[Stephen Edwards] had some time one Christmas. So he took a DE2 FPGA board and using VHDL built a pretty faithful reproduction of an Apple II+ computer. He took advantage of VHDL modules for the 6502 CPU and PS/2 keyboard, and focused more on the video hardware and disk emulation.

According to [Stephen], you can think of the Apple II as a video display that happens to have a computer in it. The master clock is a multiple of the color burst frequency, and the timing was all geared around video generation. [Stephen’s] implementation mimics the timing, although using more modern FPGA-appropriate methods.

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Learn FPGA Programming from the 1940s

We often think that not enough people are building things with FPGAs. We also love the retrotechtacular posts on old computer hardware. So it was hard to pass up [karlwoodward’s] post about the Chip Hack EDSAC Challenge — part of the 2017 Wuthering Bytes festival.

You might recognize EDSAC as what was arguably the first operational computer if you define a computer as what we think of today as a computer. [Maurice Wilkes] and his team invented a lot of things we take for granted today including subroutines (Wheeler jumps named after a graduate student).

The point to the EDSAC challenge was to expose people to creating designs with FPGAs, particularly using the Verilog hardware description language (HDL). If you want to follow along or run your own Chip Hack, the materials are available on the Web. You can see an FPGA driving a tape punch to create souvenir tapes in the video, below.

Some of the exercises are pretty simple and that’s perfect if you are starting out. The challenge uses a board with a Lattice ice40 FPGA and the open source toolchain for Lattice we’ve covered before. In fact, we’ve even done our own tutorials on the same basic device (but not the same board). Our final project generated PWM, not paper tape.

For the record, EDSAC was awesome. The execution unit was serial and processed bits that marched in one at a time over a mercury delay line. There is quite a bit of documentation and even some simulators, so if you ever wanted to get your hands into an old computer, this one isn’t a bad one to try.

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The Linux FPGA

It was never unusual to have a CPU and an FPGA together. After all, each has different strengths and weaknesses. However, newer devices like the Xilinx Zynq have both a CPU and an FPGA in the same package. That means your design has to span hardware, FPGA configurations, and software. [Mitchell Orsucci] was using a Zynq device on a ArtyZ7-20 board and decided he wanted to use Linux to operate the ARM processor and provide user-space tools to interface with the FPGA and reconfigure it dynamically.

This sounds like a big project and it certainly isn’t trivial by any means. However, the Xilinx tools do a lot of the heavy lifting, including setting up the Linux kernel and a suitable root file system. The bulk of [Mitchell’s] work was in developing user space tools for Linux programs to interact with the FPGA hardware. You can see a short video demo below.

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A Cold Hard Look at FPGAs

Researchers at the Delft University of Technology wanted to use FPGAs at cryogenic temperatures down around 4 degrees Kelvin. They knew from previous research that many FPGAs that use submicron fabrication technology actually work pretty well at those temperatures. It is the other components that misbehave — in particular, capacitors and voltage regulators. They worked out an interesting strategy to get around this problem.

The common solution is to move the power supply away from the FPGA and out of the cold environment. The problem is, that means long wires and fluctuating current demands will cause a variable voltage drop at the end of the long wire. The traditional answer to that problem is to have the remote regulator sense the voltage close to the load. This works because the current going through the sense wires is a small fraction of the load current and should be relatively constant. The Delft team took a different approach because they found sensing power supplies reacted too slowly: they created an FPGA design that draws nearly the same current no matter what it is doing.

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FPGA Clocks for Software Developers (or Anyone)

It used to be that designing hardware required schematics and designing software required code. Sure, a lot of people could jump back and forth, but it was clearly a different discipline. Today, a lot of substantial digital design occurs using a hardware description language (HDL) like Verilog or VHDL. These look like software, but as we’ve pointed out many times, it isn’t really the same. [Zipcpu] has a really clear blog post that explains how it is different and why.

[Zipcpu] notes something we’ve seen all too often on the web. Some neophytes will write sequential code using Verilog or VHDL as if it was a conventional programming language. Code like that may even simulate. However, the resulting hardware will — at best — be very inefficient and at worst will not even work.

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Retrocomputing With Open Source FPGAs

A few years ago, we saw the reverse engineering of the Lattice iCE40 bitstream, opening the door to a completely Open Source development tool chain for FPGAs. This was an astonishing amount of work from [Clifford Wolf], [Mathias Lasser], and [Cotton Seed], but since then we haven’t seen a whole lot from Project IceStorm. Now, that’s about to change, and in the coolest way possible. [hoglet] is retrocomputing on an ICE40 development board.

This is an implementation of the Acorn Atom on a myStorm BlackIce board. This board is basically just a Lattice iCE40 FPGA, a few support components, and a bunch of pin headers, some of which are in the not-so-handy Arduino pinout footprint. By porting some Acorn Atom implementations and a 6502 core to verilog, [hoglet] was able to stuff a cool old retrocomputer onto an Open Source FPGA development board. Video output is through a resistor DAC driving a VGA cable, and keyboard input is through PS/2.

Just about everything about this Open Source implementation of the Acorn works, and there’s still a lot left in the iCE40 FPGA. [hoglet] is able to run the 6502 core at 25MHz, which means just about every 6502-based system should be able to run on the BlackIce board.


TinyFPGA is a Tiny FPGA Board

We recently noticed an open source design for TinyFPGA A-Series boards from [Luke Valenty]. The tiny boards measure 18 mm by 30.5 mm and are breadboard friendly. You can choose a board that holds a Lattice Mach XO2-256 or an XO2-1200, if you need the additional capacity.

The boards have the JTAG interface on the side pins and also on a top header that would be handy to plug in a JTAG dongle for programming. The tiny chips are much easier to work with when they are entombed in a breakout board like this. Bigger boards with LEDs and other I/O devices are good for learning, but they aren’t always good for integrating into a larger project. The TinyFPGA boards would easily work in a device you were prototyping or doing a small production run.

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