Portable Apple II On An AVR

The Apple II was one of the first home computers. Designed by Steve “Woz” Wozniak, it used the MOS technologies 6502 processor, an 8-bit processor running at about 1 MHz. [Maxstaunch] wrote his bachelor thesis about emulating the 6502 in software on an AVR1284 and came up with a handheld prototype Apple II with screen and keyboard.

Prototype on veroboard

Originally, [maxstrauch] wanted to build an NES, which uses the same 6502 processor, but he calculated the NES’s Picture Processing Unit would be too complicated for the AVR, so he started on emulating the Apple II instead. It’s not quite there – it can only reference 12K of memory instead of the 64K on the original, so hi-res graphic mode, and therefore, many games, won’t work, but lo-res mode works as well as BASIC (both Integer BASIC and Applesoft BASIC.)

[Maxstrauch] details the 6502 in his thesis and, in a separate document, he gives an overview of the project. A third document has the schematic he used to build his emulator. His thesis goes into great detail about the 6502 and how he maps it to the AVR microcontroller. The build itself is pretty impressive, too. Done on veroboard, the build has a display, keyboard and a small speaker as well as a micro SD card for reading and storing data. For more 6502 projects, check out the Dis-Integrated 6502 and also, this guide to building a homebrew 6502.

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MicroLisp With Matching Parens

Lisp is a supremely elegant programming language, but you won’t find it around much today. That’s a shame; in the 80s and 90s, all the cool kids were using Lisp machines, computers dedicated to the creation and interpretation of Lisp. While the AI renaissance of the 80s is dead, replaced with the machine learning fad of today, Lisp machines have gotten much smaller. Now, they’ll fit in your pocket, and they have parenthesis matching, to boot.

If this build looks familiar, you’re not wrong. A while back, we saw a similar pocket Lisp computer based around the ATMega328 microcontroller with 32k of Flash and 2k of RAM. That’s not a lot by any measure, and a much more suitable processor for an AVR-based pocket Lisp machine would be the big boys of the ATMega family.

The new and improved version of the Tiny Lisp Computer is built around the ATMega1284. If it’s capable enough to run a 3D printer, it should run Lisp very well. With more program space and more RAM come more features including matching parens when entering code, a serial monitor interface, and a program editor – basically a text editor on the chip.

Apart from the larger chip, the circuit remains relatively unchanged. The display is still an OLED that can be had for a few dollars from the usual online retailers, and the other bits of circuitry are still just a handful of resistors, caps, and wire. An off-the-shelf FTDI module (or whatever serial chip you desire) can be added to connect to a serial terminal, and support for a PS/2 keyboard rounds out the board.

MicroLisp, Lisp For The AVR

We’ve seen tiny microcontroller-based computers before, but nothing like this. Where the usual AVR + display + serial connection features BASIC, Forth, or another forgotten language from the annals of computer history, this project turns an AVR into a Lisp machine.

The μλ project is the product of several decades of playing with Lisp on the university mainframe, finding a Lisp interpreter for the 6800 in Byte, and writing a few lisp applications using the Macintosh Toolbox. While this experience gave the author a handle on Lisp running on memory-constrained systems, MicroLisp is running on an ATMega328 with 32k of Flash and 2k of RAM.  In that tiny space, this tiny computer can blink a few boards, write to an OLED display, and read a PS/2 keyboard.

The circuit is simple enough to fit on a breadboard, but the real trick here is the firmware. A large subset of Lisp is supported, as is analog and digitalRead, analog and digitalWrite, I2C, SPI, and a serial interface. It’s an amazing piece of work that’s just begging to be slapped together on a piece of perfboard, if only to have a pocket-sized Lisp machine.

Thanks [gir] for the tip.

When You Need a Scope, You Need a Scope

Sometimes there’s just no substitute for the right diagnostic tool. [Ankit] was trying to port some I2C code from an Arduino platform to an ARM chip. When the latter code wasn’t working, he got clever and wrote a small sketch for the Arduino which would echo each byte that came across I2C out to the serial line. The bytes all looked right, yet the OLED still wasn’t working.

Time to bring out the right tool for the job: a logic analyzer or oscilloscope. Once he did that, the problem was obvious (see banner image — Arduino on top, ARM on bottom): he misunderstood what the ARM code was doing and was accidentally sending an I2C stop/start signal between two bytes. With that figured, he was on the right track in no time.

We just ran an epic post on troubleshooting I2C, and we’ll absolutely attest to the utility of having a scope or logic analyzer on hand when debugging communications. If you suspect that the bits aren’t going where they’re supposed to, there’s one way to find out. It’s conceivable that [Ankit] could have dug his way through the AVR’s hardware I2C peripheral documentation and managed to find the status codes that would have also given him the same insight, but it’s often the case that putting a scope on it is the quick and easy way out.

Encrypted USB Bootloader for AVRs

It probably doesn’t matter much for the hacker who sleeps with a bag of various microcontroller flash programmers under the pillow, but for an end-user to apply a firmware upgrade, convenience is king. These days that means using USB, and there are a few good AVR USB bootloaders out there.

But [Dmitry Grinberg] wanted more: the ability to encrypt the ROM images and verify that they haven’t been tampered with or otherwise messed up in transit. Combined with the USB requirement, that meant writing his own bootloader and PC-side tools. His bootloader will take unencrypted uploads if it doesn’t have a password, but if it’s compiled with a key, it will only accept (correctly) encrypted hex files.

Since the bootloader, including the USB firmware, is on the hefty side at 3.3 kB, [Dmitry] included hooks to re-use the bootloader’s USB code from within the target application. So if you were going to use V-USB in your program anyway, it doesn’t actually take up that much extra space. It’s a cute trick, but it ties the bootloader and user program together in a way that gives us the willies, without specifically knowing why. Perhaps we can debate this in the comments.

If you need an AVR USB bootloader, but you don’t need the encryption, we like Micronucleus. But if you need to deliver updates to users without them being able to modify (or screw up) the code in the middle, give [Dmitry]’s setup a try.

Embed with Elliot: Microcontroller Makefiles

Last time on Embed with Elliot, I began my celebration of the make command’s 40th birthday next month. We discussed using the default rules and how to augment them with your own variables defined in a makefile. Next, I’ll walk you through some makefiles that can be used for real-world microcontroller code development. This week, we’ll focus on one for the AVR platform, and later on, I’ll run through a slightly more complicated version for the ST32M series of ARM Cortex micros.

Along the way, we’ll pick up a couple of tricks, but the aim is to keep the makefiles minimal, readable, and easily extensible. Once you get a little taste of the power of writing your own makefiles, you probably won’t be able to stop adding bells and whistles — custom routines for flashing, checking the size of binaries, generating assembly listings, etc. I’ll leave the extras up to you, but you’ll eventually find that anything you do can be automated with a makefile.

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SDRAM Logic Analyzer Uses An AVR And A Dirty Trick

We often see “logic analyzer” projects which are little more than microcontrollers reading data as fast as they can, sending it to a PC, and then plotting the results. Depending on how fast the microcontroller is, these projects range from adequate to not very useful.

At first glance, [esot.eric’s] logic analyzer project has an AVR in it, so it ought to be on the low end of the scale. Then you look at the specs: 32 channels at 30 megasamples per second. How does that work with an AVR in it?

The answer lies in the selection of components. The analyzer uses a 128MB SDRAM DIMM (like an older PC might use for main memory). That makes sense; the Arduino can’t store much data internally. However, it isn’t the storage capacity that makes this choice critical. It seems [esot.eric] has a way to make the RAM “free run”.

The idea is to use the Arduino (or other host microcontroller) to set up the memory. Some of the memory’s output bits feedback to the address and data lines. Then the microcontroller steps aside and the SDRAM clocks samples into its memory by itself at the prevailing clock rate for the memory.

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