However, as we shall see, we can support rather complex indexing modes essentially for free!
Recall the layout of our execution unit:

Tuesday, July 24, 2012


XENTRAL is a simple Harvard Architecture CPU. It targets the Spartan6 FPGA present on the Digilent's Nexys3 board, but it's all written using portable behavioral VHDL1. You should be able to use your favorite FPGA, synthesizer and simulator.

All project files are available on github under the Creative Commons Attribution Non-Commercial (CC-by-NC) license.

Architecture Overview

XENTRAL is a Harvard Architecture processor. This means that code and data are handled separately.
The picture above is a very high-level schematic representation of the XENTRAL architecture. Instructions are fetched from ROM3. Based on the instruction, the control unit manages the execution unit. The execution unit performs the actual execution of the instruction, by e.g. adding the contents of two CPU registers, writing a value in RAM or performing I/O.

XENTRAL is a Harvard Architecture processor. This means that code and data are handled separately. This effectively precludes the use of self-modifying code. It also prevents the processor from loading its own program. The program must be programmed into the instruction memory. This is a common approach for embedded devices and microcontrollers, but it does not play nice with operating systems and boot loaders. As the project grows, we will re-visit this limitation.

Overview of the Execution Unit

Schematic representation of the execution unit

Tuesday, May 22, 2012

LightDHT - A lightweight python implementation of the Bittorrent distributed hashtable

I have always been fascinated by distributed data structures, but my efforts have always been dampened by the fact that they are really awkward to play with. In order to get anything that resembles real-life usage, you need to be running hundreds of nodes. Needless to say, wrangling hundreds of processes gets rather cumbersome. Fortunately, there are already thousands of people out there that are participating in one of the largest distributed data structures in operation today: the bittorrent distributed hashtable.
In order to facilitate experimentation with this wonderful system, I have written a simple python implementation of a DHT node. It is mostly (with a few exceptions -- see below) compliant with BEP0005. It should be able to act as a full member of the DHT, processing requests without disrupting the network.
Interested in playing around? Get the code on github!
From the documentation:
The aim of LightDHT is to provide a simple, flexible implementation of the Bittorrent DHT for use in research applications. If you want to trade files, you have come to the wrong place. LightDHT does not implement the actual file transfer parts of the bittorrent protocol. It only takes part in the DHT. 
The main philosophy of LightDHT comes down to two considerations: ease of use over performance and adaptability over scalability. 
In order to keep LightDHT easy to use, all DHT RPC calls are performed synchronously. This means that when you call a DHT method, your program will block until you have an answer to your request. That answer will be the return value of the function. This has the advantage that it keeps the logical program flow intact, and makes it more comfortable to use. 
In order to maintain O(log N) scaling across the network, BEP0005 (the standard governing the DHT) mandates that implementations use a bucket-based approach to the routing table. This enables the node to fulfill all requests in constant time and (more or less) constant memory. In LightDHT, we throw that recommendation to the wind. 
Since the main focus of LightDHT is reseach, we are going to keep around all the data we can. This means that we keep around every single node we know about. Since in practice the number of nodes is limited and the request rates are rather low, we do not bother keeping the routing table organized in a tree structure for quick lookups. Instead we keep it in a dictionary and sort on-demand. The performance penalty is well worth the reduced complexity of the implementation, and the flexibility of having all nodes in an easy to use data structure.

Saturday, December 10, 2011

Festive and Nerdy

You might have seen those big outdoor displays where every pixel is composed out of a little three-color LED light. In this post I'll tell you how I turned a string of those pixel lights into Christmas tree lights!

Adafruit industries, one of my favorite suppliers, recently started selling strings of a particularly advanced model of these pixel lights. Have a look at the product image, and tell me if it doesn't already look like a string a Christmas tree lights..
"12mm Diffused Digital RGB LED Pixels"
As you might have seen in my previous post, these pixels are rather easy to control from a micro-controller. However, stuffing a breadboard in your tree is not exactly festive. Thankfully, you can get these handy little tins that are just about the right size for a wall-wart plug, a teensy 2.0 with female headers (courtesy of Pieter Floris) and a big mess of wires.

A wall-wart plug, a Teensy2.0 and a big mess of wires.
Note that, thanks to the female-pin Teensy, everything is wired up using jumper wires. That's right - this is a no-solder project. Because the underside of the Teensy is not isolated, and because I have a number of wires in there that might come loose, I have isolated the interior of the tin using electrical tape. This is most likely not compliant with any code enacted after 1953 or so, so any imitation is purely at your own risk.

The only remaining question is how to fasten the plugs and wires onto the enclosure. Ideally you would use specially molded plastic caps and bits, but the odds of finding them in the right size and shape at your local hardware store are quite slim. Instead, I liberally applied black sugru around the holes I had cut into the tin, and then pressed the wires and the plug into it. The sugru just molds around the objects. After a quick touch-up I left it to cure overnight, and the result was really really good.

Now we just pop closed the tin and voila, a presentable programmable Christmas tree lights driver!

The controller and a single string of LED pixels, running a test pattern.
And here's the tree, all festive and nerdy:

Festive and Nerdy